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"Scrambling around his room to find a hiding place, he noticed a trapdoor to the basement, hidden under his rug. Without hesitation, he flung open the hatch just as his mother burst through his door and threw himself down into the unknown depths below."
Intro narration
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The Binding of Isaac is a Roguelike Top-Down Shooter made by Edmund McMillen and Florian Himsl, released in September 2011.

The story follows Isaac, a young boy who finds himself in possibly the worst situation imaginable. After his fanatical Christian mother begins hearing the voice of God, she removes all of his worldly possessions and strips him naked, confines him to his bedroom, and unhesitatingly accepts a commission to kill him in sacrifice to prove her devotion. Isaac discovers her intentions and manages to escape into the basement, where, still naked, he goes on the run as he fights off nightmares both physical and mental (using his own tears, no less), discovers memories of his own past, and ultimately confronts Mom in an effort to survive.

Gameplay follows Isaac as he descends into the labyrinth underneath his house. Isaac wanders the maze that is the basement and beyond, collecting items and trinkets to slowly amass enough power to survive a confrontation with Mom. The player visits item rooms to acquire powerful upgrades and faces off against a boss on every floor, heading deeper and deeper downwards with each room cleared. The game relies on a lot of Random Generation to make each playthrough a unique, while additional characters themed after important people from Abrahamic myth can radically change how the game plays. Variety is the name of the game here, and the sheer volume of rooms, bosses, items, characters, secrets, challenges and endings ensures that no two runs through the basement will ever be the same.

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A remake, The Binding of Isaac: Rebirth developed and published by US-based independent game studio Nicalis, which overhauls almost every aspect of the game, has since been released on PC, Vita and PS4 on November 4, 2014. It has a rather creepy trailer (and an even creepier one here). Rebirth boasts a new engine, a larger development team, even more items, better synergies between said items, and more new content. Ports for the Wii U, New 3DS, Xbox One, and Nintendo Switch have also been released.

The game also has a Prequel/Spin-Off titled The Legend of Bum-bo. The basic premise of the game is a randomly generated RPG with a focus on puzzles.

Due to the surprising popularity of the game and the already large amount of content, multiple expansions have been released for both the original game and the remake:

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  • The original game received two free content updates prior to its paid expansion. The Halloween update released on October 31, 2011 and added a new character, demon-related items and bosses, and an extra chapter with a new final boss. Following it was the smaller Christmas update at the end of 2011, which added one new boss and item.
  • Wrath of the Lamb is the very first expansion, released on May 28th of 2012. It adds a new category of collectibles called "trinkets," plus new and upgraded items, enemies, bosses, alternate areas, room types, a new character, a new ending and more. It has a trailer. Be afraid.
  • Eternal Edition is a free update to the original game, released by Florian Himsl on May 2015. The expansion features its own hard mode where almost every enemy and boss receives an Elite Mook variant known as an "Eternal Champion". It's definitely as tough as it sounds.
  • Afterbirth, the remake's first expansion, released on October 30th of 2015. Not only does it come with the usual stuff (such as new items, bosses, and a new character), but it also features Greed Mode and even more alternative areas. Some of this new content are ideas suggested by the community; this includes items, challenges, and other features.
  • Afterbirth† (pronounced "Afterbirth Plus"), a slightly smaller expansion for the remake, released on January 3rd of 2017. The expansion not only includes even more items, bosses, and characters, but also features a detailed Monster Compendium, official developer tools and a Lua API specifically intended for modding the game. It also received five "Booster Pack" updates throughout 2017 and early 2018, adding even more content to the game, mostly official versions of mods. The fifth and final was dubbed "The Forgotten Update" by Nicalis Inc., and you can watch the trailer here.
  • Repentance, another expansion for the remake, released on March 31st of 2021. The expansion is an Updated Re-release of the Antibirth Game Mod, featuring the alternate path and expanding on it with new secondary floors. It also improves the co-op mode by allowing players to use full-fledged characters rather than babies, and includes a metric boatload of quality-of-life changes to previous content. The release date trailer for this expansion can be found here.

On June 27, 2018, a Kickstarter page for The Binding of Isaac: Four Souls, a card game published by Studio71, was launched, and was successfully funded.

As well as all this, Isaac has also appeared as a playable character in other games such as Blade Strangers and Crystal Crisis.


Tropes found throughout the series include:

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  • 1-Up:
    • The 1-Up gives an extra life without drawbacks.
    • The Ankh gives you one extra life, but revives you as ???, one of the gimmick characters. This also overrides any completion marks with ???'s marks for that run, which can be good or bad.
    • The Dead Cat gives you nine extra lives, but your health is set to one heart on pickup, retaining any soul/black hearts you may have. There's nothing stopping you from earning more, of course, but you also revive with only one heart.
    • Rebirth adds several more:
      • Guppy's Collar is similar to the Dead Cat, but with a catch. It has a 50% chance of giving you an extra life when you die, and has no limit to potential revivals, but can just as easily do nothing. In addition, you come back with only half a heart.
      • The Broken Ankh trinket behaves like the regular Ankh, but activates only about 20% of the time.
      • Judas' Shadow brings you back as a shadowy version of Judas who has a higher damage multiplier than his flesh counterpart, and counts as Judas for completion and achievements. Black Judas also starts with two black hearts, but can gain red heart containers normally.
      • Lazarus starts with the ability to revive. Regular Lazarus is fairly underpowered, with average stats and negative Luck, but once he dies, he revives into a better version of himself with a nice stat boost plus the Anemic item for the rest of the game, albeit at the cost of setting his Heart Containers to 1. Unlike other extra life sources, he will revive on the spot, allowing you to potentially finish off whatever killed you instead of having to start the room over again. Any other character with Lazarus' Rags lets them revive as Lazarus as well.
    • In Afterbirth, dying in a Sacrifice Room while holding the Missing Poster revives you as the Lost.
  • 100% Completion:
    • The "Golden God" achievement is obtained for getting all the items in the game. The "Platinum God" achievement is a step further: you need to collect all items from the vanilla game and Wrath of the Lamb. If you started off with the DLC, though, you're likely to end up getting both at the same time.
    • Rebirth adds the "Real Platinum God" achievement. This requires beating every path and Bonus Boss of the game with The Lost, who cannot gain HP in any way.
    • Afterbirth adds "1001%", which requires the player to collect all the new stuff past Real Platinum God.
    • Afterbirth+ adds "1,000,000%", which requires the player to collect all the new stuff past 1001%, as well as a Cosmetic Award for completing all three save files. Chances are you'll want to simply get a save to 299% and then copy-paste its files.
  • 20% More Awesome: The video announcement for the Halloween update of the Flash version states that the game is now "20% more evil".
  • Abnormal Ammo: Isaac's tears are weird enough as it is, but through upgrades, he can shoot flies, blood, teeth, urine, chocolate milk, ghost tears, explosive vomit, soy milk, and more.
  • Achievement Mockery: A few achievements require you to do some questionable things, and you'll need to if you want your 100% Completion, or to unlock some specific items.
    • Dying enough times unlocks either the Pinking Shears (original) or the Scissors (Rebirth).
    • Afterbirth+ has the "Mr. Resetti" achievement, awarded for resetting the game enough times in a row, which unlocks half Soul Hearts. On the PS4, the achievement is instead unlocked by losing 10 runs in a row.
    • Unlocking Lil' Spewer in Afterbirth+ requires you to kill yourself with an Ipecac explosion.
  • All Just a Dream:
    • The first time you defeat Mom, the victory narration (as well as the intro, which is in the same style) is revealed to be just a story Isaac is drawing. Subverted when she promptly barges into the room.
    • In Afterbirth+, the final ending reveals the whole game was just Isaac hallucinating from oxygen deprivation before he died. It's even implied that the opening narration, and the premise that Mom was trying to kill him, was all in his head as well. However, this part is possibly retconned into being a part of a bedtime story as of Repentance.
  • An Adventurer Is You: Most of the original characters are based on a stock character class: Isaac is a Jack-of-All-Stats, Maggy is a tanky healer, Cain is a lucky thief, and Judas is a Glass Cannon attacker. Later characters tend to get more unconventional roles
  • And Your Reward Is Clothes: Beating Mom's Heart for the first time in the Flash version unlocks an ending wherein Isaac opens a treasure chest and proudly holds up a fez. This fez unlocks Judas, so it's not a cosmetic gain.
  • Anti-Frustration Features:
    • Your tears hit slightly past the edges of rocks so that it's slightly easier to hit flying enemies hovering over rocks. This way, you're less likely to be forced to wait until they meander into a spot where you can hit them.
    • Normally, treasure rooms are locked and require a key for entry. The exception is the treasure room on the very first floor, as there are few rooms on that floor, making the event of finding a key rather uncertain. With Curse of the Labyrinth, which combines the first and second floors into one, one of them will be locked but the other is still free. Rebirth unlocks both by default, which turns the curse into a benefit if you get it on your first level.
    • The No-Damage Run achievements only require you to beat the bosses. This means you can take the shortest path to the boss rooms to avoid damage. It also takes effect when you beat the second boss, so you can backtrack after getting the achievement without having to worry about taking damage.
    • Pills can have negative effects, but not lethal negative effects. Health Down pills become Health Up if you are at one heart container or less and are not Blue Baby. Bad Trip becomes Full Health at one remaining heart total, unless you're Blue Baby, in which case it's at half of a soul heart. Number One causes any Range Down to become Range Up until the Womb.
    • Later levels can feature special rocks called keystones which cannot be bombed and instead can be "unlocked" with a key to get past them. If you are able to fly, keystones will be stuck in place no matter what, preventing you from accidentally wasting keys on them (although, curiously, they cannot be flown over).
    • Rebirth is all about making things more clear/less annoying, particularly for items which were previously poorly explained and just telling you how to unlock the majority of the characters. This even extends to the game interface itself — each run lets you record seeds and play other seeds, the game keeps track of how far you've gotten with which characters (i.e. which major bosses beaten), and most significantly, autosave lets you resume where you left off if you close the game.
    • In Rebirth, if a room is literally impossible to clear (such as a single enemy being placed across a gap, but your range isn't high enough to hit it) and you don't have any explosive items to bomb open one of the doors, the doors in the room will open automatically after a while to make sure your run doesn't end prematurely.
    • Eternal Hearts are sandwiched between your red hearts and soul/black hearts on the health meter, giving the player a chance to build up a buffer to keep it safe until it can turn into a Heart Container.
    • Completing the extremely difficult Family Man challenge unlocks the extremely powerful Epic Fetus, and also unlocks the Glass Cannon challenge, which starts you off with Epic Fetus, allowing you to immediately play around with the hard-earned weapon instead of waiting for it to come up in the game.
  • Artificial Stupidity: The Meat Boy familiar will eat your enemies for breakfast, but occasionally kicks bombs in your direction or get stuck chasing after and ineffectually biting enemies that are invincible (for example, the Mask part of the Mask-Heart monster). The Dead Bird has similar issues.
  • Ascended Extra: Isaac's pet Guppy is only seen in Isaac's will and some items. If you get three of said items in Wrath of the Lamb and Rebirth, you turn into Guppy, giving you flight and the same fly-spawning ability as The Mulligan, but at a much faster rate.
  • Asteroids Monster: The bosses Envy, Blastocyst, Fistula, and Teratoma split into multiple smaller forms as they lose health. A few normal enemies such as Moters (two connected flies) have the same trait.
  • Attack Drone: "Familiar" items follow behind Isaac or rotate around him, and typically shoot alongside him, block shots, or have unique effects.
  • Awesome, but Impractical:
    • Brimstone, a Devil Room item, turns Isaac's attack into a charged blood laser with infinite range and high continuous damage. It requires paying a couple Heart Containers to purchase and can be very sluggish to charge depending on your build, along with overriding a lot of other items, so while it can be extremely powerful, it can also reduce your DPS significantly while making it harder to keep enemies away or activate tear effects. Generally, its usefulness varies by expansion/version, as it's an item that frequently gets buffed and nerfed.
    • Items that grant explosive attacks. Explosions are very powerful and can replace having to preserve bombs, but they can just as easily cause self-damage if the current run doesn't have explosion immunity. They also tend to be saddled with weird traits: Dr. Fetus replaced tears with standard bombs that are subject to physics and need to be timed right against defensive enemies, Ipecac shots fire at an odd lobbing arc, and Bob's Brain, Fire Mind, and similar items require being careful about their positions or their random activations.
    • Due to the game's randomization and the built-in "special items" pool (which gets less common as more items from it are received or seen from it in a run), certain combinations of game-breaking items are theoretically absurd but very unlikely to come up in the same playthrough.
    • When Isaac is down to his last half-heart, the Scapular item grants a last-gasp reprieve of one soul heart. When combined with the Cursed Skull trinket (which teleports the player to a safe room when they have half a heart left), the player is functionally invincible at the cost of having to re-attempt the room he died in from the start. Good luck fighting bosses under no-hit conditions.
    • Technology 2 fires a constant laser that deals low damage, at the cost of a halved fire rate of normal tears. It's widely considered the weakest item in the "Technology" series, as the laser doesn't do enough damage to compensate for the lower fire rate without substantial synergy. Additionally, since Issac fires a shot every time you hit him, you'll face Bullet Hell and won't be doing nearly enough to compensate.
    • Crack the Sky causes several beams of light to strike the room, causing massive damage to anything they hit. The problem is that it's very inconsistent; the beams are only one tile wide and strike randomly, making the instances where you use it and instantly flatten a boss the exception rather than the usual. It also does no damage against Isaac and ???.
    • The "crying detonator" combo, which requires getting Sad Bombs alongside tons of explosion/bouncing items like Bob's Brain, Anarchist's Cookbook (or Tammy's Head), Rubber Cement, and Ipecac. This produces massive spreads of explosive bouncing tears that can wipe out whole rooms and bosses, but also lets you get killed very easily. Explosion-immunity items and general invulnerability can make it much more viable.
    • Missing No. completely randomized your items and stats at the start of every floor. The chance of this turning a poor run into a game-breaking one is much less common than getting stuck with something unworkable.
    • The Mega Blast. When you use it, you release a massive blood beam for 15 seconds, letting you utterly decimate everything for that period of time, and it even persists between rooms. It takes 12 charges to use (normal active items require 6 at most) and it's very difficult to unlock (beat Mega Satan with every character, including the hidden ones). It also pushes you in the opposite direction of where you fire it, which can be counter-intuitive and can shoot you right into spikes or enemies. With enough charge-reducing items or skilled use, it can still be worth carrying for a while run.
    • The synergy between Ludovico Technique and Ipecac. While the poison and explosion damage can be very helpful as you can specifically direct it towards your target, it always comes back to you when it explodes, leaving you unable to deal the continuous damage that Ludovico Technique is known for.
    • Plan C, a one-time-use activated item that deals 9,999,999 to every enemy in the room, and then kills you three seconds later. This will let you effortlessly beat any boss, but must obviously be used with extra lives. The one exception is Delirium, as its death animation takes longer than three seconds, and the game does not count it as dead until it is over, so you'll have to start the fight over again. The only counter to this is Lazarus's Rags, the only resurrection item that respawns you on the spot instead of outside the room, but also means Lazarus will get the post-it credit for it.
    • As of Afterbirth+, Brimstone and Ipecac combined. The massive damage increase from Ipecac now works in conjunction with Brimstone's massive damage multiplier, resulting in a Brimstone beam which does 565.5 damage in total (with Isaac's base stats), enough to one-shot some of the Womb bosses. However, the tear delay increases from both Brimstone and Ipecac are combined, so the Brimstone beam takes a huge length of time to charge. In addition, Ipecac shots are fired when charging the beam. To charge it fully you have to continuously fire Ipecac, which will most likely blow up in your face, literally. To avoid damage, you're probably going to be forced to tap-shoot Ipecac, which also has a much lower rate of fire due to the synergy, and not get any benefits of Brimstone.
  • Author Avatar: Ultra Pride, a special mini-boss, is a Creator Cameo of Edmund Mc Millen (Ultra Pride itself) and Florian Himsl (the familiar following Ultra Pride), with its name being a joke about their pride for making the game.
  • Belief Makes You Stupid: Isaac's Mom emotionally abuses Isaac, and then tries to make him into a Human Sacrifice, because of her religious faith and belief God is telling her to do so. It's hard to put a positive reading on that display of "faith".
  • Biblical Motifs: The game's story, characters, and many items are themed around aspects of the Bible. Plus, The Bible itself appears as a collectible item, with the Book of Revelations as a separate item, along with Satan himself as one of the final bosses in either his original form or as Mega Satan.
  • Big Bad: The frequent updates and use of True Final Bosses make the plot lines rather convoluted, but there is an order:
    • The original game has Magdalene O. Moriah, Isaac's Mom, who tries to sacrifice her son Issac believing she hears the voice of God commanding her to, and serves as the Final Boss. It is ambiguous as to whether she is simply delusional or the fact that God Is Evil and the Greater-Scope Villain. Though either way, Afterbirth+ reveals her to be a twisted hallucination of Isaac's real mother.
    • In Wrath of the Lamb and Rebirth, the real antagonist is implied to be Satan, who was using Isaac through his deals before turning on him and is implied to have been the true source of the voice, manipulating Mom into killing Isaac. Though Afterbirth+ reveals that he too was an illusion.
    • In Afterbirth+, it is revealed that the very final boss and true villain of the story is Delirium, a monstrous melting skull that is the physical manifestation of Isaac's declining sanity and Dying Dream, and is the one creating all the monsters/nightmares that Isaac has been fighting, including Mom and Satan.
    • With Repentance shifting around story elements, the new final villains of the story are Dogma, Isaac's interpretation of the Christian televangelists that drove his mother to extreme fundamentalism, and The Beast, the Mother of Harlots and the final demon Isaac has to slay. These both precede the reveal that the game's events may be a story being told by Isaac and his dad, and represent the finale of that story.
  • Black Comedy: The entire game is front-to-back and head-to-toe with dark comedy. Power-ups include the severed body parts of beloved pets (which act as damage-ups, given that the game weaponises tears), a wire coat hanger (a common home-made abortion tool in real life which amps up Isaac's tears), several items that increases Isaac's health and attack power implies his less-than-stellar home life (Breakfast, Dinner, Lunch, Tough Love) and various (possibly) aborted or otherwise dead siblings of Isaac's including Brother Bobby, Sister Maggy, and Harlequin Baby (who act as combat companions). Hell, there's even a Secret Character in the form of a cyanotic corpse.
  • Blackout Basement: The Curse of Darkness results in the entire floor becoming darkened, with most of the visilbility coming from around Isaac's body.
  • The Blank: The angel statue that appears in Angel Rooms as an alternative to deals with the Devil has a blank, featureless face. This also applies to Uriel and Gabriel, the real angels contained within the statues. This may be a reference to the Bible passage Exodus 33:20: "But," He said, "you cannot see my face, for man shall not see me and live."
  • Bloodstained Glass Windows: The first floor on one of the split paths after Chapter 4 is the Cathedral, a blue light-themed counterpart to Sheol containing angelic enemies and lots of tough rooms.
  • Bloody Bowels of Hell: The game leans heavily towards this over Fire and Brimstone Hell, with floors like Sheol and the Dark Room being made of black stone and retaining the blood-attacking demons featured throughout the rest of the game.
  • Bloody Murder: Almost all enemies use weaponized blood to attack. With the correct upgrades, Isaac can too.
  • Body Horror: Most powerups do horrible things to you in exchange for their effects. In no particular order, you may grow tumors, become a living insect hive, acquire tons of diseases, cry blood, prop your eyes open with toothpicks, jam implants into your eyes, and so many more.
  • Bonus Level of Heaven: The Cathedral is a holy alternative to Sheol and ends with a fight against an angelic Isaac, but it's still just as tough as any other late-game level.
  • Boring, but Practical:
    • Flight is a simple effect granted by many items, but being airborne suddenly means that many dangers are a thing of the past, like chasms, spikes, and creep, and lets you grab some pickups without wasting bombs or keys. It even has a combat benefit in many rooms: being able to fly above a rock or a hole that enemies can't reach means that you're effectively untouchable by melee-only enemies, letting you pick them off safely. Even against ranged enemies, your movement is unhindered by terrain, so you have significantly more room to maneuver with. Most bullets also can't penetrate rocks, so they serve as effective cover, especially if you have an upgrade which allows you to shoot over them.
    • In the Flash version, The Battery allows you to charge up your item much faster than usual, if you stick around in combat for a while. Simply kill all but a single enemy, and dodge until you get all you can out of it. The item was substantially revised in Rebirth (now letting you store two charges of an item at once) to prevent this waiting game.
    • The Charm of the Vampire item allows you to restore half a heart for every 13 enemies you kill. One type of enemy in particular, the Globins, can regenerate back from the point after they count as a kill for the purpose of this item. It's a slow slog to max health, but there's a good chance it's worth the trouble. The same can be true when you're in the later levels and find an arcade. In fact, the more useful it is, the more boring it becomes.
    • The Book of Revelations' only instant effect is to give you a soul heart, and it also makes the Harbingers more likely to appear as a floor's boss. Soul hearts are very useful, and so are multiple Cubes of Meat/Balls of Bandages or the Pony, making it worth keeping around long enough to hit the heart cap and guaranteed decent items.
    • The Candle from Wrath of the Lamb shoots a small flame forward a relatively short distance, doing very decent damage to anything it passes through, enough to One-Hit Kill most normal enemies. It also has a short recharge rate for an activated item, replenishing within a few seconds without needing to clear the room. This makes it extremely useful against anything large, armored, or slow/immobile, and its hitbox can affect certain enemies in their invulnerable states.
    • The Bloody Penny trinket sometimes gives you a half-heart whenever you pick up a penny or other coin. While not as useful/awesome as Maggy's Faith (free eternal heart at the start of each floor) or The Polaroid (gives you an emergency protective forcefield as well as letting you into The Very Definitely Final Dungeon), those dropped hearts add up, allowing you partially farm for keys, bombs, and money in the Arcade (Blood Bank trades hearts for money, Bloody Penny gives you hearts when you get money, and Slot Machines sometimes give out bombs, keys, money, and hearts in exchange for money).
    • The Yum Heart, Magdalene's starting active item. No damage, no room-wide effect, no awesome buffs, just a free red heart every few rooms or so, which can mean the difference between staying healthy in the last areas or limping around as a One-Hit Point Wonder desperate to find more hearts somewhere.
    • The Box is a boss room item that gives you one of each consumable, a pill, a trinket, and a tarot card, but no big damage ups or HP increases (unless it spawns a soul heart, or gives you a positive pill). While it can seem like (and can very well be, depending on your drops) a Power-Up Letdown, in certain runs where the Random Number God doesn't feel like giving you much of anything, just having something like an extra key or bomb on hand can end up getting you access to the items you need to win. It's less ideal for players looking for stat upgrades from the boss room, though.
    • The Smart Fly/Revenge Fly. Certainly not the most glamorous familiar you can have, a tiny glowing yellow fly that doesn't block any shots. However, if an enemy so much as sneezes on you, it will home in onto and start ripping apart every enemy in the room as fast as it can. It can even deal a steady stream of damage to most bosses and can save you from the brink.
    • There are a few power-ups in Rebirth that allow you to break rocks or other obstacles by walking into them. It doesn't sound all that useful at first, but when you factor in all the rooms where rocks serve as cover for enemies or can hem you in, ones with pick-ups on the other side of them, and the blue rocks that can produce Soul Hearts or other items, there's suddenly a lot of ways it makes things easier or more lucrative. This also saves you bombs for damage or finding secret rooms, and one trinket guarantees you a coin drop whenever a rock is destroyed (which works very well with certain Cast from Money items, such as Magic Fingers and Money = Power). Destroying certain rocks can have negative effects, but they're outpaced by the potential gains.
    • The Arcade almost always includes a Blood Bank (which allows you to exchange life for money, with a small chance at other prizes) and a Slot Machine and/or Shell Game beggar (which allows you to exchange money for a random chance at getting pickups, including life). If there are a lot of hearts lying around a floor after you clear it, you can use the Blood Bank to turn the hearts into coins, which can be fed into the Slot Machine/beggar, which will usually give you some hearts, which can be fed into the Blood Bank for coins, and so on. With enough hearts or coins (or the Yum Heart + Nun's Habit, which charges the ability to refill your hearts every time you take damage from the Blood Bank), you can continue this process until one machine or the other blows up, usually yielding a useful item. You're also likely to collect a bunch of Bombs and Keys and probably some Pills and Tarot Cards, and there's no real downside to doing this every chance you get aside from the time investment, which dos limit some of your path options in Rebirth.
    • Several effects, like piercing shots from the Cupid's Arrow and triple shots from using Inner Eye are boring, but useful in most situations, with the boring part leaving when you get both.
    • How to Jump. It's an unlimited-use spacebar item which allows Isaac to jump over any obstacles for a short distance, essentially granting poor man's flight. It's also surprisingly good for dodging enemy attacks, especially the bosses who employ Bullet Hell. Since it has no recharge whatsoever, you can spam it with the Broken Remote (a trinket that teleports you every time you use a spacebar item) to freely access almost every room on a floor, including the I AM ERROR room.
    • Shooting the poop and fires in every room. Most of the time you get nothing, but sometimes you get hearts, money, or even the Petrified Poop trinket (which increases the chance you'll find something in the poop, even super-rare pickups like eternal hearts). When you're low on health, but have plenty of time, shooting the poop can be a life-saver.
    • The Placenta item from Rebirth. It immediately gives an extra heart container on pick-up, and will slowly restore your health at a rate of half a heart every few minutes. If you're low on health and have exhausted every other option, this item lets you gradually regenerate to full health just by leaving the game on for half an hour or so while you physically get up and do something else to pass the time.
    • Orbital familiars in general, including the lowly Pretty Fly. They don't give you health, but they do block projectiles from hitting you, quite often if you obtain three of them. Many of them very quickly deal damage on contact, making them potent boss killers if you are willing to get close. Combine with invincibility and you can wreak havoc with them.
    • Most of the items dropped after boss fights fall into this. The majority of them give a stat and/or health boost. Not too glamorous, but those really add up over the course of a run, and increasing your stats, especially damage and tears, is never a bad thing.
    • The Dollar, Pyro, and Skeleton Key max out your supply of money, bombs, and keys respectively. They don't provide any other effect, but 99 of any consumable virtually guarantees you won't run out, and all three make it much easier to obtain more directly useful items in one way or another.
  • Boss Bonanza: The Womb randomly has previous end-of-level bosses spawn in normal rooms. Sheol and the Cathedral have a much higher chance of these encounters. The Chest has a boss in almost every single room, and the same is true of its Rebirth-exclusive counterpart, the Dark Room. The Void, meanwhile, has several full-fledged boss rooms.
  • Boss Remix: There's a remix of the title screen’s music in both versions of the game while fighting an important boss: Isaac and ??? in Wrath of the Lamb and Ultra Greed in Afterbirth and Afterbirth+. Rebirth's version, "Genesis 22:10", also makes an appearance in Afterbirth+'s Void floor, whose music is a medley of the title theme, the Burning Basement theme, and the alternate boss music introduced in Afterbirth.
  • Boss Rush:
    • Many of the bosses appear on later floors as Degraded Bosses; sometimes two at once. The challenge rooms become this in later levels, and there's also special challenge rooms that only spawn bosses, even at earlier levels; these rooms are normally accessible only if the player has one or half a heart left.
    • One of the floor curses, "Curse of the Labyrinth", is a Random Event that combines two floors, meaning you'll have to fight two bosses one after the other.
    • The Chest/The Dark Room, two of the possible final floors, are mostly made up entirely of rooms containing multiple bosses that must be fought at the same time, though most of them can be defeated very quickly by this point.
    • Rebirth adds a proper boss rush: if you beat Mom inside of 20 minutes a secret room will open up in the wall leading to a giant room with four item pickups. Pick up one and you have to fight almost every boss in the game coming in pairs with no escape but victory, death, or teleporting out. Succeed and you get another item.
    • If you fight Mega Satan, you get to fight all of the Sins and the Four Horsemen, plus fallen versions of the two angels you bombed for the pieces of the key to get into the fight, all during the course of fighting the big boss himself.
  • Brain Monster: The game has enemies that are large brains that crawl around the floor, leaving a trail of blood that damages you if you step on it. There's also an even larger variant that splits into two smaller ones when it dies.
  • Breakable Power-Up: The Wrath of the Lamb DLC introduces Eternal Hearts, which give Isaac half a heart's worth of health above his normal maximum. If he loses it, it goes away for good, but if he finds a second Eternal Heart or makes it to the end of the floor without losing it, it turns into a Heart Container and permanently increases his maximum life.
  • Breath Weapon:
    • Blood/Brimstone lasers are fired from the mouths of several enemies and can be used by Isaac himself with items like "Shoop Da Whoop!" and Brimstone; Azazel starts with a short-ranged variant. These are powerful, but take precision to use.
    • Chocolate Milk allows Isaac to fire a charged shot from his mouth. Its size and the damage it does depend on how long it's been charged for.
    • Monstro sprays a burst of red tears at Isaac as one of his attacks. The item Monstro's Lung lets Isaac do the same thing.
    • Taking Ipecac makes Isaac vomit up green explosive shots. Since they're fired from the mouth they travel in an arc, which makes smaller enemies harder to hit.
  • Brutal Bonus Level: All of the floors past the standard ending point in Chapter 4 count as this, retaining the full-heart damage of the Womb floors while mixing in tougher enemies and obstacles. These include Sheol, the Catherdral, the Chest, the Dark Room, the Void, the Corpse, and Home.
  • Bullet Hell:
    • The game leans more into this shooter style as expansions progress, with late-game enemies and bosses ramping up how many bullets they fire at once. Things can get quite ugly with several shooting enemies on the screen.
    • In Rebirth, Mom's Heart has a firing pattern very reminiscent of traditional danmaku bosses, with slow projectiles that have wildly confusing trajectories. Then there's Mega Satan, especially in his second form, and Hush is practically an Extra Stage boss. Finally, there's Delirium, who manages to mix-and-match all of the above.
  • The Cameo:
    • One of the available powerups is a chunk of meat which, after collecting several, can be used to create a Meat Boy who will follow you and help fight enemies. There's an achievement for finishing him. Rebirth adds balls of bandages which become Meat Boy's girlfriend, Bandage Girl.
    • The walls of the Arcade are lined with posters of other McMillen games like Time Fcuk.
    • Certain characters from other games make cameo appearances as alternate versions of boss fights, like Steven for Gemini, C.H.A.D. for Chub, Gish for Monstro 2, and Triachnid for Daddy Long Legs. The only alternates that aren't cameos are the plot important ones, It Lives and ???.
  • Cast from Hit Points:
    • The Devil Rooms, which exchange max HP or Soul Hearts for items. There are also several items, such as Blood Rights and Razor Blade, that damage the player when used.
    • Wrath of the Lamb features spiked doors that lead to special red treasure chests, as well as sacrifice rooms and demonic beggars that reward the player for damaging themselves.
    • Several items, like the Dead Bird and Red Patch, will only activate if Isaac takes damage.
    • The Kamikaze item. You can blow anything up, at any time, but the bomb you use is stuck to you, forcing you to take damage in the explosion unless you have invincibility or explosion immunity.
  • Cartoon Bomb: The bombs used by playable characters are round, have a fuse, and are marked with a skull. Troll bombs have a trollface in place of the skull.
  • Charged Attack: The chocolate milk enables a hold-type charge attack, with the chocolate milk spit getting stronger as it's charged up. Brimstone is also hold-type, but requires a full charge to fire. In the Flash game, they interacted oddly when combined, but in Rebirth Chocolate Milk just speeds up Brimstone's charge time.
  • Checkpoint Starvation: Played straight in the Flash version, averted in Rebirth. In the Flash version, a run needs to be completed in one sitting — you cannot save your run and leave the game, due to the limitations of Flash. In Rebirth, due to being built entirely on a new engine, you can save your current run and come back to it at a later date.
  • The Computer Is a Lying Bastard: A few items in the original version of the game had mistakes in their item blurbs for how they work; for example, the Toothpicks item says "tears up" when it actually increases shot speed. Rebirth fixes all of these items, or changes the description to fit the effect.
  • Color-Coded for Your Convenience:
    • Green is the color typically associated with acidic attacks, mainly the dangerous ipecac explosive shots.
    • Most Champions are color-coded (with the exception of the size and "pulsating" variants), based upon the additional abilities they have and what items they drop when they are killed:
      • Red Champions have increased health and drop red hearts.
      • Yellow Champions are faster than their non-colored counterparts, and drop a battery.
      • Green Champions have increased health, leave a damaging trail of creep and drop pills.
      • Pink Champions have increased health and randomly launch projectiles, but don't spawn anything upon death.
      • Dark blue Champions don't have anything special going for them (besides doing a full heart of damage like other Champions), but spawn a few blue flies upon death.
      • White Champions are invincible until every other enemy in the room is killed, upon which they can be damaged normally. They drop Eternal Hearts.
      • Transparent and white Champions can move through obstacles that would obstruct it normally, like pits and rocks. They drop golden chests upon death.
      • Grey Champions have decreased health (but still deal a full heart of damage) and drop keys.
      • Pitch black Champions will fade in and out of visibility like a Null, and drop a red chest.
      • Orange Champions have increased health, will make Isaac lose money if they hit him, and drop coins.
      • Black (well, more of a dark green) Champions have increased health, explode when killed and drop bombs.
      • Dark red Champions will turn into a pile of red mush when killed which, if not killed within a short amount of time, will regenerate and begin attacking Isaac, similar to a Globin.
      • Large Champions have substantially increased health, deal two full hearts of damage, and drop "One Makes You Larger" pills. The noises they make are also a good deal lower-pitched.
      • Mini Champions have the same health but are much smaller, and drop "One Makes You Smaller" pills. Like Large Champions, the noises they make are modified; in this case, they're of a higher pitch.
  • Competitive Balance: Each character is designed with this is mind.
    • Isaac:/The Warrior aka, Jack-of-All-Stats: Isaac is not really effective at anything, yet has no real downsides. Any item can be possibly played in his advantage, and a good Isaac player will take what is given to them, and use it effectively. However, after beating Isaac as the secret character, Isaac obtains The D6 as a starting item. The item allows Isaac to transform an item into another one, meaning Isaac can have a selection of 4 items per floor.
    • Cain/The Thief aka, Fragile Speedster: Cain is primarily luck based, with good damage and speed but low HP. His Luck Foot starting items makes him luckier than any other character in the game, and can turn a run around with one good pill or spin of the Gambling Machines. Cain's strength relies on getting lucky, and if it pays out, then Cain can become exceptionally strong.
    • Maggy/The Cleric aka, The Medic/Stone Wall: Maggy has the most starting HP, but is slow and weak. To combat this, Maggy has a starting item (Yum Heart) that can restore a Red Heart every few rooms, keeping her HP high. She can also use this to enter curse rooms without losing health by regaining it using Yum Heart. Maggy's problems can also be taken care of if she gets a Devil item by sacrificing her high HP, meaning that Maggy can trounce the early game.
    • Judas/The Mage aka Squishy Wizard: Judas has low HP and good damage, but starts with the Book of Belial, which can boost his damage output to high levels, and is useful his whole run. If a Judas player can effectively make use of his offensive tactics, then Judas can be potentially the most powerful character in the game.
    • Eve/The Druid aka Critical Status Buff character: Eve starts with low HP and poor damage, but has Whore of Babylon, an item that boosts her speed, firing rate, and damage when she is at low health (which also activates at one full heart as opposed to half). Eve relies on this to be effective, and if she has a chance to obtain lots of soul hearts and lower her red hearts, Eve can be a powerhouse that can trounce even the toughest of enemies.
    • Samson/The Berserker aka Glass Cannon (changed to a second Stone Wall in Rebirth): Samson has low starting HP and somewhat terrible stats, but begins with Bloody Lust, an item that increases his damage for every enemy that dies and resets when he leaves the room. Samson is bad against bosses, but in rooms with loads of enemies, he becomes a force to be reckoned with, able to blow through enemies with ease. Rebirth reverses his abilities, making him have as much starting HP as Isaac and become stronger as he takes damage rather than as he kills, so any items that provide healing can turn him into a powerhouse.
    • ???/Blue Baby/Joke Character/Lethal Joke Character: ??? has a distinct disadvantage to anyone else: he has no Heart Containers, only soul hearts. While this may seem like an obvious downside, any and all effects that rely on having low HP automatically and permanently kick in, meaning that if he gains the right items (like Whore of Babylon), ??? can absolutely decimate any boss in the game, and sweep enemies to the side. Oh, and once you've unlocked the final levels, you are guaranteed to get your choice between two powerful low-HP items after defeating Mom. Besides this, Devil Rooms cost 3 soul hearts for everything, and HP Ups only give you more soul hearts. Rebirth also gives him slightly more damage and speed than Isaac (but not to nearly the extent of Cain or Judas).
    • Azazel/The Demon aka Glass Cannon/Fragile Speedster: Rebirth character. Starting with flying and a close-range laser, as well as the best starting speed, Azazel has to quickly switch between dodging fire and dealing in the pain up front. Starts with no hearts but three temporary black hearts, but unlike ???, can get new hearts.
    • Lazarus/The Paladin aka Master of None: Rebirth character. Lazarus starts with good health but otherwise terrible stats (including luck), but can revive with great stats but only one health. Pre-death Lazarus can't keep up with the other characters and post-death Lazarus is as fragile as Judas without the massive offense (and leaves you even worse off if it happens late in the run). The only way to make him really effective is to leverage his pre-death health to give his post-death self an advantage; either coast on it until you find a health up item and die on purpose, or go for broke by spending it all on devil deals.
    • Eden/Luck Based Character: Rebirth character. The genderless Eden starts with randomly assigned stats and a couple of items. Since the number of times you can restart as Eden is limited, you have to make do with whatever the RNG gives you.
    • The Lost/Harder Than Hard: Rebirth character who starts with good items but dies in one hit and can never gain more health, ever. If you discover the complex sequence of events needed to unlock thisnote , you deserve your punishment. Afterbirth makes this somewhat easier by giving him the Holy Mantle as a starter item once you donate 879 coins to the Greed Machine, meaning you'll only die if you take two hits in the same room. He can also take Devil Deals for free, so use that to your advantage.
    • Lilith/The Minion Master: Afterbirth character. Lilith is unique in that she starts blindfolded, which means she can't fire tears herself. However, her familiar Incubus copies her tears, which allows Lilith to still benefit from most tear-enhancing items. She starts with the Box of Friends, which creates an additional copy of all your familiars for the room, doubling her DPS (or more if you can find items that let you use the Box multiple times in one room) with Incubus alone, and even more with extra familiars. She also has the Cambion Conception which grants you a demonic familiar permanently after taking damage a significant amount of times.
    • Keeper/The Gambler: Afterbirth character. Unlocking him takes forever, so you would expect him to be overpowered, right? Nope. He uses Coins as Health, and all Hearts (as in every type of Hearts) automatically turn into Blue Flies. He also cannot increase his Max Health past 2 and all hits deal one full container worth of damage, effectively making him even more of a Two Hit Point Wonder than The Lost (who at least has flight to make dodging easier and (eventually) the Holy Mantle to absorb one hit per room). To top it all off, the Keeper's stats are all-around trash; he moves slow, fires slow, and has worse Luck than Lazarus. But to make things not completely terrible, he does come with a triple shot and you can unlock some profitable starting items for him. Repentance makes things better for him by letting him have a maximum of three hit points total (and starting with that much max health after beating Hush) and letting him buy devil deal items with money.
    • Apollyon/The Assimilator: A new character added in Afterbirth+, Apollyon begins with fairly below average stats and the Void item, which destroys pedestal items and gains their abilities, if the destroyed items are active ones, or grant Apollyon random stat boosts, if they're passive. This means that while Apollyon starts as a rather weak character, with a bit of luck, he may end up with an active item that has a plethora of uses at once, or with massively buffed stats, potentially making him unstoppable in the late game.
    • The Forgotten/Mechanically Unusual Fighter: Added in the final booster pack to Afterbirth+, is one of the most unusual characters in the game. For starters, they are two characters in one: The Forgotten and the Soul. The Forgotten themselves mainly uses a melee attack via a bone club, which can be charged up to be thrown like a boomerang, and can only gain bone hearts. The Soul functions similarly to the Lost minus being a One-Hit Point Wonder, but cannot move a certain distance from the Forgotten, and can only have Soul Hearts. Key to playing them effectively is knowing when to use each character, since they have separate health bars and both die when one dies.
    • Bethany/White Mage: Added in Repentance, Bethany starts with the Book of Virtues, an item which spawns a blue wisp that orbits her and can block damage or fire as she does, but uniquely can synergize with just about any spacebar item in the game. She also gains more uses out of her space bar items by accumulating charges when she gains soul hearts, but she cannot use soul hearts to add to her health, leaving her reliant on red hearts. She also has a bigger chance of getting angel rooms over devil rooms.
    • Jacob and Esau/Tag Team: Added in Repentance, Jacob and Esau are two characters in one, though unlike the Forgotten, both are active at the same time. They both move at the same time and place bombs at the same time, but you can hold a button to only move Jacob in case you need to reposition him. They also do not share items, making choice key to which one you give certain items to, but they also can each carry a spacebar item, effectively giving you two spacebar items available at the same time. Like the forgotten, Jacob and Esau have their own health bars and both die when one dies.
  • Confusion Fu:
    • The Dead Sea Scrolls gives the effect of a random spacebar item per use, though it has a small pool of ones it can pick.
    • Any Rainbow item (including the 3 Dollar Bill) will have randomized tear effects that change every three seconds, such as the Rainbow Baby having a random effect for every individual tear. The Pride Day Challenge in Afterbirth has you equip all of them at once, allowing them to stack with different random combinations.
    • The spider enemies from Wrath of the Lamb have quick and erratic movements. Their movements are impossible to read and thus are the bane of prospective No-Hit runs.
    • Eden in Rebirth starts with randomly generated stats and items.
  • Crapsaccharine World: The game has cutesy Thick-Line Animation, adorable characters, and horrific monsters they have to beat to avoid becoming a red smear on the basement floor. Edmund says he did this intentionally to make the game more palatable.
  • Creepy Basement: Isaac's journey starts in the basement of his house, filled with flies, spiders, and deformed humans, and things only get worse the deeper he goes.
  • Critical Annoyance: Averted. The game alerts you that you have half of a red heart left by having the heart blink and by the character peeing themselves a little every time they enter a new room (except for Eve), but these aren't distracting. There are several items that activate when Isaac reaches critical status which alert the player as well.
  • Critical Existence Failure: Isaac is perfectly fine when at half a heart, besides peeing himself when entering a room. However, dying causes him to explode into a mist of blood in the Flash version, or collapse to the floor as his ghost leaves his body in Rebirth.
  • Critical Status Buff: The Whore of Babylon item gives a major damage buff when you're reduced to half a red heart or have nothing but soul hearts.
  • Cursed With Awesome:
    • Literally with the Whore of Babylon item, which has the quote "What a horrible night to have a curse."
    • Devil Rooms take your hearts and give you cool items in return.
    • Challenge Runs, introduced with Wrath of the Lamb, remove Treasure Rooms but give you a specific item set to start, which for some challenges can make the game a breeze.
    • Dead Cat immediately lowers your HP to one but gives you nine lives; while you can gather soul and black hearts to make up for it, those will be gone after you lose a life, which makes the whole lives thing pointless unless you're really skilled. However, it is insanely practical for the Lost, who cannot pick up any hearts and will always be at one HP regardless, which makes it a must-have for his runs.
    • Sometimes Curse of the Maze will take you exactly where you're trying to go and bypass having to clear certain rooms.
  • Cybernetics Eat Your Soul: One of the items you can buy at the cost of heart containers in the Devil Room is Technology.
  • Cycle of Hurting: Respawning in exactly the wrong spot can result in your remaining Dead Cat lives being taken away by a Slide that you cannot avoid because of the Item Get! pose.
  • Damn You, Muscle Memory!:
    • Regardless of whether you prefer W-A-S-D or the arrow keys to move, odds are you're going to trip up at least once from using both of them. Additionally, Q is "switch to your last-used item" in many games, but here, it uses your pill or tarot card, and E is "use item" or "interact with surroundings" most of the time, not drop a bomb where you're standing. The main problem with all this is that the Flash game doesn't let you just re-bind the keys to something more comfortable.
    • If you use a controller to play the PC version, you'll quickly notice that the left trigger for using your special item is easy to mix up with the Vita version's left shoulder button that is used for dropping bombs; the special item button gets instead remapped to right shoulder button. BOOOOOOOM!
  • Darker and Edgier: Edmund's previous works like Meat Boy are rife with Black Comedy as is, but The Binding of Isaac is significantly darker than any of them in tone and themes even if it plays much of it for humor.
  • Dark Is Not Evil: Dark Bum's item description is "he wants to take your life", which tends to have the player bracing themselves for a crippling Power-Up Letdown. His effect is actually literal, because he picks up unused red hearts and converts them into soul hearts. This is almost always an excellent trade (especially if you're playing as ???, who has absolutely no use for red hearts), making Dark Bum a desirable item
  • Dark Reprise: Repentance has "Dread", a horrifically dark remix of "The Calm" which plays after killing Mom's Heart in the Mausoleum, much like its Antibirth counterpart Spinning Intensifies.
  • Dead Character Walking: A patched glitch in version 1.3 of the Flash game could do this. If you had only half a heart left aside from ???, who has no natural hearts and took one full heart of damage, rather than dying like normal, you would instead act as if you had constant mercy invincibility; touching or getting hit by Greed would still sap your money, but you never took damage at all, were counted at low health, and could use health taking machines, like the devil beggar or blood donation, constantly with no penalty, while technically dead. Picking up a full heart would restore you to half a natural heart.
  • Deal with the Devil: The Devil Rooms allow the player to exchange Max HP for items. Wrath of the Lamb introduces the demonic beggar, which damages the player in exchange for the possibility of dropping an item.
  • Death of a Thousand Cuts:
    • Maxing out tears in the early floors can result in this, depending on your base damage. Isaac with full tears and no damage upgrades can take down high level bosses such as Loki and Monstro II competently by swarming them with tears. Even more deadly with piercing, homing, or curving tears, or Standard Status Effects tears, whose effect chance is calculated on a per-hit basis.
    • The thin Odd Mushroom deliberately invokes this by increasing rate of fire greatly while reducing damage.
    • Technology 2 provides a near-constant laser at the cost of 35% damage.
    • Rebirth has the Soy Milk item. It cuts your damage down to a measly 20%, but you can fire dozens of tears per second.
  • Degraded Boss: Many bosses can reappear late in a run as a "normal" enemy.
  • Denial of Diagonal Attack:
    • Under normal circumstances, Isaac can only fire in the four cardinal directions, though there's some wiggle room using momentum to make the tears move at an angle.
    • Afterbirth adds Marked, which automatically fire tears at a target on the floor controlled by the player (like Epic Fetus but with tears).
    • Inverted with the "R U a Wizard?" pill effect and The Wiz item in Afterbirth, which make you fire tears only diagonally, denying you from attacking horizontally and vertically.
    • Afterbirth+ adds the Analog Stick, which allows the player to fire diagonally or straight at will. If playing with an analog stick or mouse, then you can fire in any direction you want.
  • Desperation Attack:
    • The Whore of Babylon item, which makes the player much stronger when having only half a red heart or only soul hearts. Eve starts the game with this item and can gain the benefits if she has a full red heart remaining. It's not a Desperation Attack if ??? or The Lost picks up this item, as it is always active.
    • Wrath of the Lamb introduces a sort of desperation room, normally accessible only if the player has one heart remaining. These rooms always contain a special item... but they're also challenge rooms that spawn bosses when the item is taken. Again, ??? and The Lost can always enter these rooms. Unfortunately, these rooms were massively Nerfed in Rebirth, probably because they were easy to abuse. Items can still show up in these rooms, but it's more likely there will be a plain old chest.
  • Detachment Combat:
    • When using the Pinking Shears or Transcendence (the latter is permanent), Isaac floats around as a severed head. The former also turns his body into a secondary attacker, but only works for the current room.
    • The Scissors does the opposite, and has your head become the familiar while you control the body. The severed head fires tears, though despite missing the head the body can fire tears as well.
    • The Peeper pops out one of your eyes, which bounces around the room, hurting everything it touches.
    • Guillotine turns your own head into a rotating shield that deals high contact damage. It becomes a bit finicky though since it also moves the area you shoot tears from.
    • Leprosy causes chunks of flesh to fall off when you take damage, which then orbit around you and can damage enemies, as well as block projectiles.
  • Developers' Foresight: Has its own page.
  • Difficult, but Awesome:
    • Dr. Fetus allows you to shoot bombs instead of tears. On one hand, it can hurt you and it takes good timing. On the other hand, you have limitless bombs (enabling you to clear out rocks and get to items and secrets more easily), and enemies and bosses die really easily. This is taken Up to Eleven with the Ipecac Syrup in Wrath of the Lamb: it does more damage than bombs and poisons on hit, but its arcing path is very difficult to work with. Rebirth introduces items like Host Hat and Pyromaniac block explosive damage or cause it to heal Isaac, making bomb attacks always worth using.
    • Eve's starting kit is based around her having potentially high damage output, but needing to manage health and avoid hits to use it well.
    • Certain orbitals, such as Cube of Meat and Distant Admiration, can kill bosses relatively quickly if you can stay close enough to deal Contact Damage without taking damage.
    • Mom's Knife, has incredible attack power, but terrible range unless you charge it (and there's no indicator of how much you've charged it, so that's basically guesswork). Furthermore, since the knife does constant damage while in contact, the trick is throwing it so the target is at the apex of its flight, doing the most damage before it returns. Too much and it will graze them for less damage, while too little is none at all.
    • Brimstone upgrades your damage, can hit multiple times per shot, and ignores all terrain, but it takes a couple seconds to charge up and it cannot be fired before it's fully charged. Moreover, it doesn't knock back enemies. However, anything which upgrades your tear fire rate shortens the charge time, and the Chocolate Milk item allows you to fire it without charging it up fully. It's powerful on its own on the early floors, but requires damage upgrades to remain useful later in the game.
    • Wrath of the Lamb added Polyphemus, an item that lowers your fire rate and improves your damage significantly. Combined with Brimstone, it makes the laser shots charge even slower, but kill most bosses in one hit, and the final bosses in around 10.
      • The Gravity/Brimstone combo in Rebirth. Instead of leaving floating tears that eventually fire unless you release the attack key, it leaves behind timed Brimstone orbs that fire with the full force of a single Brimstone shot. While you have overall less control over your Brimstone, you can charge and fire it much faster since you don't need to wait for the first shot to run out before charging the next.
    • Diagonal-walking through spikes without taking damage. Requires practice, but mastering this little exploit is handy for getting free items that are otherwise inaccessible. Rebirth keeps this as an Ascended Glitch, though requiring slightly different timing.
    • The Keeper. He starts with triple shot (good), horrible speed, luck, and rate of fire (bad), uses coins as health and cannot increase his max HP beyond two coins (horrible), which makes him a very difficult character to play as. However, you can eventually unlock the Wooden Penny (a one room recharge active item which has a 50% probability of spawning a coin upon use) as a starting item for him, which obviously acts as a healing item of sorts and lets you exploit the hell out of sacrifice rooms. In Afterbith, sacrifice rooms were revamped to offer rewards depending on how many times you step on its spikes on the current level, and they have a chance to either send you to the angel room, spawn an angel room item, or both. If you find a sacrifice room in the first floor and leave a few coins laying around, it is possible to practically start the run with overpowered items such as the Sacred Heart or the Godhead (if you've unlocked it). Sacrifice rooms eventually spawn Angels, making getting the key to Mega Satan's room much easier, and further uses can teleport you directly to the Dark Room, meaning that if you're skilled enough (or are just crazy lucky), you could potentially beat Mega Satan without having even beaten the first floor.
  • Disc-One Final Dungeon: Chapter 3, where you fight Mom, is only the final part of the first completed run of the game. The following area, Chapter 4, was the original Very Definitely Final Dungeon, where you fight Mom's Heart/It Lives. A Halloween patch added Sheol, accessible form a devil room on the Womb 2 or freely after It Lives has been unlocked, where you fight Satan. Wrath of the Lamb adds the Cathedral as an alternate final floor, with Isaac as its boss. The final patch of Wrath of the Lamb added the Chest, a follow-up to the Cathedral that ends with ???. Rebirth would add a dark counterpart to the Chest, the Dark Room, plus a True Final Boss accessible from either floor. With Afterbirth+, a new dungeon was added: the Void. Due to the nature of accessing it, the Void can be reached after nearly any earlier floor to serve as a final level.
  • Disc-One Nuke: Due to the randomized nature of the game, it's completely possible to get powerful items like Brimstone or Epic Fetus on the first couple of floors.
  • Double Unlock: When you manage to unlock a secret item, you're only told it now has a chance to appear in the game. You still need to find it physically to use it.
  • Downer Ending: The first ending has Isaac about to suffer another murder attempt by his mother. Most subsequent endings imply Isaac was Driven to Suicide after his family fell apart and his mother started abusing him. The Afterbirth+ final ending flat-out confirms it, though Repentance features a different, less dark conclusion.
  • Down the Rabbit Hole: The trapdoor in Isaac's bedroom floor leads down to much more than a simple basement.
  • Dungeon Bypass: The "We Need to Go Deeper" shovel allows you to dig down to the next dungeon level without finding and beating the boss (and, if you're not judicious about using it, without picking up the powerups you'll eventually need to win the game). This can also be used to get to Sheol early.
  • Dummied Out:
    • The "Oh No!" / Explosive Diarrhea pill is probably one of the few unintentional examples, disappearing from the game sometime around the Wrath of the Lamb update. The pill returned not only in a patch to the original game, but also in Rebirth.
    • In Afterbirth, the first patch for it removed I.V. Bag from Greed Mode due to a possible unintended combo happening (I.V. Bag + Bloody Penny/Contract From Below/Piggy Bank/etc. with an endless supply of 3 cent red hearts from the shop = endless money). Eden can still spawn with it, however.
    • Afterbirth has an unused boss known as "Skinless Hush", implied to be an alternative form of an existing boss. Both the boss' code and spritesheet are in the game's files, but the spritesheet has only one finished sprite and, if forced to spawn in-game, the boss has absolutely no AI attached to it. In reality, it was a Red Herring for data miners.
  • Dynamic Difficulty: Individual runs don't change depending on how well/poorly you do, but the game itself slowly changes the more times it's beaten, starting with unlocking more levels/bosses after Mom. After a while, more advanced versions of bosses replace their predecessors and other bosses change too, such as the Harbingers appearing after Mom is beaten for the first time. Most notoriously, beating the game many times would steadily increase the chance that The Fallen would randomly replace a regular boss and would frequently appear on the first or second floors. On the other hand, unlocking more and more items with each completed run and experience with the game mechanics counteract this, keeping things more-or-less fair.
  • Early-Bird Boss: The Fallen is unlocked as a random boss battle after beating the game for the first time. It is essentially a more dangerous version of Krampus that splits into two at low HP, giving you double trouble. Its presence is completely random and it can strike on the first two floors, often before you have any real means of killing it besides Death of a Thousand Cuts. If it shows up later, you're probably powerful enough to deal with it handily. In Rebirth, The Fallen's HP is decreased on earlier floors, which makes things more fair.
  • Earn Your Bad Ending: The vanilla ending is pretty happy until it's revealed that it was All Just a Dream, and Mom's still coming for you. Most of the endings after that pretty much go downhill from there.
  • Earn Your Fun: More harder levels and gimmicky characters become unlocked and many of the stronger items in the game become available in the item pool once you beat the game in several ways, most ways being stupidly hard, such as beating all the bosses on hard mode with only one hit point.
  • 11th-Hour Superpower:
    • A fully upgraded Cube of Meat (or Ball of Bandages in Rebirth) can only be obtained through standard means on the final floor of Chapter 4 (Womb/Utero/Scarred Womb), since only a single Harbinger can spawn per area and four copies of the item are needed; even with the bonus floors unlocked, there isn't much game left by then. However, Meat Boy/Bandage Girl will crush almost everything remaining with ease if you can stay out of the way for more than a few seconds.
    • You're guaranteed some of these in one of the last secret floots, The Chest, as long as you have keys — four golden chests are guaranteed in the starting room and every chest in the level has an item in it.
  • Elite Mooks:
    • Enemies can come in alt-colored "champion" versions that will almost always drop an item when killed. Different colors have different meanings, which wasn't consistent between enemies in the Flash version but is more standardized in Rebirth. Bosses can be champions as well, which can vary how they fight by a large amount.
    • The original game's Eternal Edition features even tougher white-colored enemies/bosses, which have several unique tweaks to their behavior that make them far more dangerous than usual. For example, the normally easy Monstro splits into two, then four smaller Monstros when he takes enough damage. Fortunately, these "Eternal" enemies only appear in the game's hard mode, and they are completely absent from Rebirth.
  • Enemies with Death: Death is one of the Harbingers. a collection of bosses. He's the last one to appear, as part of the Chapter 4 floors' boss pool.
  • Enemy Roll Call: The ending shows the enemies crudely drawn on paper during the credits. Rebirth removed these, however.
  • The End... Or Is It?: Just as Isaac appears safe, his mother appears in the background behind him, holding a knife. One's doubt of final victory becomes apparent as all your victory means is that The Womb is unlocked and mother merely "sleeps"; in all versions of the game, there's a lot more left.
  • Empty Room Until the Trap: Each level may have one specially marked room with a chest or item that, if you open/take it, will trigger three waves of enemies. Wrath of the Lamb offers a different version where you fight two degraded bosses in a row.
  • Evolving Weapon: The Cube of Meat, an item that only drops from the Harbinger bosses. Every cube you get increases how effective the Power-Up is, capping at four. It also reminds one of a certain skinless boy after the second cube. Rebirth adds the Ball of Bandages, which turns into Bandage Girl as it ranks up.
  • Evolving Title Screen: The title screen (in the Flash version) or the save file picture (in Rebirth) changes as you reach various milestones in the game, like killing the Final Boss or collecting every item.
  • Exploding Barrels: Wooden barrels can be found in some rooms, and shooting them a few times makes them explode like bombs. Some can be pushed by walking into them.
  • Explosive Stupidity:
    • Many items (such as Loki's Horns, Rubber Cement, and Cricket's Body) do not synergise well with Ipecac, usually because they make your explosive shots travel in an unusual path which leads to them blowing up right next to you. The Suicide King Challenge enforces this by starting you with Ipecac + My Reflection, so your shots boomerang around and come back to hit you in the face.
    • Dr. Fetus doesn't have as many bad synergies as Ipecac, but the ones which it does are arguably even worse because the bombs explode after a time delay rather than on contact with an object. This means that even if you aim your bombs perfectly, they'll still come back to hit you.
    • It's entirely possible to simply hurt yourself with your own bombs, especially if you have bomb effects which make the explosion radius larger.
    • Exploding familiars like BBF and Bob's Brain are likely to hurt you because you can't control when they explode.
  • Eye Scream: The Peeper gouges an eye out of Isaac's head, while the Lead Pencil and 8-Inch Nails are jammed into one or both of his eyes, respectively.
  • Fartillery:
    • Fatties and their variants can fart to push Isaac away from them, though this doesn't deal damage. Mega Fatty ups this with his fart attacks which do deal damage and will also send out a spray of poop bullets or spawn Dips.
    • Several items exist which make Isaac fart, including all the bean items, Butt Penny, and the Bad Gas pill. The effects of these vary, but many of them poison enemies.
    • Afterbirth adds the Farting Baby which farts when it is hit, and may charm, poison or knock back enemies.
  • Faustian Rebellion: The items Satan sells you at the cost of your maximum health often end up making beating him a lot easier.
  • Faux Horrific: At the beginning of each level, we see Isaac remembering some awful torture he has been through. In one memory, he is sitting on the toilet and notices there is no toilet paper left — but his horrified reaction is the same.
  • Feed It a Bomb: A useful strategy for dealing with Chub is to get it to charge at you then drop a bomb in its path, causing it to eat it.
  • Flies = Evil: Flies are a very common enemy, and several other enemies vomit flies. You can even get in on the action with certain items.
  • Flip-Screen Scrolling: As a game that takes cues from the original The Legend of Zelda, most rooms take up one screen's worth of space and flip over to the next after entering a door. Rebirth adds some bigger rooms that use traditional scrolling.
  • Flunky Boss: A few bosses get their difficulty from swarming the player with enemies that act as meatshields at the very least. The Duke of Flies, the dark-shaded version of Gurdy, and Mom herself are the best examples of flunky bosses that summon enemies during battle. However, Monstro is the best example of a flunky boss that can't summon enemies. In the Flash version of The Binding of Isaac, Monstro can spawn with two Black Flies in his boss room. Rebirth expanded the amount of monsters Monstro can spawn with, including two Attack Flies, two Gapers, two Hoppers, two Mulligans, or four Frowning Gapers. Later expansions let Monstro spawn with two Cyclopias or four Ministros.
  • Foreshadowing:
    • While they usually just feature Isaac getting humiliated in some way, one of Isaac's between-level nightmares shows him suffocating inside of a chest, foreshadowing the Sheol ending and ???'s possible origins.
    • The description for the Suicide King card is "A true ending?" and the card itself kills you. The later endings of the game (pre-Repentance) make it clear that Isaac suffocates himself in his toy chest; in other words, the "true ending" of the game is Isaac's suicide.
  • Freeze-Frame Bonus: Ending 13 has a hidden and hard-to-read message that appears for just an instant before it returns to the title screen. It reads "GET IN THE BOX!", a Shout-Out to Time Fcuk.
  • From Bad to Worse: One of the rewards for clearing Chapter 4 enough times is making the game more difficult, appropriately known as "Everything is Terrible!".

    G-L 
  • Gainax Ending: Given the general ambiguity surrounding the story, most ending cutscenes are very abstract and confusing beyond the intro and initial ending. There's also a few gag endings, mainly the ones gotten from opening the treasure chest at the end of The Womb.
  • Game-Breaking Bug:
    • One of the items introduced in the Halloween update of the Flash game, Brimstone, causes Isaac to shoot enormous streams of blood from his mouth. Since such a radically different item was very new to the mechanics of The Binding of Isaac, certain item combinations could lock you out of firing it at all. The boss of the level added at the same time as Brimstone was also initially immune to it due to a bug.
    • In the first version of the Halloween update, The Headless Horseman would sometimes replace the boss of the new level. This wouldn't crash the game, but this would horribly distort the music and you miss out on fighting the True Final Boss.
    • In Wrath of the Lamb, the Random Event "Curse of the Labyrinth" combines two floors from the same tier into an "XL" version, causing the player to have to fight two bosses, one after the other. It is possible for the base Final Boss to be the first boss fought in the Depths/Necropolis XL. Normally, when the boss is defeated, it becomes impossible to leave the room, and if the boss is fought first in the XL version, then the game would not recognize the level has been completed. This left the player stuck in an inescapable room, unless they can use a teleportation item. A similar thing could happen with Mom's Heart in the Womb/Utero. Thankfully, this bug was fixed in the first patch.
    • Wrath of the Lamb's had a bug at launch that caused the Polaroid to replace Isaac with the wrong True Final Boss when brought to the Cathedral. The boss screen shows ???'s image with the name "Daddy Long Legs", and the fight itself was with Triachnid, an alternate version of Daddy Long Legs in the same vein as past references to Edmund McMillen's games like Steven, C.H.A.D., and Gish; Triachnid was intended to be Dummied Out but accidentally replaced a plot-important boss. Conveniently, Wrath of the Lamb was the first expansion to start elaborating on Isaac's dad (Triachnid is a father in his origin game), so it wasn't widely realized as a mistake until patches restored ??? and Triachnid to their intended places.
    • The v1.3 update to Wrath of the Lamb had a severely exploitable one. Using a Blood Donation Machine or Demon Beggar to reduce your health to zero made you completely invincible until you picked up a heart. This made the No-Damage Run achievements pathetically easy. Luckily, it was patched mere hours after the update.
    • The v1.03 update to Rebirth has a nasty one (that some people initially thought was an intentional change just to make the game harder). You can not bomb into special rooms (Treasure Room, Shop, Sacrifice Room, etc.) from Secret Rooms. This can lead to Unwinnable situations where you can use The Moon or Telepills to enter the Secret Room and find out that it only borders special rooms, giving you no way out.
  • "Get Back Here!" Boss: The second stage of the Mask of Infamy can only be hurt in the back, and it turns and zigzags very quickly. Inverted with Lust and Super Lust, where you have to run away as fast as you can to avoid taking Collision Damage.
  • Ghost Butler: Every new room you enter with enemies in it will shut all the doors and won't let you leave until you defeat all the monsters within or blow the doors open with an explosion to force your way out; you can also use items like Dad's Key or a Get Out Of Jail Free card. Leaving rooms this way without eliminating every monster will respawn them all to their original numbers if you re-enter the room. Trying to bomb doors in the Chest or the Dark Room won't work, though unlocking items will.
  • Glowing Eyes: Some powerups, mostly angelic/demonic items like Sacred Heart, give Isaac glowing eyes to signify their power.
  • Good Wings, Evil Wings: The Bible and the Holy Grail give you white wings. Lord of the Pit and any item giving you a demonic effect combined with Transcendence give you black wings.
  • Gorn: Most of the bosses and enemies are quite disgusting, along with several powerups.
  • Grossout Show: This one runs the full gamut: piss, poop, vomit, blood, Body Horror, fetuses, and so on.
  • Guide Dang It!:
    • Items aren't labeled and even picking them up doesn't give the player any idea on what they do. The Tick is an example of an item designed to trick new players, but haplessly picking up items in any run could end up ruining it entirely, such as Libra severely reducing the player's DPS on the final floor, certain cursed items teleporting the player out of boss fights when hit, or items that change Isaac's firing pattern/range making it dangerous for him to hit anything.
    • There's no guide on how to unlock some of the special characters, several of which require at best counter-intuitive behavior. This was fixed in Rebirth, which details how to unlock them (and changes some of the conditions), while adding its own, even more obscure special unlock.
    • Special items are those that make other special items show up less often if you pick them up. There is absolutely nowhere in the game that tells you this mechanic, and it took a long time before the Special stat was even discovered, and that led to all new strategies being used in order to get good special items. Then Rebirth happened to change how they worked.
    • What lowers the chance of getting a Devil Room / Angel Room at the end of the floor changed in Repentance. All the way up to Afterbirth+, taking any kind of damage to red health would lower the chance. In Repentance, taking damage from Curse Rooms doors or stepping on spikes in a Sacrifice Room no longer reduces that chance. While it is an incredibly useful change, it also makes absolutely no sense for veteran players or for new players. Veteran players have learned for years to not take any kind of damage to red health and new players will certainly take red health damage from other sources than Curse Rooms / Sacrifice Rooms (and it requires them to notice that the chance does not lower, which they may not see if they play without the Extra HUD) before, and will learn to avoid taking red health damage. Moreover, there is nothing in the game that hints at it besides outright taking damage from these sources and realizing that "Oh, I still got my devil room chance".
    • Downplayed regarding the Ace of X cards. In Afterbirth+, the Ace of X cards turned all pickups in a room into another kind of pickup (hearts, keys, bombs, coins). In Repentance they now turn ennemies into their respective pickups as well. Veteran players have learned the effect of these cards in Afterbirth+ and have absolutely no way of figuring out the change if not by sheer luck or accident.
  • Guilt-Based Gaming: If you try to quit the game, Isaac will ask you "Are you sure you want me to die?"
  • Hammerspace: It's not specified where all the coins, keys, and bombs are stored, especially since Isaac is as naked as the day he was born.
  • Have a Nice Death: Dying on a run displays a last will from the character on the retry/quit screen, displaying what they died to, where they died, and what items they collected.
    Dear Diary, Today I died. I was killed by this thing [picture of monster] in some [descriptor of area]. I leave all that I own [items] to my pet cat Guppy. Goodbye, Cruel World! -XOXO [character name]
  • Heart Container:
    • Items that add one extra red heart to your health can be found here and there, often in boss rooms, although only one of them resembles a heart (and an anatomically correct one at that) with most others being food. Some even grant two max HP, and/or restore all your health when you get them.
    • Eternal Hearts act as half a Soul Heart when you first pick them up (albeit sandwiched between your Heart Containers and your actual Soul Hearts on your HP gauge), but transform into full-on Heart Containers if you manage to hold onto them until the next floor or pick up a second Eternal Heart.
  • Hearts Are Health: Your typical straight example, copying The Legend of Zelda exactly. Notably, there's also the <3 item, which is a Heart Container that resembles an anatomical heart rather than a Heart Symbol.
  • His Name Really Is "Barkeep": ???. It's clear that the character is a suffocated Isaac, but for distinction the game always uses ??? to refer to them.
  • Hitbox Dissonance:
    • The flies have a larger hitbox than their visible size. This makes it easier to hit them, but it makes it easier to misjudge how close you're allowed to get without getting hurt, not helped by their tendency to suddenly jerk towards you.
    • The Transcendence pickup removes your body, turning you into a floating head that can fly over rocks and pits. It does not remove your body's hitbox. Getting hit even causes your body to briefly flash into view, just in case you were wondering why you were taking contact damage while hovering over that narrow crevice.
    • The tears' hit box is their shadow, not the tear itself. This is normally a moot point, but bosses like Monstro can be a bit counter-intuitive without knowing this fact. It also makes Ipecac a learning experience, as the arcing tears will detonate despite logic suggesting they would sail over obstacles or enemies. Knowing this makes the Suicide King challenge in Rebirth somewhat easier.
  • Hoist by His Own Petard: Almost all bosses with explosive attacks are vulnerable to said attacks, though most will usually fire them in such a way that they won't be damaged by them. Wrath is a notable example, as his only attack is just as dangerous to him as to the player, moreso if the player has homing bombs. Pride is a notable exception, being immune to his own troll bombs.
  • Holy Halo: Numerous kinds of halos are available as angel room items. There's The Halo (a gold circle hovering above your head), there's Scapular (which is the older interpretation of the halo: a glowing light behind Isaac's head), and there's the Holy Grail (like the Scapular, but it's a many-pointed star of light instead of a bright haze).
  • Holy Water: Holy Water is a passive item that causes a bottle of water to spawn behind Isaac and follow him around like a familiar. Taking damage will break the bottle and create a puddle of water that harms non-flying enemies, while in Repentance, the bottle is hurled at enemies and breaks on contact instead.
  • Inexplicable Treasure Chests:
    • Treasure chests are everywhere in the basement of the house, and they're even more inexplicable in The Womb.
    • It's possible to unlock a treasure chest and find another smaller treasure chest inside, and although very rare, it's possible that chest has another still smaller chest inside it! Which could happen while you're inside a chest!
  • Infinity +1 Sword: Many of the best items in the game can only start appearing in runs after accomplishing very difficult end-game tasks.
    • Isaac's secret starting item. You need to beat the game's (initial) true final level ten times, which is quite a lengthy and dangerous task. And after that, you need to beat it once again with the resident Joke Character, which is nothing short of an extremely painful ordeal. But if you manage to accomplish this mission, your reward will be The all-mighty D6 dice, an activated item capable of transmuting any collection related item(s) into other random item(s).
    • The Halloween update to the Flash version has Mom's Knife, an absurdly powerful item that can only be found in runs after killing the update's new True Final Boss with Isaac.
    • Rebirth adds plenty more unlockable, powerful items, with the biggest one being Godhead, which increases all tear-related stats, makes your tears homing, and gives them a damaging halo. It's unlocked by beating all the end-game bosses (except Mega Satan) and Boss Rush on Hard with The Lost.
    • Beating Mom's Heart on Hard with The Lost unlocks the strongest 2-player familiar, which is completely invincible.
    • The Stop Watch. It requires donating 999 coins to the shop, which on average requires two hundred playthroughs. However, while the unlocked-by-default Broken Watch only works at random (and sometimes it speeds up enemies instead), the Stop Watch is permanently active. This was nerfed in Afterbirth, requiring the player to be hit once (Holy Mantle hits count) to get the Stop Watch to activate, and only for the current room.
    • Afterbirth adds Mega Blast, earned by defeating Mega Satan with every character in the game. It gives Isaac access to Mega Satan's giant blood laser, which lasts for fifteen seconds and persists through rooms. It has a 12-point charge bar and batteries only fill it by three. It will melt any boss in its way and fires with such intensity that it can push Isaac backwards.
    • Afterbirth+ adds Delirious, unlocked by beating Delirium with any character once. When used, it summons charmed bosses to fight for you, even Hush if you're in a 2x2 room. Like Mega Blast, it has a 12-room charge and only appears in one item pool (Angel Rooms), though oddly one battery will charge it fully.
    • The Death Certificate in Repentance is generally the absolute final item a player can unlock in Rebirth, as it requires getting all Hard Mode completion marks with every character in the game, including their alternate Tainted forms. The unlock is well worth it even if there isn't much to unlock afterwards, as grabbing the Death Certificate teleports you to a secret area containing one of every unlocked item in the game, letting you pick one (more with exploits) before warping back.
  • Interchangeable Antimatter Keys: In classic Zelda style, any key fits all locks, but disappears once used.
  • Interface Screw: Curse of the Lost disables your minimap, which can be a real pain if you get hit with it in the later levels which have dozens of rooms. Rebirth adds a Curse that hides the health bar.
  • Invincibility Power-Up: The My Little Unicorn and the single-use Chariot tarot card both protect from and cause Collision Damage. The Gamekid item turns the game into a short round of Pac-Man. The Book of Shadows prevents the player from taking damage but doesn't deal contact damage.
  • Ironic Nursery Tune: After beating "It Lives!" for the first time, a distorted version of "Jesus Loves Me" replaces the normal credits music. It's also used in the trailer for Rebirth.
  • Item Get!: Isaac will briefly hold up any upgrades, space-bar items, pills, and cards that he finds. This can be hazardous if he picks these up while there are enemies nearby.
  • Jack-of-All-Stats: Isaac starts as this, having very balanced stats but no starting items other than one bomb; he's generally a Master of None compared to the other specialized characters. He moves away from this once he unlocks the D6 as a starting item, giving him better control over items.
  • Joke Character:
    • ???, unlocked by beating the true form of Chapter 4's final boss, which itself unlocks after beating the original version ten times. He starts with the same stats as Isaac (not even Cain or Judas's improved offensive stats) except all his hearts are temporary ones like soul hearts or black hearts, which can't be healed like regular hearts can. He can't earn regular hearts, as all health upgrades are converted into soul hearts. He also starts the game with The Poop, which creates a pile of poop. His lack of heart containers prevent many items from doing anything.
    • In Rebirth, there's an even larger Joke Character in The Lost. He starts with no health and cannot pick up any health, meaning that you will always die in one hit unless he has extra lives or a form of invulnerability. His only boons are being able to fly you can fly (which Azazel, a much stronger character, can do innately) and being able to take devil deals for free. Afterbirth buffed him by giving him Spectral Tears and the D4, allowing The Lost to re-roll his build, and makes him start with the Holy Mantle after donating enough coins through Greed Mode, guaranteeing him an extra hit per room unless he reroll it away. He got buffed again in Repentance, by making his Holy Mantle effect intrinsic, meaning it cannot be rerolled away. He also traded the D4 for the Eternal D6, which acts just like Isaac's D6 aside from its chance to cause rerolled items to vanish. Unfortunately, devil rooms were changed to only let him take one item.
    • Keeper, the third and final joke character, was added in Afterbirth as the final reward from Greed Mode's Greed Machine. He's slightly more survivable than The Lost because he can take two hits and has a triple shot, but can only restore health with coins, meaning his survival is very luck-based even after unlocking starting items that can benefit him. He can't earn any extra health, making devil deals mostly useless for him. Repentance lets him have up to three hearts and turns devil deals into coin exchanges, removing some of his weaknesses.
  • Joke Item: The Poop, ???'s starting item, which just drops a pile of dung where you're standing and can be used once per room. It can be used for cover, to block an enemy chasing you, or for bridging gaps with a bomb, but virtually any other item is more useful in any situation. Rebirth and its expansions add items that can make it spawn different-colored poops (golden poop that drops a lot of money, black poop that acts as a Smart Bomb, or white poop for a buffing aura) as well as a trinket that makes poop drop better items, making it Not Completely Useless if one of the synergies is obtained.
  • Jump Scare: Polycephalus, a cluster of screaming eyeless heads, frequently digs down and suddenly reappears with a unique scream.
  • Kaizo Trap:
    • Due to the way Mom's attacks work, you can still be hit when you kill her. Her foot is preceded by a shadow, and if she dies while the shadow is present, the attack hits that spot instantly.
    • When Mom's Heart/It Lives is defeated, it releases a circle of blood shots. They aren't unavoidable, but a distracted player can be damaged or even killed if not careful.
  • Kill Enemies to Open: Every room containing enemies will automatically shut itself down until you kill them all. It's possible to force your way out of a room containing enemies by hitting the door with an explosion, either yours or one from an enemy. This tactic must be used sparingly because bombs are a precious commodity most of time unless your have an item that replicates them, and doing so doesn't take care of the enemies inside the room, so if you walk back into it, you'll either have to deal with the enemies or use another bomb to escape once again. Also, this tactic doesn't work in boss/mini-boss battles, because their doors are blast-proof. It also doesn't work on doors in the Chest or Dark Room.
  • King Mook: Many bosses are tougher versions of enemies found on their floor, such as Chub for the Charger maggots and the Mask of Infamy for the Mask + Heart enemies.
  • The Krampus: Krampus was added as a boss in the Christmas update to the Flash game and has remained since then. He's an alternate version of the Fallen who will randomly appear in the Devil Room (10% chance at first and 40% at later visits) in a similar fashion to Greed/Super Greed for shops and secret rooms, and drops either a Lump of Coal (a range-based damage upgrade) or the Head of Krampus (an active item that replicates his four-way Brimstone shot).
  • Last Ditch Move:
    • Some enemies explode, while some others spawn bullets when killed. Certain bosses can do so as well, such as both portions of Steven exploding into bullet spreads.
    • Daddy Long Legs tries to crush you with its body as it dies.
    • Due to glitches, sometimes the effect of final bosses dying can actually hurt you.
  • Last Lousy Point: Obtaining Platinum God (and, in Rebirth, Real Platinum God) requires touching every item in the game at least once. Some of these items, such as Sacred Heart, are so rare and are tucked away in areas that are so rarely accessed that it can take forever to find that last lousy item without some form of infinite rerolls. Rebirth adds some other items into the mix, such as every Secret Room exclusive item (Skeleton Key and Black Lotus are two) due to the fact that Secret Rooms rarely contain items anymore. Perhaps most infamous is Steven, which was originally removed from every item pool, meaning it was only attainable as a rare boss drop from Steven himself and could not be rerolled from Little Steven; Afterbirth put it back into the treasure chest pool and into Greed Mode, though Repentance removed it from the former again.
  • The Legions of Hell: Isaac has to fight a number of demons during the course of his descent through the basement, usually as bosses. One of the unlockable characters, Azazel, is a demon himself.
  • Lethal Joke Character: The soul heart mechanic of ??? lends itself to some amusing exploits that give him an edge. He registers as having no health due to lacking red hearts, so he always has the maximum devil/angel room change, can always enter challenge rooms, and gains permanent use of low-health effects like the Polaroid and Whore of Babylon. He also benefits greatly from items that affect red heart drops or consume them, such as the Mitre and Dark Bum.
  • Lethal Joke Item:
    • The Bible grants flight for a room and doesn't recharge for another six (barring other items to increase charge time), but kills the final boss and the original two True Final Bosses in one use. However, using it on Satan just kills you, and against every later boss it'll just deal no damage.
    • The Rosary has a fairly disappointing obvious effect (adding three soul hearts), but it can often be used to cheat the devil out of some of his nicer items without dying and has the side effect of making a Bible more likely in shops, guaranteeing an easy final boss if you get it. Plus, when playing as ???, it's one of the best health upgrades available.
    • The Dead Cat item makes it so you gain eight extra lives, at the cost of setting your max HP to one when you pick up the item and whenever you die. Normally this is a pretty lousy trade-off, but if you only have one heart at the time or are playing as certain characters (Judas, Samson in the Flash version, or especially ??? and The Lost), it's basically free lives.
    • The One Makes You Larger pill is normally pretty useless, just upping the size of your sprite. With the Forgotten, however, the range of his melee attack is determined by sprite size, so the pill gives allows him to hit enemies from greater distances. This is especially useful for exploding enemies, making them safer to melee.
    • The Spindown Dice added in Repentance rerolls all items in a room to the previous item in the list (in other words, one item ID lower). Assuming you know what items will reroll into (or look it up), you can reroll some very powerful items from common things. For example, if you donate enough to a Blood Bank to spwan a Blood Bag item, it can be rerolled into a free and guaranteed Brimstone, since Brimstone's item ID is one below Blood Bag.
  • Light Is Not Good: The Cathedral is unusually bright for a Binding of Isaac area, but it's just as likely to kill you as any other area in the game, if not more so.
  • Living Apart: In later levels, some of the enemies include living brains, rolling guts, or disembodied hearts.
  • Losing Your Head:
    • Some of the enemies can attack the player even after losing their body or head or they just get separated.
    • Transcendence removes the player's body, allowing you to float over rocks, pits, spikes, and damaging liquids, being the most basic flight-granting item in the game. The Shears and the Hanged Man Tarot card also do this, but only for one room.
    • The Guillotine from Rebirth turns Isaac's head into a familiar, making him fire tears while orbiting around it. If you can get past the Interface Screw, the head not only protects the body from bullets, but the item is also a stat boost.
  • Luck-Based Mission:
    • Every single playthrough, due to the randomized nature of the game. You can never guess if items will complement each other, or if they'll even be useful, including boss drops and shop inventory. Key and bomb drops can either leave you with a surplus or lock you out of chests and treasure rooms if you fall short. The good stuff in the shop costs just enough that you probably won't be able to pay for it until you've gone down three or four levels, when Greed loves to lay its ambushes. Pills are always random, and are just as often bad as good (unless you have the Lucky Foot or PhD). Slot machines and the like won't pay out half the time without good luck. There is always a chance that you will reach the end of the game with a much more or less powered up character than you expected through no fault of your own. On the extreme, it's possible to get absolutely no useful upgrades, all bonuses being activated items that you do not want, or things your character doesn't benefit from at all. On the other hand, it's possible to get an item combination that absolutely dominates everything. That said, due to the way items are unlocked, each successful playthrough means it is more likely items will be useful, and the fact that the game gets longer means more items to find. The first playthrough is often the toughest.
    • Acquiring four Cubes of Meat requires you to fight and kill all four Harbingers in a single playthrough. Unfortunately, they don't usually all show up on the same run. Acquiring the Book of Revelations early on almost guarantees fighting all four of them, but finding it early enough is another luck mission on itself. It's not even a complete guarantee, as Conquest can replace Death on the Womb/Utero and the Headless Horseman can replace anyone, and they always drop Pony items. In Rebirth, it's even worse, as the Harbingers now have an equal chance of dropping either a Cube or a Ball of Bandages, which also has an achievement for acquiring four. It's overall much easier to use items in later expansions like the Potato Peeler or Box of Friends to get the achievements.
    • Bosses also contribute to the luck-based nature of the game. Not all of them are equally difficult, and depending on a character you choose and powerups you get, some might be a cake-walk (Larry Jr., Loki) and some an endless source of frustration (Gurdy for a slow-moving character, Monstro II for people who dislike close-range combat, or The Bloat for pretty much everyone). Since not getting hit (which is more difficult if you get a boss you are faring poorly against) is almost a requirement for getting access to the devil room, which often allows one to balance the odds, the difficulty snowballs depending on how much the Random Number God loves or hates you.
    • There's no way to tell if a shop or a secret room will spawn Greed/Super Greed instead.
  • Luck Manipulation Mechanic:
    • The dice items allow the player to reroll various facets of the game. In the original, the player could reroll permanent items (D6) or pickups (D20). Rebirth and its expansions adds the ability to reroll enemies, the player's own items, stats, rocks, and everything at once, plus one die that duplicates a random pickup in a room and one that lets you select which effect to use.
    • The Book of Belial guarantees a Devil Room/Angel Room on that floor, provided that floor meets the conditions for spawning one (it won't work on the first floor or past the Womb). Rebirth adds the Goat Head, which is a permanent item with the same effect.
    • The Book of Revelations ups the chance of the player encountering one of the Harbingers (and acquiring a Cube of Meat/Ball of Bandages in the process) in the Boss Room, with repeated use on that floor making it all but guaranteed. The Headless Horseman and Conquest can still screw up an attempt to construct Meat Boy/Bandage Girl, though.
  • Luck Stat: Luck affects the chances of getting a drop after clearing a room and the chance of firing special effect tears (for example, shooting a tooth with the Tough Love item). Items like the Lucky Foot can make slot machines pay out more frequently, but this is independent of luck (though the Lucky Foot still boosts luck itself).
  • Ludicrous Gibs: The fate of most enemies you destroy is exploding into chunks of blood and guts. The notable exceptions are Isaac, ???, Mega Satan, Afterbirth's Little Horn, and Afterbirth+'s Big Horn. There's also a seed effect that makes enemies explode into even more gore.

    M-R 
  • Macrogame: Each playthrough is different than the previous one, but as achievements are unlocked, more and items and floors begin appearing.
  • Made of Explodium: Bob's Brain, Bob's Rotten Head and Big Beautiful Fly (BBF) are all reusable bombs. In addition, Ipecac gives Isaac the ability to shoot explosive vomit from his mouth. The reason why any of these things explode is a mystery.
  • Magikarp Power:
    • In terms of items, the Wafer makes anything that hurts you do exactly one half-heart of damage. It does almost nothing for the first half of the game (where pretty much only Champion enemies, spikes, collision with bosses, and your own bombs hurts for more than that), but starting at the Womb, everything does a full heart of damage, so the Wafer effectively doubles your health (and any health pickups).
    • Using Guppy's Paw (which trades heart containers for three soul hearts each) isn't so good at the start of the game since red hearts with which to fill your heart containers are easy to come by, unlike soul hearts. However, using it later in the game is beneficial as both are scarce and functionally similar. Spending all of your heart containers will give you three times your max health, whether your heart containers are empty or not. Additionally, doing so will give you all benefits of the Lethal Joke Character.
    • Believe it or not, ???. Mom always drops The Polaroid when killed. Besides being required to access the Disc Two Final Dungeon, the Polaroid casts a protective shield wherever you have 1/2 of a red heart or less. ??? always has less than 1/2 of a red heart. That's right folks, the Lethal Joke Character has the greatest life expectancy in the late game.
    • The Bible. It allows you to fly for a single room. While useful, there are items that grant this effect permanently and don't take up your special item slot. It also has the slowest recharge rate possible, so you only get flight once every six rooms (without upgrades). But, if you use it on Mom and Mom's Heart/It Lives, they die instantly, allowing you to bypass two of the hardest bosses in the game and making the No-Damage Run in each section a lot easier. Just don't use it on the final bosses.
    • The Cube of Meat (and Ball of Bandages) start as this. Their familiars help with blocking hits and damaging enemies, but are otherwise unremarkable for a boss fight reward (specifically the Harbingers). With each Harbinger defeated, though, the Cubes or Balls slowly takes form as the more useful Meat Boy (or Bandage Girl if you collect the Balls). Level 4 Meat Boy/Bandage Girl is especially powerful; the hard part is somehow assembling one and getting substantial use out of them, since finding 4 Harbingers that drop the same pickup isn't that likely to happen without holding onto the Book of Revelation for several levels.
    • The D10 in Rebirth. It rerolls any enemies in the room, so using it in the early floors means you'll potentially have to face enemies from much later floors with the paltry upgrades you might have acquired by then. However, the opposite is also true, if you hold unto it until late floors like the Womb, it might turn the local populace into enemies from much earlier floors, making them trivial to dispatch and turning what might have been a challenging room into a breeze.
  • Malevolent Architecture: Useful items may be stuck behind rocks or pits, and in many cases it's not worth expending a bomb to get to them. In later levels, this gets more and more common, in addition to rooms being laid out to make attacking enemies more difficult (and them attacking you easier) and eventually adding random spikes.
  • Marathon Boss:
    • If your damage output doesn't scale as you progress through the game, later bosses can take an obscene amount of effort to kill. This is especially true of Isaac and ???. And then there's Mega Satan of Rebirth, who takes forever without the Chaos Card due to being a Boss Rush as well as a regular boss.
    • Afterbirth adds in Hush and Ultra Greed, who are unique in that their HP is scaled so you won't wind up with a combo that can kill them in twenty seconds. No matter how well-prepared you are, expect to spend a lot of time killing them.
  • Maximum HP Reduction: Deals with the Devil, Black Market deals, Health Down pills, and certain items can take away heart containers from you. This isn't too serious, though, as there are numerous ways to gain heart containers, and soul hearts can usually give you HP beyond the maximum. Blue Baby and other soul heart-only builds never experience this, since they only have one to three soul hearts (which are typically easier to find than extra heart containers) taken away whenever they'd normally have heart containers cut off.
  • Meaningful Name: The Wrath of the Lamb expansion's name foreshadows two of the bosses introduced in the game (i.e., a Messianic Archetype version of our main character, and one of his alter egos). The titular Lamb itself makes an appearance in Rebirth, if you bring the Negative down to Sheol. You are taken to the Dark Room area, where you'll have to fight the Lamb
  • Memento MacGuffin: The Polaroid, which depicts Isaac's family before the madness started. Getting knocked to half a heart with it on gives Isaac a protective shield. Defeating the Cathedral's True Final Boss while holding it takes you to the Chest, where you fight the Blue Baby. Rebirth adds its counterpart, an unrecognizable picture of Isaac's family called The Negative. Instead of a shield, it deals damage to all enemies when Isaac is brought to half a heart, and upon beating Sheol, it brings Isaac down to the Dark Room to fight the Lamb.
  • Mind Screw: A whole lot in this game, but one moment stands out. In Ending 6, Mom's fist flies out of a chest that was spawned by destroying her heart, which was inside her womb, which was inside her corpse in the first place. So Mom was hiding in a chest in her heart in her womb inside her to begin with. Yeah. Figure that out.
  • Mood Dissonance: The Arcade rooms are very nicely furnished and bright, with a very chipper 8-bit remix of the first level theme. It's a bizarre contrast to the rest of the game.
  • Mood Whiplash: Ending 20 in Afterbirth+. It acts as a Mind Screwdriver, showing Isaac's descent into suicidal insanity, and the last shot has Isaac walking away in the afterlife while a somber One-Woman Wail plays in the background... and then, like all endings, the triumphant credits theme starts playing as the credits roll.
  • Mook Maker: Several of the bosses, though not all of them. Also one of the enemy types. It Lives is a Degraded Boss Maker.
  • Money for Nothing:
    • Once you reach the Womb, money has very little use, as there are no shops to be found anywhere in there.
    • Subverted if you happen to get the Money=Power upgrade, which is does Exactly What It Says on the Tin, increasing your attack power in direct proportion to the number of coins you have. Also, you can still find beggars, slot machines, and even arcades in The Womb to dump off your hard-earned gold in exchange for hearts, items, and keys.
  • Monstrosity Equals Weakness: Satan and the Lamb compared to Isaac and ???. Much more monstrous...and far, far easier.
  • Motif: Suicide and self-harm. A lot of the passive items hurt Isaac physically on his sprite, and some items (like Judas' Shadow or Lazarus' Rags) require you die in order to make the most of them. The later bosses factor into this too, with Isaac basically killing different versions and/or representations of himself. The motif makes a lot of sense when the endings reveal that the whole game is Isaac's Dying Dream as he's suffocating in the chest due to perceiving himself as full of sin.
  • Multiple Endings:
    • Depths Ending: Isaac is saved from Mom when a Bible drops on her head. However, this is then revealed to have all been a story Isaac was drawing. The real Mom proceeds to break into Isaac's room holding the knife she wants to use to murder him.
    • Womb Endings: Isaac opens a chest to find various items inside, culminating with what appears to be his own rotting corpse.
    • Cathedral Ending: Isaac looks in the mirror and realizes he is corrupted with sin. He then looks at his toy chest.
    • Sheol Ending: Isaac's Split Personalities, ending with a demonic looking one, flash before him. Isaac decides to climb into the toy chest in his room, possibly to kill himself.
    • Chest Ending: The collapse of Isaac's family is shown through a series of Polaroid photos. Ending 20 shows that Mom and Dad were constantly fighting, causing the latter to leave and the former to start abusing Isaac.
    • Dark Room Ending: Isaac's mother is shown wandering outside, searching for the now missing Isaac.
    • Golden Door Ending: Isaac is shown after locking himself in his toy chest, flashing in and out of a demonic, grinning self-image, ending with him permanently staying as a demon.
    • Blue Womb Ending: Isaac's mother discovers Isaac's remains in his toy chest, before cutting to Isaac waking up in the underworld, with a shadowy, demon-like figure looming over him.
    • Greed Mode Ending: Isaac is trapped in a cave-in, and hangs himself shortly after. It then shows a Shopkeeper in the same room, who cracks a grin after a moment, implying that Isaac has become another Greed.
    • Void Ending: More of a Mind Screwdriver than anything else, it explains some things: Isaac's father left his family, and Isaac committed suicide in the chest because he believed he was full of sin. The whole game is a Dying Dream, and the last shot is Isaac walking into the distance in the underworld...
    • Corpse Ending: "Mother" is revealed to be a drawing by Isaac, but Isaac is interrupted by his real mother, who sees the drawing and is hurt by it, kicking Isaac into the closet as punishment. As Isaac is breathing faster and faster, Satan can be seen behind him...
    • Beast Ending: The Beast is driven into hell by a holy light, and Isaac ascends to heaven, seeing his own life from the end to the beginning. However, it was revealed that the entire game, including Isaac's suicide in the chest, was a bedtime story by Isaac's father, with the help of Isaac himself. After being asked if this is how Isaac wanted the story to end, Isaac agrees for his Father to tell another story, this time of Isaac and his parents instead of just Isaac and his Mother.
  • Mundane Made Awesome: After everything that just happened, Isaac gets rather surprisingly excited over the items in the endgame chest, including a hat, a nail, a quarter, a fetus in a jar, a wafer, and some bling.
  • Mutually Exclusive Powerups: Most of the powerups which completely alter the nature of your tears can't be used in tandem, though this depends on which version of the game is being played. The most notable items are Brimstone, Dr. Fetus, Epic Fetus, and Mom's Knife, which variously override one another depending on when they're picked up. Rebirth and its expansions added interactions for most of these combinations, averting the trope.
  • My Name Is ???
    • The last unlockable character is simply called "???". Fans have nicknamed him "Blue Baby".
    • The Blue Womb is named "???" when you enter the area.
  • Mythology Gag: Several of the bosses and items are references to other works by Edmund. Namely, the Cubes of Meat, Dr. Fetus and Mega Fetus, the Super Bandage, Larry Jr., C.H.A.D., Steven and Lil' Steven, blobs that look like Gish, Gish himself as a boss, and Triachnid in Wrath of the Lamb.
  • Name of Cain: One of the unlockable characters, not surprisingly considering all of them have Biblical names. While not outright evil, Cain is somewhat shady-seeming — he wears an Eyepatch of Power and his abilities make him resemble a "rogue" class.
  • Night of the Living Mooks: There are various undead enemies Isaac must defeat. See also Non-Human Undead, below.
  • Nintendo Hard: Much like the Roguelikes it's based on, Isaac has no continues. You start with one life and additional lives are very, very unlikely to show up once along multiple playthroughs, much less a single one. If you die, you start all over again. The configurations of rooms and upgrades are randomized, and in some situations damage will be unavoidable. Survival depends on reflexes and calculated risk. Wrath of the Lamb takes this even further! If you thought the vanilla version was merciless, you won't survive the new horrors you'll face here.
  • No Cutscene Inventory Inertia: No matter what the player has picked up, the character at the cutscenes is still what he was at the beginning. For that matter, it'll still be Isaac no matter which character you're playing as. Although, considering the achievements imply most of the characters are Isaac wearing a Paper-Thin Disguise of some sort, it may be fitting after all. The only exception to this is Lazarus, who will be crying blood after he's resurrected.
  • No-Damage Run:
    • A no damage run of each area will earn you a Steam Achievement.
    • The Devil/Angel Rooms can only be accessed by playing through a level without taking any red heart damage and no damage at all against the boss. The exception is Basement II, in which the Devil Room is guaranteed so long as you don't take red heart damage.
    • Enforced with The Lost, who literally has no health and dies if hit by anything.
  • No Ending: If the player has gotten all of the Womb endings, then going to the chest when it appears again (such as during Challenge runs) will just take the player straight to the end credits.
  • No Ontological Inertia: Averted; any flunkies spawned by a boss will not die when the boss does (except in the case of Mom) and must be killed in order to clear the boss room.
  • Non-Human Undead: Remember those horrific monsters Isaac must defeat? They come back from the dead later.
  • Non-Indicative Difficulty: The "Challenges" stick you with various item loadouts and usually don't allow treasure rooms. Depending on the loadout, the game can either be hilariously easy or unspeakably difficult.
  • Obvious Beta:
  • Offing the Offspring: Unsurprisingly, since the game was named after the original (the Choice of Abraham kind).
  • Old Media Are Evil: The plot and imagery are inspired from inflammatory Christian propaganda from The '80s, such as the Jack Chick comics.
  • Ominous Latin Chanting: "Enmity of the Dark Lord", the song that plays when you fight Satan and "Lament of the Angel", the Cathedral's background music generate this impression using gibberish. "Everlasting Hymn", Rebirth's Cathedral music, has real Latin, as does Ascension, the music that plays during the Blue Baby fight.
  • One-Hit Point Wonder:
    • If you take a Devil Room item without having the regular hearts to pay for it, but still have soul hearts when you do, Isaac is reduced to zero health (no hearts or soul hearts) and will die from a single hit. Any health upgrades you collect instead pay off the debt, after which they can be acquired normally. You can, however, collect more soul hearts for a buffer. Rebirth altered this so you only lose your red hearts, keeping soul and black hearts.
    • The Lost lives off this trope, literally starting with no health and incapable of increasing it. Hope you brought your Holy Mantle.
  • Onion Tears: Once you find the Sad Onion powerup.
  • Otherworldly and Sexually Ambiguous: The multiform Eden is neither male nor female.
  • Ouija Board: Serves as a power-up that gives Isaac ghost tears that move through objects.
  • Our Zombies Are Different: Most of the enemies in the game behave like zombies and seem to be undead of some sort, and they all come in different shapes and sizes, many of them with various horrific appearances. In addition, some bosses have posthumous forms making them a more borderline case of this trope.
  • Palette Swap: Enemies with a different color palette. They're stronger but drop items upon death.
  • Paper-Thin Disguise: It's heavily implied that the unlockable characters are just Isaac wearing these. Word of God angrily confirms.
  • Photo Montage: The 13th Ending shows various Polaroids of Isaac and his family's life before the events of the game.
  • Plunger Detonator: The Remote Detonator item.
  • Potty Failure: The item "Lemon Mishap" allows you to lay down a puddle of pee that damages enemies, and the item "Number One" replaces your tears with a rapid-fire stream of urine. Also, certain pills will give you bad gas or literally explosive diarrhea. There's also the boss monster Peep, who uses urine as his main weapon, and Mega Fatty, who can attack you by farting out diarrhea or small turds.
  • Pun:
    • For example, a certain pill makes Isaac shit bombs note .
    • The Skeleton Key, an item which gives you 99 keys (the maximum and more than you'll ever need) has a skull for a handle.
    • A lucky foot usually belonged to a rabbit before becoming lucky - not so in the case of the Lucky Foot item.
    • In real life, Ipecac causes projectile vomiting. In the game, it causes Isaac to literally vomit projectiles.
  • Power Up Letdown:
    • This can apply to quite a large variety of traditionally "good" items in the game if you're in possession of certain other ones thanks to the abundance of counter-intuitive and/or broken synergies. A common example being tear upgrades literally negating the effects of other tear upgrades, such as Ludivico Technique disabling Ipecac's explosions or, in Rebirth, Tractor Beam restricting Marked shots back to the traditional 4 cardinal directions while steel keeping the control scheme and autofire.
    • The Lost Contact can serve as this, situationally. It makes your shots intercept regular enemy projectiles, destroying both. In many cases, this is good. However, if your rate of fire is too low, and/or the enemy's is too high, it can make it incredibly hard to land a hit. The fact your shots block enemy shots effectively means that enemy shots can now block yours. Certain extremely fast-firing enemies can simply bore through your attacks and continue to hit you with impunity while preventing you from striking back.
    • The Tick, a randomly spawning trinket that makes bosses start out at anywhere from 5 to 15% less health from full, is fairly useful. What makes it bad is that it latches onto you and can't be removed by anything, making the negatives of it far outweigh the positives, and since the the Polaroid is needed to reach The Chest, picking the Tick up by accident when you can't find or buy mom's purse basically makes the game Unwinnable by Design. Luckily, Rebirth retcons the Polaroid - and it's new counterpart the Negative - into permanent powerups, allowing the Tick and the Photos to exist in harmony. It even buffs the Tick with a health-recovering effect.
    • Despite being considered by the game as an upgrade, Shot Speed is generally regarded as nothing but a negative due to making it harder to lead shots to hit at diagonals, zero synergy with any item in the game, and actively detrimental to items like homing tears (tears have a limited turning radius; the slower the tears are, the sharper the turns they can make) and Lump of Coal (tears get stronger the longer they're in the air, so faster tears won't gain as big a damage bonus because they'll hit the target sooner. Many players actively skip shot speed ups, only possibly taking Stem Cells (which has an HP upgrade attached to it) or Cat-o-Nine-Tails (which has a damage upgrade attached to it).
    • A high speed stat, as well. You'd think it'd be useful, but once you get four or five, it becomes very difficult to control the character and prevent yourself from careening into spikes, monsters, fires, shots, etc. Less obvious/problematic with flight. High speed can also make it very difficult to aim tears while moving.
    • Any item that either conflicts severely with items you already have (Mom's Knife after finding a Fetus item, for example), do not benefit your character at all (Yum Hearts for ???, anyone?), or is an activated item found after finding your activated item of choice.
    • My Reflection, which turns all your shots into boomerangs that follow you. These have abysmal range (and any range pickups you get make them boomerang further, which is pretty much useless) and behave in goofy ways which can be hard to predict or keep track of because they follow you rather than a trajectory. They could theoretically be useful, but you'll probably just get hit more by having to think about them.
    • Collecting Number One or the thin Odd Mushroom after already collecting many fire rate upgrades. Both of these items are extreme increases to fire rate at the cost of range or damage, but if you've already got a high fire rate...
    • The Dead Cat item. You have eight extra lives, yes. You also have your health bar reduced to one heart (and any health bar increases you find vanish on respawn), and if you die you respawn outside (with your secondary item still used up if you used it and any Soul Hearts still gone) and enemies fully heal. Good luck beating any bosses you couldn't kill on the first try. And those eight extra lives are going to need to last you the entire rest of the game.
    • The Coin Purse. Its only effect is an automatic four pill spawns. Nothing says those pills have to be good. For every time you get four stat-up pills, there are at least three when you get a net stat down or else it's a wash, and you can't tell which you've got unless you've found the PhD pickup, or be able to only get good effects with Virgo.
    • Prior to their revamp in Afterbirth, sacrifice rooms usually weren't worth the effort - at most, you'll get a chest.
    • Spelunker. Yay, you can find secret rooms automatically! But you still have to bomb them open, and the X-Ray Glasses do the exact same thing except they also open the room for you.
    • Lord of the Pit, which grants flying and a minor speed boost. It can be found in a Devil Room and costs just as much as Spirit of the Night, which grants flying and spectral tears, the latter far more useful than the speed boost in almost all cases.
    • Level 3 and 4 Meat Boy might be seen as this depending on your situation. On the other hand, you get another familiar that deals contact damage and homes in on enemies, meaning later bosses like Isaac and ??? take damage faster since they never move, allowing Meat Boy to camp over them the entire fight. On the other hand, the two bosses also utilize Bullet Hell tactics, so having that extra orbital to block shots with might be more important.
    • For some reason, the +40 damage bonus from Ipecac isn't affected by the damage multipliers from Polyphemus, Proptosis, Sacred Heart, Bloody Lust or Crown of Light. It also doesn't fully apply to burn or poison damage, instead it only adds +4 damage. Similarly, the damage bonus is ignored in many Ipecac synergies, to the point where some synergies (such as Ipecac + Mom's Knife) have lower DPS than Ipecac on its own.
    • The general opinion of Bob's Brain. It's a minion/follower that charges into enemies in a straight line when "fired" along with Isaac's tears, exploding on contact. It will do this even if the enemy is right next to Isaac, causing him to take Splash Damage. It also doesn't explode on the return trip if it misses, making it hard to aim, and doesn't synchronize well with tears. BBF (Big Beautiful Fly) also falls into this category for exploding upon contact with enemies (regardless of location) though at least BBF's movement is predictable and can be accounted for.
    • Thought that Samson or ??? wasn't worth the effort in the original? The crappy reward-to-effort ratio for The Lost, a genuine Joke Character, is even worse. The same goes for the reward for completely filling up the Greed Machine, The Keeper, who is the bottom of the barrel.
    • The Suicide King special tarot card requires beating one of the more infuriating challenges to unlock and has the splendid effect of dropping 10 items... and killing you.
    • The Lord of the Flies transformation gives you flight, and makes all enemy flies into friendly blue flies. That's about it... At least it's free flight, but Guppy is far superior.
    • The new transformations in Afterbirth range from helpful to nearly pointless. Fun Guy, for example, is just a free Health Up.
    • Tiny Planet, which makes your shots orbit you at a fixed distance instead of going forward... which makes it almost impossible to hit anything with more than half your shots, forces you to fight at close range, and makes knockback useless for defense. At least it works differently with a few alternate attacks (notably Brimstone).
    • Curse of the Tower, unless you have an item that provides bomb immunity or invincibility after being hit. It spawns a handful of troll bombs any time you take damage, dealing lots of damage to anything in the room—but possibly also damaging you again. And it will destroy any blood donation machines or demon beggars you try to use.
    • If you manage to defeat one of the hardest bosses in the game with the Keeper, one of the hardest characters in the game, you are rewarded with... him starting with a single penny in future runs. Gee, thanks.
    • The Wiz allows Isaac to shoot tears out of both eyes at once, but also forces him to shoot diagonally (effectively a permanent "R U A Wizard" effect). Given that the game is designed around shooting in the cardinal directions, it's generally more of a hassle than a benefit.
  • Promoted to Playable: Some of the hidden items from the original game are now unlocked from the start in Rebirth. Other items now have different unlock requirements, such as The Nail. And inverted with a few items that didn't need to be unlocked in vanilla, such as the Wire Coat Hanger.
  • Purposely Overpowered:
    • The Stopwatch. Unlocking it requires you to put 999 coins in the Donation Machine (which can take dozens of runs since the Donation Machine will randomly jam and refuse further donations for that run). Once you have it, it slows down all enemies and their bullets for the duration of that run, making it a near-cakewalk. It got hit with a huge nerf in Afterbirth via a patch, reducing it to an on-hit, single room effect (still pretty good against bosses, but not quite as useful overall).
    • The Mega Blast is Brimstone or Krampus' Head on crack. It gives you Mega Satan's giant Brimstone laser, which lasts a solid 15 seconds and persists between rooms, allowing you to steamroll through numerous rooms before the laser runs out. However, this requires beating Mega Satan with every character, including the Joke Character AND it's the only item in Afterbirthnote  with a 12-room charge (and batteries only refill 25% of the charge bar), so you'll only be able to use it once every two floors, or once a floor if you're lucky. With the addition of Void in Afterbirth+, you can cut the charge time in half.
    • Mama Mega! is this. When used, it deals kills every normal enemy in a room (if an enemy spawns another upon death, you have to kill them yourself), does massive damage to bosses, destroys all forms of rocks (but not Keepers or rocky chests), opens all Secret and Super Secret Rooms, and even opens the paths to Boss Rush and the Blue Womb if you've passed the time limit for either. Best of all, it lasts for the entire floor on which it is used. It basically gives you an entire "free" floor with nothing standing in your way. The only downside is that it can only be used once before going away.
  • Press X to Die:
  • Purposely Overpowered: The Wrath of the Lamb expansion adds a lot of incredibly powerful new items and effects that would be extremely abusive on the vanilla version of the game;note  You'll need every one of them if you want a chance to survive (not win, just survive!) the horrors you'll find on the new floors.
    • Samson in Wrath of the Lamb, who is basically Judas with a slightly lower base attack. (He is skilled at fighting mobs of enemies, but that's not much consolation if you have to fight one of the few bosses that doesn't summon punching bags.) He is rebalanced and further differentiated from Judas in Rebirth.
    • Guppy. While the ability to fly and spawn blue flies is pretty good, it requires a three item combo; one of these may need to be the above-mentioned Dead Cat, and you'll almost definitely need to trade in some health in a Devil Room for it. In comparison, the three item combo of Wafer+Habit+Yum Heart/Book of Revelations/The Nail makes you functionally invincible for the entire game, and there are two item combos that are significantly more powerful than Guppy.
    • The Stop Watch in Rebirth. It takes forever to unlock, but it gives all enemies and their bullets the slowdown effect. For the rest of the run. Sadly, this was nerfed to an on-hit, single-room effect, given how easy it is to obtain once unlocked, but is still a pretty good effect even after the nerf.
    • Mega Blast in Afterbirth. It also takes forever to unlock, but it's a massive Brimstone laser which has insane damage and persists between rooms. It has the honor of being the only item in the entire game with a 12-room charge that isn't instantly filled by a battery.
  • Powerup Magnet: There's a "Magneto" powerup which attracts powerups even through rocks and over gaps.
  • The Power of Hate: Rebirth adds The Negative, which is The Polaroid's dark counterpart. It damages all enemies when Isaac reaches his last hit point.
  • The Power of Love: The Polaroid, a picture of what seems to be Isaac and family, shields Isaac on his last hit point.
  • Psychopathic Manchild: Several bosses give this impression due to the goofy, yet deranged smiles they make when attacking.
  • Quirky Miniboss Squad: The Seven Deadly Sins are this, each one a personification of its namesake.
  • Random Event: The Curses introduced in Wrath of the Lamb, which are randomly activated at the beginning of a level. Curse of Darkness makes the map useless, while Curse of the Lost increases the size of a floor from what it would normally be and removes the pattern for Secret Room spawning. Curse of the Labyrinth combines two floors of the same tier — including two item rooms and two bosses.
    • Rebirth changes the Curses around and adds some new ones.
  • Razor Apples: The "Apple!" item, added in Afterbirth+, makes Isaac spit out razor blades along with his standard tears.
  • Recursive Ammo: The Parasite gives Isaac the ability to split his shots when they hit something.
  • Reset Button:
    • Forget Me Now, an item unlocked by defeating Satan with ???. It starts you over from the beginning of the current level with a new layout and new monsters, including the boss. Why would you want to use this? The reset level also has new items, giving you more opportunities to power up your character. The only problem is that Forget Me Now is a one time use spacebar item, forcing you to abandon your current spacebar item and beginning the reset level with that item slot empty.
    • One of Rebirth's Dice Rooms has the same effect as Forget Me Now.
  • Revive Kills Zombie: The Bible, while normally dealing no damage, kills Mom and Mom's Heart instantly. Inverted with Satan, using the Bible will instantly kill you instead.
  • Rewarding Vandalism: Rarely, destroying poop and putting out fires drops an item. Rocks with a small X on them (usually identifiable by their blue-tinted color) will also drop an item when bombed. (Destroying lots of poop this way unlocks The Bean, and blowing up enough rocks unlocks The Small Rock.) And then there's the secret rooms, which are revealed by blowing up walls.
  • Roguelike: Things like level layouts, monsters, items, and most bosses are randomly generated — though it's also level dependent (i.e. you will never find a Caves boss in the Basement, unless the boss could appear in both). Wrath of the Lamb even randomizes what level you'll be sent to by providing alternate stages. For example, in the original version, the first two levels would always be the Basement. After the expansion, each of the first two levels will be either the Basement or the Cellar.
  • Rummage Sale Reject: Given that every passive item picked up includes a cosmetic change to go with it and that these visuals stack graphically as you collect more items, Isaac's appearance by the end of your run is guaranteed to look like this in normal play... when it's not just straight-up Body Horror.

    S-Z 
  • Save Scumming: Because the game is randomized, you can instead do this on startup to get preferential powerups instead of garbage (the latter being more likely). This is discouraged in Rebirth by tracking your current winning (or losing) streak — restarting counts as a loss.
  • Scare Chord: Played on certain endings and when you enter the Devil Room.
  • Secret Character: ???, who is actually Dead Baby from one of Edmund Mcmillen's earlier works.
  • Segmented Serpent Larry Jr. and the Hollow are made of multiple segments, each of which can be individually damaged. They can split into multiple entities if a segment is broken and there are two or more remaining segments on either side.
  • Self-Deprecation: The creators of the game appear as a rare variant of one of the Seven Deadly Sins. Namely, the the butt-ugly Ultra Pride, due to a feeling of pride and satisfaction at finishing their game.
  • Sequential Bosses: Particularly the True Final Bosses. Special mention to Satan, the only boss with multiple health bars. Rebirth tops Satan with Mega Satan, who, including all of the Degraded Bosses he summons, has twelve separate phases.
  • Sequence Breaking:
    • It's possible to reach Sheol without beating the game ten times either by using the "We Need To Go Deeper" item or taking the hatch in the (possible) Devil Room in the supposedly last stage. Rebirth allows you to use Angel Rooms to reach the Cathedral.
    • While it might be a bug, it's currently possible to get credit for beating the game as ??? long before you can even unlock him...by using The Ankh, an item that gives you an extra life, at the cost of transforming into ??? on respawn. Considering you can't unlock the character until you've beaten the (constantly growing and changing) game 10 times, finding this particular item, and getting to the final boss, and then dying to respawn can save you a lot of time and trouble, as you keep most other powerups when you come back... meaning you don't have to play a full game with what is essentially a joke character to unlock a powerful item for Isaac.
  • Serial Escalation: This game has issues with one-upping its own final dungeons every couple of patches.
  • Set Bonus: As noted in Ascended Extra, acquiring 3 out of 4 specific cat-related itemsnote  in a single playthrough will transform Isaac into his deceased cat, Guppy. Not only does it make you look hilarious / adorable, but it grants you the abilities to fly and spawn blue flies whenever you hit enemies.
    • Rebirth adds a few more cat-themed items, and also adds a new transformation, the Lord of the Flies. It's a bit of a letdown, though.
    • Afterbirth adds even MORE transformations, including Mom (grants you a Mom's Knife familiar) and an Angel (flight plus some soul hearts).
    • The Eternal Edition of the original game adds an angel transformation if you get seven health upgrades from eternal hearts in one run.
  • Seven Deadly Sins: Personifications of these are present in this game as minibosses. Beating all seven unlocks a new item. Super versions appear in Wrath of the Lamb.
  • Shout-Out: Lots of them.
  • Skeleton Key:
    • Gives 99 regular keys, which is probably all you'll need and then some.
    • In Wrath of the Lamb, the Golden Key allows you to open any door in the floor where you get it without spending any of your Interchangeable Antimatter Keys, then vanishes after the floor is exited.
    • A variation of this is the Dad's Key active item: Upon using it, every single door in the room is unlocked. Every single one. Even the door to Mega Satan in Rebirth.
  • Smart Bomb: The Death tarot card, the Necronomicon, and one of the functions of the Sun tarot card will damage every enemy in the room when used. In Rebirth, black hearts serve this function when depleted.
  • Snarky Inanimate Object: Fortune Teller machines will usually give cryptic predictions when used, but sometimes they'll get really sassy with you instead. Rebirth played it up significantly and gave the Fortune Teller many more fortunes to pick from, a lot of which are directly insulting the player ("You are playing it wrong, give me the controller" comes to mind).
  • Sorting Algorithm of Weapon Effectiveness: For the most part, items unlocked later aren't any more powerful than those available at the beginning... except the familiars, where the ones available at the start fire vanilla bullets and those acquired later have much more exotic and useful attacks. And the health-increasing items, where the unlockable ones generally either heal you or give you some soul hearts as a bonus.
  • Spikes of Doom: These appear in later levels in some of the rooms.
  • Spiteful A.I.: Some players say that the game has a "troll engine" that manipulates the random drops you get just to laugh in your face, especially if you tempt fate by grumbling about never having enough money for shops, and then the game gives you a Steam Sale (half-off everything in the shops) right after you've fought Greed in the very last shop in the game.
  • Stealth Pun:
    • The item "Mom's Eye" makes your tears fire in two directions at once — in other words, Mom really did have an eye in the back of her head!
    • The 3 Dollar Bill's name comes from an old saying that something is "as queer as a three dollar bill", back when "queer" meant "strange" and not "homosexual". The Bill also makes Isaac happy, or to use another term, gay, back when the word meant "happy, of good cheer" and not its modern meaning. But keeping with the more modern definition of "gay", it gives Isaac a random tears effect every three seconds, which the game describes as "Rainbow Tears". The Afterbirth challenge themed around it is even called Pride Day.
    • The Ipecac item makes you shoot green bombs out of your mouth, which is literal projectile vomiting.
    • The Guardian Angel spins around you and blocks incoming shots, thus protecting you.
    • Bringing the negative down to Sheol takes you to the Dark Room after beating Satan, which was a place where photographs were processed back in the day.
    • Picking up a Cube of Meat the first two times causes it to orbit Isaac and block projectiles for him. It's a meat shield!
    • The Latchkey item gives you Luck and Keys.
    • The Hallowed Ground item spawns glowing white poop with a protective aura. The term "holy shit" has never been more appropriate.
  • Standard Status Effects: Paralysis, slowdown, and poison, among others. Interesting case, since only Isaac gets to inflict them on enemies (with the exception of the Death boss).
  • The Stinger: "Get in the box!"
  • Stuck Items:
    • Your reusable item can only be replaced with a different one, though depending on the character, you might start out empty-handed. Likewise, you can only drop a one time use item by picking up a different one, which can be stressful if you end up holding one you know would be detrimental to your health if you accidentally used it.
    • Wrath of the Lamb adds trinkets, which can only be exchanged if you find another one. Trinkets are passive abilities, so this usually isn't a problem — but there is actually one trinket that cannot be removed no matter what. This is not a bug, although it is a literal bug; the trinket in question is a Tick. Ticks are hard to remove!
      • Even worse, since the Tick can't be removed, this prevents you from picking up the Polaroid, a trinket needed to access the Very Definitely Final Dungeon.
      • Rebirth finally allows the player to drop Trinkets...except for the Tick.
      • Afterbirth finally provides a way to take off the Tick: picking up a Match Stick. Which allows you to scare off the Tick by heating it off.
  • Summon Bigger Fish:
    • Monstro's Tooth causes the boss Monstro to fall and stomp an enemy on screen for massive damage. It doesn't work on Monstro himself.
    • The High Priestess Tarot Card summons Mom's big fat leg to stomp directly on a random enemy for major damage. Against Mom, this card summons another foot to stomp you. Maybe because Mom has two legs...?
    • Rebirth adds the Mom's Toenail trinket. Every minute, Mom's foot comes down to hit an enemy...and if there isn't one, it hits you instead.
  • Suspicious Video-Game Generosity:
    • The truly, verily, most definite Final Dungeon as of last Thursday features four golden chests, containing four powerups without any other price than the keys needed to open them. You are going to need them.
    • In general, Wrath of the Lamb buffs up the pool of available items, trinkets, powers, etc. with many powerful additions. You will most certainly need them...
  • The Swarm: Wrath of the Lamb introduced the Swarmer enemy, which appears as a husk of a humanoid head animated by a swarm of flies that are apparently using it as a nest..
  • Tactical Suicide Boss: A few.
    • In Wrath of the Lamb, Gurdy's boils are sometimes replaced by guts that spew explosives instead of regular projectiles. These will damage her.
    • Loki drops Boom Flies that, if your offensive capabilities are strong enough, you'll immediately detonate on top of Loki.
    • A canny player can push Wrath's bombs back at him, dealing significant damage. If you've got the Homing Bombs powerup, you won't even need to do that.
    • One of the new bosses in Wrath of the Lamb, Pin, can actually damage itself with its green exploding projectiles.
    • It's possible to have Mom's foot step on another part of her body, dealing a large amount of damage.
    • Satan goes down quite quickly by destroying the flying worm bombs he spawns when he's near them.
    • One of the champion versions of the Hollow is a grey version that spawns Boom Flies for every section destroyed. These can be used to blow up more sections and spawn more Boom Flies until the boss is dead.
    • The champion version of the Carrion Queen spawns little Hearts instead of the dangerous red poop. Said Hearts try to avoid Isaac and damage the Queen when destroyed.
  • Take That!:
  • Take That, Audience!: The title screen, after clearing the game 200 times.
  • Take Your Time:
    • There is no time limit of any kind, and you can backtrack as much as you want within a floor.
    • Downplayed in Rebirth, which has a Boss Rush room that's only accessible by beating Mom in under 20 minutes. Similarly, Afterbirth adds an exclusive boss fight that requires The Womb to be cleared in under 30 minutes.
  • Tears of Blood: Items such as haemolacria give Isaac these, and this time it isn't a bad thing. It means your tears will do more damage.
  • Tears of Fear: Isaac's default mode, given his crapsack childhood of abuse, harassment, and attempted filicide. When Isaac picks up a demonic upgrade, these frequently become Berserker Tears. When he gets the 3 Dollar Bill, he gets Tears of Joy.
  • Teeth Flying: The Tough Love item, appearing visually as brass knuckles, allows Isaac to randomly spit teeth instead of tears. If Isaac has items which increase the visual size of his tears, he'll also shoot larger teeth.
  • Teleportation Misfire: Telepills have a very small chance of teleporting you to an "I AM ERROR" room rather than a random room on the floor (so named due to its only inhabitant being a random guy that says "I AM ERROR). Not that it's necessarily a bad thing, since it usually contains some goodies and a trapdoor to the next floor. Certain other teleportation items in Rebirth and its expansions are also hard-coded to teleport you to I AM ERROR under certain conditions.
  • Teleporting Keycard Squad: Taking an item or opening a chest in Challenge rooms triggers three waves of enemies.
  • There Is No Kill Like Overkill: There are a slew of item combinations that let you deal more damage than is actually necessary, even for bosses. Orbiting knife chains, explosive tear barrages, forming projectiles so gigantic a single tear will fill up the entire screen, the list goes on. Hell, the Game Breakers in this series are so numerous that it warranted its own page.
  • Toilet Humor: There are quite a few items related to urine and feces, piles of poop are ubiquitous throughout the game, and several enemies are living embodiments of the same.
  • Too Awesome to Use: Some of the Tarot cards and the Full Health pill, usable only once. Possibly some of the more powerful subweapons which can only be used once every six rooms. The epitome is the Sun Tarot Card. It is a full health pill, Necronomicon, and one-floor Compass and Treasure map, all rolled into one. It's agonizing deciding whether to use the card to see all a level has to offer, or save it for emergencies or for the final boss.
    • Averted if you manage to get your hands on a Placebo/Blank Card, which let you reuse your pills and cards on a short cooldown, at the cost of your active item slot.
  • Turns Red: All four of the Harbingers, Gemini, Peep to a minor extent, Carrion Queen, and Mom's Heart. Some are actually less difficult after turning red.
  • Treasure Is Bigger in Fiction: Coins in this game are pretty large.
  • Troubled Fetal Position: Isaac does this in the floor transitions and at the start of each floor.
  • True Final Boss: The Binding of Isaac is notorious for constantly one-upping the game's final boss with each content addition. For a full summary of the true final boss progression in the games:
    • Mom was the final boss of the flash game, but after killing her once and seeing the credits, subsequent runs move onto a Brutal Bonus Level (the only one in the game at the time), the Womb, which ends with Mom's Heart. After killing it around a dozen times, Mom's Heart is permanently replaced with It Lives, making it the true final boss of that version.
    • The free Halloween update added a new floor, Sheol, accessed after It Lives is killed. Satan, the boss of Sheol, is the true final boss of the base Flash version.
    • Wrath of the Lamb added a split path after It Lives, with the Catherdal leading to Isaac, an alternate final boss. By winning enough runs against Isaac and unlocking the Polaroid, Isaac becomes demoted: depending on the version, he is either replaced by ???/Blue Babynote  or the game progresses onto the Chest, a new final floor for the fight with ???. Satan remains as an alternate final boss.
    • Rebirth retained ??? as a possible final boss, while Satan got demoted with the addition of a counterpart to the Chest, the Dark Room, with The Lamb as its boss. A third, tougher final boss, Mega Satan, can be reached by unlocking a door in either the Chest or the Dark Room with the angel room key pieces.
    • Afterbirth technically has no new final bosses, aside from Ultra Greed in Greed Mode. The new superboss, Hush, is accessed at the end of Chapter 4 and ends a run upon its first defeat, but connects back to the main path on subsequent kills.
    • Afterbirth+ adds one new floor, the Void, reached from a randomly spawning portal (only guaranteed after killing Hush), which has a new final boss, Delirium. Killing it grants the then-final ending, making Delirium the true final boss of Rebirth until another expansion was developed.
    • The alternate path in Repentance features a sidequest that unlocks its exclusive final floors and boss, Mother. Defeating it once allows another secret to be reached on any path, whichs leads to a floor containing two more final bosses, Dogma and The Beast. These are considered the definite true final bosses of the game (so long as no more expansions get added), and beating them unlocks one last ending.
  • Underground Monkey: In both new model and palette swap flavor. Even a few bosses come in variants, some of which are Shout Outs to other games.
  • Unstable Equilibrium: Zig-Zagged.
    • In general, having better items means doing better overall and and allows the player to take more risks, which in turn may improve the player's position. Doing well on the first floors greatly increases your chances of getting through the entire game. On the other hand, it's not as easy to turn things around if the first floors treat you especially badly.
    • Devil/Angel rooms are most likely to appear when you don't take red heart damage. If you're powerful and/or have many soul hearts, you can get even more powerful.
    • Both the Self Sacrifice Rooms and the Cursed Rooms reward you with items/pickups, but require you to damage yourself. On the one hand, they're not a good idea if your HP is low. On the other hand, they're not a good idea if you have soul hearts either, since the rewards are rarely worth them. Overall, they're still a balancing factor, since soul hearts are bound to pop up even on below average runs.
    • Blood donation machines give you with money in exchange for hearts. This alone rewards you for doing well, but if you use it enough, you get an item that not only gives you a heart container, but will also replenish the hearts that you spent. If you do really well on health, and have many hearts laying around the level, you get even more health.
  • Unwinnable by Mistake:
    • Thanks to a teleportation item (either pills or the Teleport! item) and some rotten luck, a player can find themselves stuck behind a room full of rocks, with no bombs to break them, making it impossible to continue, and forcing them to quit out of their current run.
    • It happened to the game author himself. In his co-commentary with Northernlion, he mentions that, after getting a certain combination of power-ups and power-downs, his firing range was too low to reach some monsters standing behind a pit — and killing all enemies is required to leave a room.
    • The Wrath Of The Lamb expansion has issues with this, with XL levels being the main culprit. The player may teleport into an area with no exits, or the game may not spawn the hatch to the next level if certain bosses are fought first in the Sequential Boss fight.
    • Wrath Of The Lamb also adds a Challenge Run reel, one of which is called the purist. The point is that you have to defeat the Disc Three final boss without access to item rooms. The game forces you to go up to the Disc Three Final Floor by having no trapdoor to the other Disc Three Final Floor, only a beam of light that takes you to where the challenge wants you to go. However, if you, by any means, manage to teleport yourself out of the beam of light before the new level loads, the beam will disappear from the boss room and you'll be permanently stuck on the Womb/Utero 2 with no means to get out. This could, however, be deemed Unwinnable by Insanity, as players would either have to be either very clumsy or insane to teleport just as they enter the light.
    • Taking Telepills in The Chest can teleport you into an empty room that is impossible to escape from, instead of teleporting you into an I AM ERROR room like it is trying to. Unless you managed to grab another teleport before using them (either through using the pills right before you run into another Q item to take it with you or bringing "Teleport!"), you're stuck and have to forfeit the run. This is fixed in Rebirth; the I AM ERROR room in The Chest or Dark Room has a hole which resets the stage.
    • It's entirely possible to end up having a spacebar item (which appear on pedestals) appear from a chest which is in the middle of a path you need to walk through to get to the boss room... when you've already got a spacebar item, there's no other way to get to the boss room and you don't have any means of getting around the pedestal. You try to rush past the pedestal, you do the item grab pose and the pedestal pushes you back the way you came from. Now there's another item on the pedestal, but you're otherwise in exactly the same position...
  • Urine Trouble:
    • The aptly named Number One upgrade replaces his tears for a urine attack instead. This dramatically increases fire rate, but sharply decreases range.
    • The Lemon Mishap item, which leaves a yellow puddle that damages enemies. The Lemon Party pill is an upgraded version, which creates a much larger puddle at the cost of not being reusable without the Placebo item.
  • The Very Definitely Final Dungeon: The Chest, a floor made up entirely of boss fights in almost every single room, with ??? (The Blue Baby) as the final boss. Rebirth adds the Dark Room, a dark-themed counterpart to the Chest. Its boss is The Lamb. Afterbirth+ adds the Void, in which every area of the game is merged into one level. It has many bosses, with all but one (Delirium) being pulled from other areas.
  • Virtual Paper Doll: Almost all of the upgrades Isaac can pick up will change the look of his sprite.
  • Turns Red: Boss!Isaac (and Blue Baby), on the last third of their health, gain angel wings and suddenly gain the ability to summon Angelic Babies and use a "beams of light" attack similar to Crack the Sky, along with firing more homing shots.
  • Versus Character Splash: Happens before boss fights.
  • Violation of Common Sense: Want God to notice your good deeds and reward you? Murder a beggar. note 
  • Vomit Discretion Shot: The Hematemesis and Vurp! pills. They both have names which imply that they make Isaac vomit, in that hematemesis is the medical term for vomiting blood and vurp is a portmanteau of vomit and burp, but this is not shown.
  • Vomit Indiscretion Shot: Ipecac causes the character to become violently ill and shoot out explosive vomit. Similarly, the Horf! pill makes Isaac fire a single high-damage Ipecac shot.
    • Several bosses including Mega Fatty and Mama Gurdy can attack by vomiting on Isaac.
    • There's a milder example in Ending 6, which unlocks Ipecac. Isaac falls into the chest, which presumably contains Ipecac, and climbs back out. He then becomes nauseous and vomits into the chest. He has his back to the camera, but some of the splashback is still visible, not to mention the resulting explosions.
  • Womb Level: Appropriately titled The Womb, and its alternate, Utero.
  • World of Symbolism: Downplayed. A lot of the stuff is just creepy, but when you start viewing it through the eyes of an abused child... The choice of the item powerups is even worse:
    • HP ups being mostly spoiled milk and dog food, implying what Isaac is usually fed. "Nutritious" foods which give Isaac two hearts of max health include raw liver and buckets of lard.
    • Items used to beat/punish Isaac increasing his speed. (Wooden Spoon, Belt)
    • His pets all dead, presumably killed by Mom. (Tammy's Head, Max's Head, Dead Bird)
    • The Virus you often receive for beating Lust is AIDS/HIV.
    • The Brother Bobby, Sister Maggy, and Wire Coat Hanger items, implying that Isaac is a failed abortion or unwanted child.
    • The other characters being alter-egos of Isaac. He is so lonely he has to make them up in the first place (or he made them up to cope with what's happening to him).
    • The Polaroid might imply that Isaac's mom might not always have been the psychopath we know her as.

Tropes exclusive to the remake and its expansions include:

    open/close all folders 

    A-M 
  • Absurdly Spacious Sewer: The Downpour and its alt version the Dross, added in Repentance.
  • Adult Fear: The idea that your son going missing before finding out that he killed himself because he believed he was the cause of his father leaving is a very disturbing thought.
  • Alternate Reality Game: The Missing Poster was intended to be used for this. What it does is obscure enough (you need to kill yourself in a Sacrifice Room while holding it), and if someone manages to figure out its effects, the Missing Poster would be used to generate a small piece of the Game Over screen. Piecing them all together would give hints on how to unlock The Lost. Unfortunately, how to unlock the reward for the ARG was discovered by data miners, which Edmund was bitter about.
    • Edmund learned his lesson and tried again. Following the release of Afterbirth, he left a clue 109 hours after the initial release. The first clue started with the icon for a new Achievement added to Afterbirth in a patch, named "Generosity". This led to a clue hunt involving a phone number, Twitter, and some digging. The end result of the ARG led to the release of a new unlockable character, the Keeper. Edmund had prevented data-mining by simply not adding the character to the game until the ARG was completed.
    • However, he did drop some hints in the game files about a new character, along with red herrings, including a spritesheet showcasing the Keeper as an old man instead.
  • All the Worlds Are a Stage: The Void, the final area of Afterbirth+, is a combination of all previous areas of the game. Any room can be from any area, there are around seven bosses aside from Delirium that can be from any area, and the boss drops can come from any item pool in the game.
  • In 'Repentance', collecting Dad's note brings Isaac up through the floors he already went through, ending at Home. Unlike the void, however, this is only a revisit to the floors you went to in this run, so you do not see any floors from the Womb onwards.
  • Ambiguous Gender: A new character, Eden, is gender-neutral to go along with their theme of randomization.
  • Ambiguously Evil: The soundtrack when fighting Satan/Mega Satan is called Hericide, which is the killing of one's lord/master/teacher, and in the 16th ending Isaac turns into a demon.
  • And Your Reward Is Clothes: The reward for beating Mom's Heart/It Lives! on hard mode, Mega Satan, and/or completing all marks (defeat every major boss plus the Boss Rush) on Hard Mode on a character? A different selectable skin for co-op babies. Slightly downplayed in that these babies have special abilities, though these are generally minor, such as dealing contact damage and dropping pickups upon death. Except for The Lost, who unlocks a special baby that doesn't take damage for killing Mom's Heart/It Lives! on hard mode, and unlocking the extremely powerful Godhead if you manage to clear all Hard Mode marks with him.
  • Animation Bump: The game is a slightly pixellated Retraux style. The endings, however, are very fluidly animated.
  • Anti-Frustration Features:
    • When you hold up an item, you're still able to move. This means you are much less likely to get hurt if you accidentally walk over a trinket/consumable during a fight, and prevents you from getting stuck if there's an active item blocking your path.
    • If the boss room entrance is facing the top, the trapdoor to the next room will be closed to prevent you from accidentally walking into it. Likewise, if your speed is almost too high to handle, you'll go over the trapdoor instead of into it to keep from going in there because you're too fast.
    • Blowing a hole through a wall to a Secret Room is now permanent; previously, one had to use another bomb to re-open the room after leaving it. Particularly helpful for cases where the room had a Slot Machine or something similar and one needs to leave to get more coins elsewhere.
    • Items now show a short tagline upon pickup that give a sense of what they do. Many just explicate what the item does, while others are more cryptic but still helpful. This can save a newcomer a trip to the wiki.
    • You can now rebind keys as you see fit, so you have a solution if you keep blowing yourself up because E is "open door/chest" in other games or you keep wasting items tapping Q or Space.
    • For the first couple of seconds upon entering a room, BBF will destroy enemies without exploding, preventing unavoidable damage in that way.
    • Curse of the Labyrinth, if it occurs on the first floor, now leaves the Treasure Rooms unlocked as opposed to requiring keys for both. Previously, this curse could easily rob a player of two items, as the low number of rooms could potentially provide no keys to unlock them.
    • The Polaroid and its evil counterpart, the Negative, are now item pickups instead of trinkets. This allows you to pick up the Tick without sacrificing a Brutal Bonus Level run. Afterbirth added Missing No., which rerolls every passive item you have at the start of each floor... except for the Polaroid, the Negative, and the Key Pieces for Mega Satan, for the same aforementioned reason.
    • Greed mode's wave spawning can be de-activated by sacrificing half a heart to hit a button surrounded by spikes, but if you're playing as The Lost it's free, since they're a One-Hit Point Wonder.
    • In large rooms, enemies with laser attacks will generally not fire at Isaac from off-screen to prevent the player from taking unavoidable damage.
    • Normally, when you're doing a Victory Lap, you can't get achievements nor completion marks. However, Delirium - which is probably the hardest boss in the entire game - is one of the few exceptions to this rule. This means that you can do a first run to power yourself up, defeat The Lamb, start a Victory Lap and then try to fight Delirium, which significantly improves your chances of victory.
    • If you happen to get a certain Secret, but the achievement isn't given to you for some reason, you can visit the Secrets page and the game will send a check back to the platform's servers to ensure that it's given again.
    • The final chapter of Repentance, Isaac's Home, features a Sequential Boss who has so many phases that you're given not just one, but two opportunities to heal; first by sleeping in Isaac's bed before fighting Dogma, then automatically before The Beast sends its Horsemen after you. For the second time, if you're lacking on Red Heart containers, you will be given some Soul Hearts to compensate.
      • To access that final chapter, you need to get either the Polaroid or the Negative. But to get them, you must defeat Mom - and her room has no exit door; therefore, you need a way to teleport out it. For that purpose, there's a guaranteed way to obtain a a Fool card: break the marked skull on Depths II/XL or its alternate versions. This doesn't work in Mausoleum or Gehenna.
      • Likewise, to unlock the alternate version of a character, you need to use either the Red Key or a Cracked Key in a certain spot the final chapter. The first time you reach that final chapter, you are guaranteed to find the former right before the Sequential Boss. Alternatively, you can spawn the latter by leaving a trinket in a treasure room or a boss room. After you go through the special door and climb back back to that particular floor, re-enter that room and you'll find a Cracked Key on the ground.
  • Announcer Chatter: Afterbirth has a Large Ham one for calling out cards and pills. A patch made him come out at random instead of being on by default.
  • Arc Number: 109, the number of coins originally supported by the Greed donation box. The developers teased players about it and misled them into a wild goose chase for secrets, but what it meant is that a certain amount of content was simply blocked off until a patch came out after 109 hours. Coincidentally, 109 also happens to be the in-game ID of the item Money = Power. 109 is also the pickup quote for the Dataminer active item in Afterbirth+.
  • Arrange Mode: Afterbirth introduces the Greed mode, which is a gauntlet mode where you fight 10 waves of enemies per floor and receive coins, which can be used to gain pickups and items. Each floor also has a Curse Room, a silver treasure room that spawns items from the boss pool, and a gold treasure room that spawns items from the treasure room pool. Beating the extra 11th wave gives you the Devil/Angel room. After the Final Boss, an "Ultra" version of Greed, is killed, a Greed Machine spawns, which allows you to deposit coins into it like a Donation Machine. Doing so unlocks stuff for the main game, including a secret character.
  • Artistic License – Biology: The Frail's second form is skeletal, even though it's based off of a worm and therefore shouldn't have an internal skeleton.
  • Art Shift: The endings added to the game after Rebirth's launch, especially the Final Ending, have a noticeable change in visual style, favoring thicker outlines akin to the original game.
  • As the Good Book Says...: Rebirth's theme is titled Genesis 22:10. The verse? "Then he reached out his hand and took the knife to slay his son."
  • The Artifact: The starting room still displays the Move, Attack, Bomb, Item diagrams, but not the keybindings, which can now be changed on the menu.
  • Ascended Glitch:
    • A certain Womb room would spawn two Larry Jrs and a Chub, but due to engine constraints the Chub would be without her third section and unable to spawn enemies. When the room was ported over to Rebirth, this "buttless Chub" was left in faithfully.
    • The Liberty Cap still retains its oddball ability to sometimes act as a compass instead of a mushroom item.
    • You can still walk diagonally through spikes and not take damage if you get it pixel perfect. In addition, one possible Curse Room layout has a Red Chest guarded by four "Eternal" fireplaces, and you can walk through those to open the chest if you're careful enough, though you can also blow them up with bombs.
    • Conquest could randomly replace War in the Depths 1 in the original game due to a glitch. Here, Conquest is both a Depths 1 and Womb 1 boss.
  • Ascended Meme: The in-game description for the Cancer trinket is now "Yay, Cancer!", recognizing a meme that spawned from the people expressing happiness at receiving Cancer (a useful trinket) and other improbable phrases that only make sense in The Binding of Isaac.
    • Additionally, beating Mega Satan as ??? unlocks a multiplayer baby called "Blue Baby".
  • Back from the Dead: Rebirth continues the trend from Wrath of the Lamb of making undead versions of existing bosses, in this case applying it for nearly all normal bosses. Note that being undead usually makes each boss even more dangerous than before.
  • Big Head Mode: Entering "T0PH EAVY" as your seed gives your character a head that's at least 5 times bigger than their body. On the other hand, entering "T1NY D0ME" gives you Small Head Mode.
  • Big "NO!": One trinket is a crossed circle whose description reads "NO! Never again!" It removes space bar items from the item pool, which is good when you're collecting stuff to randomize with the D4.
  • Black Market: In Rebirth, finding a hidden passageway beneath rocks may sometimes lead to a modified shop which lets you pay hearts for shop items instead of coins. It also has a trap door to the next level, all three beggars, and sometimes a slot machine (type varies).
  • Black Speech: The incomprehensible, guttural chanting in the Sheol theme could count.
  • Bondage Is Bad: A new item in Rebirth, "Gimpy'', is a black leather gimp mask. It's found in the Devil Room for the cost of one heart, or in regular Item Rooms.
  • Bonus Boss: Afterbirth plays with this in regards to Hush. You do have to beat it at least once with every character, but unlike Mega Satan, this boss does not end the run. Rather, it is an optional diversion that can net you some extra items at the cost of fighting an extra boss, and once you beat it, you go right to Sheol or the Cathedral. Averted in Afterbirth+, where you have to beat this boss to unlock its Final Boss.
  • Boss-Only Level:
    • The Blue Womb contains two Item Rooms, a Shop, and a single 2x2 room with Hush in it.
    • Home only has a replica of Isaac's house in gameplay, with no special rooms. Simply sleep in Mom's bed and go to the television to fight Dogma. It is a downplayed example in that the backwards trek through the previous floors is the "stage" and Home is simply the boss area.
  • Boss Rush:
    • Accessible by beating Mom in under 20 minutes. You have to fight through a total of 30 bosses in 15 pairs, but you're given a free item (out of four choices) to start and a second item if you win (whether they're helpful or not is another matter).
    • Mega Satan summons the Four Horsemen, all the Super Sins and the Angels during the battle.
  • Bookends: The trailer for Rebirth, linked above, has the (rather creepy) song titled "Jesus Loves Uke" playing in the background. The (presumably) last trailer, for The Forgotten Update, has it playing in the background as well.
    • The game's intro was narrated by the narrator. The Final Ending in Repentance uses the same style.
  • Bowdlerise: Inverted in the credits, which thank everyone who played the hell out of the original, with "heck" being struck out and replaced with "hell.".
  • Bragging Rights Reward:
    • Prior to the Afterbirth expansion, there was no practical reason to beat Mega Satan other than seeing the final ending. There were no unlocks for doing so, no achievements for doing so, not even a mark on that character's boss kill list. All you would receive was a new picture for your save file.
    • With the exception of the Lost, who has an item unlock for beating every main boss, Hard mode has no item unlocks, and its associated achievements only require you to beat Mom's Heart/It Lives to unlock co-op babies. Beating the Brutal Bonus Levels is purely for sport.
    • The unlock for beating the Bonus Boss with Keeper? Said character starts with a penny. Of course, you're earning that penny by killing what is arguably the hardest boss in the game with a borderline Joke Character. (Repentance, for what it's worth, subverts this—that penny instead becomes a permanent third coin heart, something very handy given how difficult it is to get actual coin heart upgrades mid-run.)
    • In order to obtain Godhead, you need to collect collect all the completion marks on Hard mode for a bonafide Joke Character, one of the hardest tasks in the game. It's a fantastic item that ranks among the best in the game, with tons of positive synergies, but at that point do you even need it?
    • Repentance adds the Death Certificate, which is unlocked by getting every Hard mode completion mark for every single character, which totals out to a whopping 334 completion marks between thirty-four characters. Said item effectively lets you choose from any item in the entire game, and can be combined with other items like Abyss or Moving Box to bend the game over your knee with impunity, but you'll likely need to put in hundreds of hours to even get to that point.
  • Bullet Hell:
    • The battles against Mom's Heart/It Lives, Isaac, ???, and Mega Satan offer (a mild example of) this.
    • Afterbirth adds a true example of this, Hush, an extra boss that spawns hundreds of enemies and normalizes the damage it takes, meaning even an overpowered build can't just cheese it.
    • Afterbirth+ has another fine example in the form of Delirium, the new True Final Boss. He combines complex bullet patterns with impredictable shapeshifting and erratic Teleport Spam. Add his massive amount of HP and you have a pretty challenging boss in your hands.
  • Call-Back: One of the new items in Afterbirth is actually an old item: Lusty Blood is just Bloody Lust as it worked in the original Isaac.
  • Cat Scare: The final stage of Greed Mode is named Ultra Greed. At the start of the stage, there's a boss room. You enter it, expecting a hard boss battle...and then a single Monstro (or a pack of Greeds) appears. Ultra Greed is in the next boss room.
  • Cast from Money: The Magic Fingers is an item that instantly deals a small amount of damage to every enemy in the room every time you put a penny into it. It has no cooldown time, so if you have lots of money (or are lucky enough to find the Dollar), you can spam it...until your money runs out, anyway.
  • Challenge Run: Challenges, introduced in the Wrath of the Lamb expansion, make you play the game with various conditions that make the game much harder. They usually consist of a run where you start off with a certain set of items and are unable to gain any others.
  • Chest Monster: Afterbirth+ added a chest type that looks normal but becomes spiked when you stand right next to it. After some complaints, the chests were redesigned so that the difference between them and standard brown type was more obvious.
  • Classic Cheat Code: Some seeds have interesting effects, such as BRWN SNKE (causes Isaac to leave a trail of poop wherever he goes) or B00BT00B, described below. Taken even further in Afterbirth, where all those special seeds are now available in a list after you put one in, although the icon that indicates the list seems to indicate an Easter Egg, and can be chosen for any play, as well as removing the ability to progress in unlocks with some. They've all but been named cheatcodes. There is even one that makes Isaac invincible: ALM1 GHTY
  • Collision Damage: Isaac takes damage from pretty much everything that moves. The player can cause this if they use My Little Unicorn (or the Chariot tarot card) or the Virus powerup. The former is temporary and provides invincibility as well, while the latter is permanent but doesn't prevent damage in return.
  • Cool Hat: The Host Hat, an item introduced in "Afterbirth". It's a Host that Isaac wears on his head. It protects Isaac from explosions, and has a chance to block incoming shots and counter with the Hosts' signature three-bullet spread.
  • Color-Coded for Your Convenience: Champion enemies have traits based on their suit of color, and all of them do one heart of damage rather than half. Dark green means an enemy explodes at death and drops a bomb, pale red means massive HP, dark red means regeneration with a double-heart drop, gray deals double damage and drops a key, light blue means it explodes into bullets on death, green leaves a trail of green creep and drops a pill, and dark blue generally means "less dangerous, but can still hurt you" and gives friendly flies.
  • Completion Mockery: getting 100% Completion with both Expansion Packs installed unlocks the 1000000% achievement, represented by a picture of a stop sign and the description "Just Stop!"
  • Contractual Boss Immunity: Delirium is the only boss with a single phase that is immune to the Chaos Card. It isn't immune to Plan C, which destroys everything in a room and kills you three seconds later, but its death animation takes longer than three seconds. Only Lazarus or a character with Lazarus' Rags (both of which respawn the player in the same room) can make use of this strategy, limiting any unlocks to Lazarus alone.
  • Combinatorial Explosion: Part of the update to the game is that this can now happen. The formula is no longer "pick up anything that can help you, hope for Mom's Knife or Brimstone" for a win that seems assured; now any number of items can assure victory in combination.
    • Two wildly differing examples are Midas Touch + Book of Shadows + Nine-Volt, which basically gives you the ability to go running around turning enemies into coins every 2 rooms, and Ludovico Technique + magnetic tears + shielded tears, which give you a single controllable projectile that pulls in enemies and shots while disabling all shots that touch it meaning that you have control of basically a black hole against your enemies.
    • Another mild game-breaker is homing shots plus Technology 1 or 2. The laser beam actually curves itself around to hit enemies.
    • A further combo involves Guppy's Hairball and How To Jump. Jump over enemies to avoid them, and the hairball will plow through them. The momentum will also cause the hairball to bounce about well beyond its normal range.
    • And pretty much anything plus Brimstone. Homing Brimstone? (if combined with Spoon Bender) Triple Brimstone? (if combined with Third Eye) Ten beams firing simultaneously in all directions...Brimstone? (if combined with Tammy's Head).
  • Co-Op Multiplayer: You can sacrifice one of your hearts to allow another player to create and control a familiar. Some of these familiars have special abilities, and some are unlocked by beating Mom's Heart on Hard Mode.
  • Counter-Attack:
    • Evil (black) hearts inflict the Necronomicon effect on all enemies in the room when a full black heart is depleted. The Negative also does this when Isaac reaches half a heart.
    • If you pass a certain size threshold through size-up items, Isaac will retaliate against damage with a short-distance rock wave.
  • Cursed With Awesome: One of the random curses that you can get inflicted with is "Curse of the Labyrinth", which essentially merges two floors and forces you to fight two bosses in a row at the end. If you get it on the first floor, both treasure rooms will now be unlocked, sparing you the need for one key. On the other hand, later floors can potentially rob you of an additional Devil/Angel Room that could have spawned for both floors.
  • Damage Reduction: Afterbirth added scaling damage reduction as a game mechanic for end bosses, lowering the damage a player deals to them based on the player's damage output and making it much harder for an overpowered run to simply steamroll them. The main users of the mechanic are Hush (both forms, but it's much more noticeable in his second phase), Ultra Greed (again, both forms), and in older versions, Big Horn. Delirium does technically also have damage scaling, but only while it's transformed into another boss; it's generally not noticeable since the reduction ramps up as Delirium takes damage and resets to zero whenever it returns to its default form.
  • Damage-Sponge Boss: While there were already bosses who had this as their gimmick, this is taken to extremes as one of many gimmicks for the two new major bosses of Afterbirth, Hush and Ultra Greed, who both have health that eclipses the health of every other boss in the game and even normalize the damage you deal to them, meaning that something that would one-shot every other boss in the game will only deal chip damage to them. Afterbirth+ subverts this with Ultra Greedier, who has less health than Ultra Greed, yet plays this straight with Delirium, who doesn't need normalized damage because he has more HP than even Hush.
  • Damn You, Muscle Memory!:
    • An odd variant—the game's new engine will trick some people up. Rebirth's new engine is extremely powerful, able to handle insane amounts of stuff on the screen at any time at a rock-solid 60fps even on lower-end video cards, but this has some unfortunate effects on people transitioning from the lower, glitchier framerate of the first. People who are accustomed to the half-second pause when transitioning into a room will routinely screw themselves over on hazards, and attacks are much harder to dodge thanks to a lack of Hitbox Dissonance.
    • Most throwable items are used by pressing the appropriate key and aimed using the keys for firing tears. The Chaos Card, however, is automatically thrown in the direction Isaac's body is facing, meaning that it is aimed by walking in the direction you want to throw the card and then hitting the "use item" key. Woe betide the player who saves one far a particularly tough boss, forgets how to aim it, and ends up throwing it into the bottom wall.
  • Darker and Edgier: Even though the regular game is fairly grim on its own, Rebirth has a much more oppressing and bleak atmosphere, compared to the cartoony and lighthearted (if drenched in Black Comedy) atmosphere of the first.
  • Dark Is Not Evil: The Dark Bum, a demonic version of Bum Friend with a description of "He wants to take your life"... meaning he wants to take those red hearts that you can't take because your health is full and turn them into soul hearts (and the occasional spider. Being sick sucks). His usefulness (even after two nerfs) and fairly common appearance rate has pretty much made him an Ensemble Dark Horse, complete with Twitter page.
  • Death Dealer: Completely literally with the Chaos Card, which instantly kills anything it hits when thrown. It can even one-shot bosses! Except Delirium. It doesn't work on him.
  • Developers' Foresight:
    • The various insects you see—not the enemies you're supposed to kill but the actual pixel-tiny insects that add to the atmosphere—are interactive. Shoot or step on one and they'll turn into small puddles of blood. If you shoot a fly, tiny pixels of blood will fly off and they'll actually spiral downwards to the floor! If you have the Soul, bugs will be steered away from you.
    • If you're at half a heart left and you enter a new room, Isaac will pee a little at the entrance showing how terrified he (and possibly you, the player) is at what dangers this new room may present that could end your run.
    • In Afterbirth, the Flush item (which normally has no effect on bosses) instantly kills Dingle, Dangle, and Brownie when used.
    • When you defeat Ultra Greed, he turns into a giant gold statue. Midas's Touch will instantly do this, lowering his damage reduction.
    • In Afterbirth+, Glyph of Balance alters the drops of champion enemies so that you'll get what you need the most. If you're almost dead, its first priority is giving you a red heart - unless you have no heart containers, in which case it'll give you a soul heart instead.
  • Difficult, but Awesome:
    • Anti-Gravity Tears can be very difficult to use, but when used well you can simply drag any enemy that chases you through a path of tears, set traps for enemies that love to jump on your head, and burst down enemies that only expose themselves to damage every so often. Goes Up to Eleven with Brimstone: It can be difficult to make every laser hit, but you can now fire full-power Brimstone shots as fast as you can charge them.
    • Bob's Brain is notorious for blowing up in people's faces and generally being more trouble than it's worth. However, if you can keep track of its respawn time, keep repositioning so it doesn't immediately blow up in front of you, and Lead the Target so it hits what you want, it can function as a high-damage grenade launcher that continually replenishes itself. Particularly skilled users can even time it so that it blows up enemies when they're near bomb-able locations, saving on bombs in the long run. It shows up more frequently than a lot of items, giving players more incentive to learn how to use it properly.
  • Downer Ending: The endings added by the expansions make it clear Isaac thought himself to really be corrupt by sin and was driven to suicide. The 20th ending in Afterbirth+ puts everything more or less into perspective.
    • Isaac starves and suffocates to death inside the chest while hallucinating the entire game. His mother believes him to be missing and only finds out the truth when there are only bones left inside the chest. We then see Isaac walking into the underworld, followed by a demonic shadow.
    • The Greed mode's ending features Isaac getting trapped by a cave-in and becoming Greed after hanging himself. In the Greedier ending, his head then comes off and a fountain of bugs gush out of it. How this fits with the other endings is anyone's guess...
  • Dual Boss:
    • Certain bosses have champion versions which feature two of the boss, but each only has half the health of the original.
    • The "Double Trouble!" mechanic can randomly pair two bosses together, usually two of the same boss but occasionally different ones. This includes champion bosses, which means both versions will be doubled.
  • Dunce Cap: The Wiz powerup in Afterbirth is a dunce cap that gives the player character permanently crossed eyes (similarly to the "r u a wizard" pill).
  • Dying Dream: Afterbith+ reveals that the whole game is this. Even his mother attempting to kill him was allegedly imagined.
  • Early Game Hell: The hardest parts of Greed Mode are often the early levels, when you have little health and only average tears and damage. Once you've gained a few items, your chances of survival increase dramatically.
  • Earn Your Happy Ending: The final ending of Repentance has the narrator, Isaac's father, summarize the tragic and twisted themes of the game and its expansions. Just as the story is about to truly end he stops, reminding Isaac that he's the one telling the story and that it doesn't have to end like this. His father suggests a happier ending, to which Isaac agrees. Isaac's father asks his son if he's getting sleepy, to which Isaac responds that he is. His dad begins the bedtime story again, only this time instead of being about Isaac and his mother it's about Isaac and his parents.
  • Eldritch Abomination: In Afterbirth, collecting three Devil items transforms you into Leviathan (a mini-Cthulhu), which grants flight and a few extra black hearts.
  • Enemy Scan: The Spider Mod in Afterbirth gives every enemy (and segments if applicable) a visible health bar so you can track how much damage they've taken. It also displays how much damage your weapons are doing. Finally, the spider familiar itself wanders around randomly and can cause random effects to any enemy that touches it.
  • Escort Mission:
    • Isaac's Heart makes Isaac immune to damage, but generates a heart familiar which takes damage in his place. Enemies will target the heart instead of Isaac, so it has to be protected. On the plus side, Isaac can use any self-damage item for free, except Sharp Plug.
    • The Guardian challenge gives you a character with invincibility, flight and a powerful sword to stab monsters with. The catch? You must protect a Punching Bag with Isaac's Heart attached to it. The Punching Bag takes damage, not the heart, and you have no control over how the Punching Bag behaves. You have to escort him all the way to Womb II. Most of the time, it is impossible to prevent the dude from getting hit a handful of times, though at least it tries to avoid traps in rooms. Usually. It's especially bad when you end up with charger-type bosses that can't be knocked off course, which means the Punching Bag is guaranteed to take hits.
  • Everything Fades: While no enemies leave behind corpses upon death, they do explode into blood and bones upon death, which is cleaned up upon leaving a room. It's especially noticeable in the Boss Rush if you're quick enough to beat it in order to leave and be able to enter it again, as the room will likely be caked with gore once you're done.
  • Exact Words: The Suicide King card's description is "A true ending." From a Certain Point of View, it does just that, since it drops several items then kills you.
  • Feelies: The Nintendo Switch boxed release includes a fully illustrated instruction manual, which parodies the original Legend of Zelda game manual, as well as some stickers featuring some of the game's characters and enemies.
  • Fire/Water Juxtaposition:
    • The new alternate chapters for the Basement and the Caves in Afterbirth, Burning Basement and Flooded Caves.
    • Invoked even further with the Repentance DLC which is set to add Antibirth's Downpour (A flooded sewer during a thunderstorm) and Mines (A fiery mineshaft surrounded by lava) into the mix.
  • Flunky Boss: A number of new bosses spawn monsters while fighting them, such as Brownie summoning Corn Dips (Or Chargers, if you're fighting the Champion version). Mega Satan kicks it up a notch and can summon bosses in the form of any of the regular Sins. And occasionally The Hollow.
  • Game-Breaking Bug:
    • There seems to be a game crashing bug that crashes the game when enough damage is done to Mega Satan that they die without changing forms. It's rare, but it appears to be very consistent.
    • Sad Bombs give bombs a Tammy's Head effect. Godhead adds a giant halo to each tear. There's a glitch that allows one to let loose as many bombs as the screen will handle. Combining all three is the only way outside of Mega Satan to crash the game.
    • In Afterbirth, Ending 17 is supposed to be unlocked by defeating Hush at ???, but it only works once. On replays the player receives the exits to the next chapter instead. The problem back when the expansion was released is that imported saves from Rebirth wouldn't get the ending chest if they already had Chapter 5 unlocked. Trying to enter ??? as some characters would always crash the game, too.
    • Alluded to by the Afterbirth item GB Bug, which has the description "Game breaking bug, right away!" It adds a "glitchy" familiar that bounces around the room inflicting random status effects on enemies.
    • Delirium is meant to be completely immune to the Chaos Card. but when it gain multiple hitboxes, it becomes vulnerable. Which isn't supposed to happen and the game crashes. Time to hope you brought good items.
  • Gameplay and Story Integration: One of the new rooms is a bedroom where Isaac can sleep in a bed to recover health. In the center of the room, there is a rug. Blow it up with a bomb to reveal a hatch to the next floor or a crawlspace. You may want to rewatch the intro at this point.
  • Glass Cannon: Afterbirth added a literal Glass Cannon, which grants a powerful, reusable shot but also reduces Issac's health to just half a heart. A patch nerfed it hard so it takes away all three kinds of hearts.
  • Glass Weapon: In Afterbirth you can create the Glass Cannon, which is exactly what it sounds like, in every sense. It's incredibly powerful, but lowers your health to just 1/2 heart, meaning that you'll be killed in a single blow if something hits you.
  • Green Around the Gills: Picking up Ipecac gives Isaac this appearance, which is fitting since it's a emetic.
  • Guide Dang It!:
    • Unlocking the Lost would be a tall order for any one player to accomplish using only the resources made available in game, so much so that the process was ultimately figured out by some people data mining and spoiling it; this unsurprisingly prompted an upset response from creators Edmund McMillen and Tyrone Rodriguez, who noted it was meant to prompt an ARG-type response with the community working and speculating to solve it. The process was meant to be a community effort, which probably would have worked eventually, but it is still fairly obscure. You either need to spend a long time going in circles and following vague clues to figure this out, though it should be noted that knowing what to do isn't half of it. note  The unlock method has been significantly shortened (but made more luck-based) in Afterbirth: Die with any character in a Sacrifice Room, while holding the Missing Poster. The difficulty is now getting both items to appear in a run, since seeded runs do not work. However once you unlock them, you can use them in any run afterwards.
    • Two stats are not shown on the stats screen, or elaborated upon at all. You have "Evil / Faith", which is a sliding scale determining what kind of special deal (Deal With The Devil or Deal With The Angel) you get when you qualify for one, and Shot Height, which is used as an alternative to range in some items that claim to increase it (notably increasing Shot Height as Azazel is a bad thing and makes his range shorter). Afterbirth+ added a Found HUD option that displays the former among other stats, but not the latter.
    • Different damage upgrades are "worth more" than others. Furthermore, some simply add to your damage stat (Damage +1 for instance), while others give you a multiplier instead, or on top of, adding to it, which is significantly better most of the time. The highest of both types are provided by, respectively, Ipecac (+40 damage) and Tough Love (3.2 damage multiplier for each tooth.)
    • Special items are back! Not only was this completely unexpected due to Edmund outright saying they would be removed, they now work differently. Instead of the huge list of Special items from the original, there are now only fifteen: Polyphemus, Mutant Spider, Brimstone, Sacred Heart, Soy Milk (no longer Special in Afterbirth), Pyromaniac, Lil Brimstone, Ludovico Technique, Dr. Fetus, Epic Fetus, the D6, Mom's Knife, Stop Watch, the D100, and Godhead. What's more, the Special counter is changed based on sight, not pickup, meaning seeing some of the bad Special items will make it less likely you'll get good ones.
    • Lazarus' entire playstyle is bizarre. He starts with a negative Luck stat (-1), and greatly benefits from dying as early on in the run as possible, since the item he starts with (Lazarus' Rags) give him a damage bonus when he revives.
    • Unlocking the Blinding Baby requires you to use The Sun card via the Blank Card activated item. Either you get this one by accident or never figure it out on your own.
    • Getting to the Boss Rush and Blue Womb/Hush fight are not elaborated upon aside from one vague hint from the Rules Card ("THE WALLS WILL HARDEN OVER TIME. TIME IS THE ESSENCE") and a clock appearing on the "progress bar" — and the clock only applies for the former. Mom needs to be beaten before 20 in-game minutes, and It Lives! (Blue Womb is locked prior to Mom's Heart transforming in to It Lives!) needs to be beaten before 30. Players going in blind are likely to stumble upon the game's Bonus Boss by accident after a good run.
    • Afterbirth+ follows up Rebirth's secret character with its own: the Forgotten. While the mechanics of the Broken Shovel can easily be learned, the way it's obtained is not. The first boss must be beaten in under one minute, causing the first room in the floor to get a moving shadow of a shovel over it. Then that room has to be bombed. Nothing really hints at killing the first boss in just a minute (and because of the strict time limit, it's unlike to do that by accident unless an outstanding item shows up at the beginning), and the only clue about bombing the first room in the game is that after successfully killing the first boss, Mom's voice will be heard.
    • The Dice Rooms in Afterbirth have their effects determined by the number of pips on the floor, and can reroll things from your current load out to all the pedestal items on the floor to the floor itself. What are these effects, and which numbers are they assigned to? The game doesn't tell you. You'll have to find out by using them or looking it up. Downplayed in Afterbirth+ where the Dice Rooms now have markings that hint at what they do (for example, an image of a D4, D6, or Forget Me Now for the aforementioned three). Of course, if you're new to the game you probably won't know what those markings actually mean.
  • Hitbox Dissonance:
    • The wall-crawling spider type enemies have a damage hitbox that's extremely confusing, placed at least an inch away from the front of the actual model. Since this hasn't been fixed and the crawlers are often protected by a pit, this could be intentional to keep the game from becoming unwinnable by having extremely low range. Still, this can be frustrating when Brimstone and diagonal shots like tiny planet don't hurt them despite visibly touching them.
    • This can happen to Isaac himself through size-altering pills. "One Makes You Larger" does not increase the size of Isaac's hitbox, but it does increase his visual size. "One Makes You Small" does decrease Isaac's hitbox along with his size.
  • Instant Runes: The Black Powder in Afterbirth causes a trail of black powder to appear where you walk, which dissipates after a certain distance. Forming a circle with the powder forms a glowing red pentagram which damages any enemy that touches it.
  • Interesting Situation Duel: The "Challenges" give you certain combinations of items that can drastically alter the way you play. For example, the "When Life Gives You Lemons" challenge mentioned below gives you Lemon Mishap (a Joke Item) and lowers your stats, but also gives you the Nun's Habit, Nine-Volt and a single Lemon Party pill. Since your stats are so bad, you'll have to rely on leading enemies into the damaging puddle dropped by Lemon Mishap until you get better items (but due to the Habit and Nine-Volt, you can use the Lemon Mishap in almost every room, more often if you take damage).
  • Interface Screw: This game adds three new interfaces curses on top of Curse of the Lost (minimap disabled). Curse of the Unknown removes your health bar, forcing you to keep a mental tally. Curse of Darkness reduces the ambient lightning to almost nothing, though the player and most enemies glow. Curse of the Blind turns every pedestal item (activated and passive items) on the floor into a question mark, including Devil/Angel Room items and shop inventory. It'll make you think twice about picking something up.
  • Jack-of-All-Stats: Invoked by the Libra item, which actively balances your stats once you collect it. Can turn the player into a Master of None or Master of All, depending on what items it is mixed with.
  • Jump Scare: Ending 3 a.k.a. The Rubber Cement ending.
  • Kaizo Trap:
    • Mom's Heart now explodes into a radial bullet burst when killed. It's easy enough to dodge if you know it's coming, but a surprise nonetheless.
    • Devil Rooms in Afterbirth have a chance to contain enemies that you have to kill in order to leave.
    • Killing Ultra Greed turns him into a gold statue. If you're in Greedier Mode, this statue will begin to move, starting a Sequential Boss fight. On your first playthrough of Greedier, it waits a bit before starting, but on all runs after that the fight starts immediately.
  • Last Lousy Point:
    • Rebirth is much worse about this than the original, especially with Secret Room items. Since the Secret Room is less likely to spawn items combined with more possible variations, most players resort to exploiting the item pool mechanics to reroll Secret Room items from Beggar drops or mushroom spawns. Another annoying item is Steven, which is now impossible to find except by killing Steven in a main boss fight (already rare to begin with), and only has a 1 in 5 chance of spawning compared to Little Steven. Rerolling also doesn't work on Steven drops, so it has to drop normally.
    • Applies much less in Afterbirth: Greed Mode's restocking shops mechanic and expanded shop item pool means it is possible to see the secret room items there. As for Steven, it now drops in Golden Chests.
  • Lethal Joke Weapon: Because of the improved item synergies, many mediocre items can become the keystone to a weird yet highly effective setup. Bonus points if several if your items are bad but they become effective in tandem with each other.
    • Lemon Mishap and Pinking Shears, two mediocre-at-best items in Wrath of the Lamb, have been buffed to the point that they can singlehandedly kill bosses.
    • "A Card Against Humanity". You'd be surprised at how badly some bosses handle a room full of poop. Becomes even better if you can remotely attack through Ludovico Technique, ???'s Only Friend, or Robo-Baby 2.0. The poop itself dramatically increases the likelihood of finding a Petrified Poop trinket, which will in turn make all the remaining poop drop a lot of collectibles.
  • Logic Bomb: The ? Card is a card that copies the effect of your active item. The Blank Card is an activated item that copies the effect of your card. If you use either of the two while holding both, both get consumed and you logic-bomb your way into the I AM ERROR room.
  • Long Song, Short Scene: The Blue Womb/??? area gets a dang nice song, even though it is simply a single room before a boss fight.
  • Luck-Based Mission:
    • Eden is entirely built around this trope, starting with random stats and items. However, using them requires "tokens", which are acquired by defeating Mom. This is to prevent Save Scumming for a set of good items, though it's easy enough to earn a large surplus if you use other characters consistently.
    • "Dice Rooms" are rare rooms with a giant die on the floor. Stepping on it may re-roll Isaac's items, reroll the pedestal items on the current floor, reroll the pickup items on the current floor, restart the current floor, or several of the above at once. Using such a room can be the ultimate gamble.
  • The Load: Familiars have the potential to screw you over to an even greater degree than before, especially when "free-roaming" and "explosives" are dangerously mixed. Special mention goes to the BBF (essentially a friendly Boom Fly) which combines both and is liable to blow up in your face if the enemy it collides with happens to be near you.
  • Luckily, My Shield Will Protect Me: The Trinity Shield blocks all bullet attacks from Isaac's front, moving with Isaac.
  • Magikarp Power:
    • One of the new ideas added to the game is the possibility of crippling yourself by picking up a bad item early, Tiny Planet and Bob's Brain to name two, in the hopes that later in the run you'll pick up an item that goes complements them really well, a high attack speed or piercing shots for tiny planet making it better than the pretty inaccurate item it is normally. One could argue it's part of what makes the game more fun than before, just the pure number of items that you might pick up and hold on to in the hopes they work exponentially well with another item you find on the run.
    • The Void item added in Afterbirth+ has two properties. This first, which is moderately useful, allows it to destroy any passive items in a room and covert them into random stat boosts, with a chance for a slight range reduction. The second, which applies to this trope, allows it to destroy any spacebar items and permanently copy their effects. There is no limit to how many items can be absorbed this way, and all of them will trigger each time Void is used. This can include generating Soul Hearts, Black Hearts, Eternal Hearts, mapping a floor, using Blank Card and its combos, Placebo and its combos (most importantly, infinite charge), four-way Brimstone from Krampus, and so forth. The only things it can't copy are spacebar items that are consumed upon use.
    • Also in Afterbirth+, the Smelter item. It takes 6 rooms to charge, and each use removes Trinket(s) and permanently applies its effect to Isaac. If you happen to find Mom's Box or a Fortune Teller machine, you'll quickly become unstoppable.
  • Marathon Level: The new Very Definitely Final Dungeon, the Void, is this. It's as large as two XL Womb floors put together and has 5 to 8 boss rooms besides the Final Boss fight, one of which always contains another "final" boss. Good luck finding your way there if you get hit by Curse of the Lost.
  • Mr. Seahorse: Immaculate Conception and Cambion Conception make you progressively more pregnant as you fulfill their conditions to spawn a familiar. While the latter is one of Lilith's starting items and works with her thematically, you can pick them up as any character. Then there's also the fact that every character is meant to be Isaac anyway...
  • Murder-Suicide: Invoked by a new item in Afterbirth+, the Plan C. It instantly kills anything in the room (even final bosses)… and kills you shortly afterwards.
  • Mushroom Samba: The Wavy Cap item (known as Black Mushroom in Antibirth), added in Repentance. It's an active item that can be used multiple times in the same room, and each use increases your fire rate... at the cost of making the screen and sound progressively more distorted with each use. Enough uses will make the game look like an LSD trip. Watch it in action here.
  • Music Is Eighth Notes: The Dingle boss whistles to summon mooks. The audio is accompanied visually by a few eighth notes.

    N-Z 
  • Nerf: An inevitable result of re-balancing the game from Wrath of the Lamb, several items and one character (Cain) were weakened for balancing purposes. On the flipside, Samson received a much-needed buff.
    • The Bloody Lust mechanic change to powering from Samson taking hits instead of getting kills is a significant change of strategy. To compensate for this, Samson starts with more health than the original game and Bloody Lust applies its damage buffs for the entire floor rather than per room. The original effect of Bloody Lust is emulated with the Lusty Blood item.
    • An Afterbirth patch nerfed Lilith's Box of Friends and Azazel's Mini Brimstone laser's power and charge time. Another patch un-nerfed Azazel's damage, but made his charge time worse.
    • Afterbirth also nerfed Judas's Book of Belial to only increase the chance of getting a Devil/Angel Room when held instead of guaranteeing one whenever possible.
    • A patch for Afterbirth made a very controversial nerf to the Stopwatch. Instead of having it slow down every enemy permanently, it was changed to activate only after taking damage and having its effect only last for one room. Many people were visibly upset by this, and in a later patch, while it kept its flaws, it was changed to also activate if Holy Mantle's shield was broken, which at least still makes it a very useful item when playing as The Lost.
    • The Void and the Smelter have the power to absorb active items and trinkets, respectively, allow the player to gain the effects of the absorbed items permanently. At the time they were introduced, this included things that should be limited to one use, such as Diplopia, allowing players to do things such as endlessly duplicate and absorb items or gain practical immortality through the Missing Poster trinket. This was patched out in short order, limiting single-use items to a single use even if they were triggered by being absorbed.
  • New Game+: In Afterbirth+, if you defeat the Lamb as your Final Boss, you're given the option to do a "Victory Lap". You start from the first level with all your items intact. This can be done endlessly, and the game tracks how many times you repeat. Most achievements won't work in this mode (only the "U Broke It!" achievement can be earned on a Victory Lap). However, there is a twist. On the third lap, you are turned into the Lost, minus his Holy Mantle and D4, and shops no longer appear. The game also randomly takes away some of your items.
  • Nice Hat: Afterbirth introduces the item Host Hat, which puts a Host on Isaac's head and renders him immune to bombs. The item's in-game description is even "Nice hat!"
  • Nintendo Hard: A "hard" difficulty level has been added, which significantly dials up the amount of enemies and champions while giving you less powerful items. Afterbirth+ does this for Greed Mode, which is called Greedier Mode.
  • No Fair Cheating: Level seeds allow you to repeat a powerful run, but block any achievements. Special seed effects that make the game easier (such as making Isaac invincible or causing all enemies to become permanently charmed) will also block achievements. Similarly, every challenge run only allows you to unlock the associated achievement. Finally, the "Rerun" mode blocks any achievements, other than ones associated with it specifically.
  • No-Gear Level: In order to retrieve the second piece of the Knife in Repentance, you have to travel through an obstacle course with all your passive and active items nullified, your bombs disabled, and any held consumables removed. Once you get reach the end of the course and retrieve the piece, you have to traverse back through the course with more obstacles added and an invincible ghostly Mom constantly trying to ram you every ten seconds or so. The one thing you're allowed to keep is flight if the character innately possesses it, which makes the course a lot easier.
  • Not Completely Useless: Normally, Range upgrades are very underwhelming compared to damage or tears upgrades due to to most characters already having adequate range; Azazel inverts this by having high damage output right out of the gate but very low range. Downplayed in that Range upgrades only affect his mini-Brimstone to a small degree, but stacking more than one of them is still hugely helpful for the lategame bosses.
  • Not the Intended Use:
    • The key pieces from the Angels are supposed to be used to unlock the True Final Boss. However, if the player's objective is the regular boss of that floor, then these items can be rerolled with the D6, allowing for an extra Angel Room item. Players can also bet on getting Dad's Key later to sidestep the problem, but this is less likely.
    • Dead Cat (which gives you nine lives… at the cost of only having a single heart container for each life) can be used to purchase every Devil Room item for basically free: you pay for the Devil Room item with your last heart, you die, you get resurrected, you go back into the Devil Room, pick up the other Devil Room item, die again, get resurrected again, continue to the next floor with 7 lives remaining and two free items! If you're good at dodging, you probably don't need all nine lives anyway.
  • Obvious Rule Patch: Since the Keeper uses coins for health, any items that drop coins on taking damage are given a chance to not drop anything exclusively for him.
  • One-Hit Kill:
  • Pajama-Clad Hero: The PJs "make you feel cozy", which shows by healing Isaac to full health and giving him four extra soul hearts on top of that.
  • Reality-Breaking Paradox: The ? Card is a card that copies the effect of your active item. The Blank Card is an active item that copies the effect of your card. If you try to use them with each other, you get teleported to the I AM ERROR room, implying that the game just gave up on trying to figure out what happens.
  • Recurring Boss:
    • Depending on your choices and luck, you may fight The Fallen as a boss four times in a single run: once at a random level, once during the boss rush, once as a Double boss in the Womb, and once guarding Satan. Not counting the times it shows up as a regular room enemy and its slight recoloring Krampus.
    • Similarly, the Greed/Super Greed battle is almost guaranteed to replace one of the shop rooms or secret rooms you've striven to unlock. Bonus points if the two show up on consecutive floors. He's so common, that a trinket, Greed's Rib, added in Afterbirth, prevents any form of Greed from appearing.
    • There are four completely different Gurdy bosses for various stages in her life cycle, which can be fought in each of the four main areas of the game. You can potentially fight them all in one run.
    • In Greed mode, you are naturally quite likely to fight several copies of Greed and/or Super Greed on the penultimate level, then fight several more copies at the beginning of the final level. And then you fight Ultra Greed.
  • Red and Black and Evil All Over: The color scheme for Gehenna, the Mausoleum alt floor in Repentance, which looks like a Satanic torture dungeon.
  • Retraux: Rebirth's art style is intended to resemble a colorized Gameboy game. Additionally, there is an arcade filter available by using the B00BT00B seed code. Afterbirth adds a Retro Vision pill which pixelates the screen on and off for a minute or so.
  • Rock Me, Asmodeus!: Hericide, Satan's new boss theme, as well as the boss theme for Mega Satan.
  • Save Scumming: Intentionally discouraged with the increased item synergies in the hopes that players stop immediately restarting when they get a bad item early on. Then again, the seeding tools provide for a form of this if one gets particularly lucky with their game's generation and want to retry after dying partway through.
    • Also discouraged with Eden. Eden starts with random stats and items, so selecting Eden requires and consumes an Eden Token gained by beating end-game bosses. But if you're a good enough Isaac player, you'll have more Eden Tokens than you know what to do with.
    • By forcing the game to quit before the Game Over screen comes up, you can reload your last suspend save, but never collect Guppy's Collar while doing this. The game forces 1-up effects when you save scum and the Collar works by either reviving or killing you.
    • Afterbirth+ punishes repeated resets with the Mr. Resetti achievement: it unlocks half soul hearts for the next runs.
  • Schizophrenic Difficulty: The Void. All rooms here are taken from random floors, meaning that you can go from fighting eight Monstros to killing a couple of flies or vice versa at the drop of a hat.
  • Schmuck Bait: There's an unlockable card called Suicide King. Upside, it spawns a bunch of cool pickups. Downside? You die.
  • Secret Character:
    • A new one for this version. "The Lost", a One-Hit Point Wonder incapable of gaining any hearts and starts with flight. Unlocking him is a real Guide Dang It!: the old way was to die to Mullibooms as Isaac on the basement, then to your own bombs as Maggie in the caves, then to Mom as Judas, then to Satan stage 2 as Azazel, with no deaths in-between and in that order. In Afterbirth, you just have to kill yourself in the Sacrifice Room with the Missing Poster in your inventory.
    • Another one was added in Afterbirth: The Keeper. He uses coins for health and converts all hearts into blue flies. He can only have up to 2 coins worth of health at a time, yet while he can't gain any, he can still lose them. On top of that, he's slow and fires triple shots at reduced speed.
    • Repentance takes this Up to Eleven by having a Secret Alternate character for every character in the roster, including the Secret Characters listed above! To get a character's alternate, you need to use the Red Key or Crooked Key in a certain room in Home as that character.
  • Sequel Difficulty Spike:
    • Zig-Zagged. Rebirth adds a lot of new content while updating older items from the original, which makes things both more difficult and easier thanks to the new items but larger item pool. The shop gets a major revamp with a lot of very useful items to potentially buy. The main bosses are a straight example, all of them (barring Mom) having been given new moves and Bullet Hell abilities which make the fights twice as hard as they were in the original game.
    • The Afterbirth expansion adds new features to the level design such as cramped rooms and trap rooms. Multiple bosses at cramped rooms don't mix well. And certain trap rooms only give you a split second to react before a bomb explodes in your face. There are also trapped treasure rooms which might require you to use a bomb to reach the item.
    • Repentance reworked Hard Mode so that soul hearts are much rarer and shops are much more likely to be of smaller variants, even when fully upgraded. In any difficulty, items that were single-handed run-carriers back in Afterbirth+ and earlier have been nerfed to being average at best, in particular most sources of soul/black hearts have either had their given health toned down or removed. The alternate path pulls no punches and requires fast thinking to avoid getting hit. Edmund himself said that the reworked Hard Mode would be difficult even to seasoned veterans and suggested that they learn the game all over again.
  • Silliness Switch: Some special seeds you can type in have strange and sometimes hilarious effects. For example, FART SNDS turns all sound effects in the game into, you guessed it, fart sounds. Find a nickel? Pffrt. Open a door? Ffrrp. Take damage? Prpfff. Another good one is SLOW 4ME2, which increases or decreases the background music tempo depending on how fast you're moving. As of Afterbirth, inputting a special seed permanently unlocks its effect, and any number can be toggled in a new menu of Easter Eggs.
  • Solid Gold Poop: In Rebirth, any run-of-the-mill poop piles you find have a chance to be golden poop piles, which drop a bunch of coins or the Counterfit Coin trinket when destroyed. Having The Poop active item and Midas' Touch will let you spawn these every single room.
  • Speed Run Reward:
    • Beating Mom in under 20 minutes grants access to the special Boss Rush room. The little clock that's visible during each loading screen shows how much time is left until this room expires, and you can bring up the map to check your current time.
    • The Speed! challenge requires you to beat It Lives in 16 minutes, or every 10 seconds you take a half-heart of damage. On top of that, all enemies and their bullets move twice as fast.
    • In Afterbirth, defeating Mom's Heart/It Lives in under 30 minutes grants access to The Blue Womb and the Bonus Boss Hush.
    • In Afterbirth+, one achievement (and a new item) can only be unlocked by beating the Lamb in under 20 minutes.
  • Spread Shot: Where to start? Several enemies use these, and there are many items that allow the player to fire in spreads.
  • Standard Status Effects: The Iron Bar item adds a Concussion tear effect, which confuses enemies. The Charm tear effect causes enemies to actively seek out and damage other enemies, if they are capable of doing so (enemies stuck on a certain path, such as rolling guts, don't seek out other enemies); the charmed enemy can still damage you, however. The Fear tear effect causes enemies to slowly run away from you, and stops some enemies from firing at you. The Fire tear effect deals some brief continuous damage after hitting an enemy, and causes occasional fiery explosions. The Slow and Poison effects return from the original game. They're unchanged except affected enemies now have an icon above their heads.
  • Sticky Bomb: Explosivo gives you a chance to fire tears which stick to enemies and then explode.
  • Stone Wall: One of the new items, Punching Bag, gives you a friendly Mulligan that acts as a tank for you.
  • Stuck Items: You're still stuck with whatever is held in your use slot like in the first game until you find another usable item. However, it's thankfully averted with pills, trinkets and tarot cards, which can be dropped by holding down the map button (or the left Ctrl button on your keyboard). But even the map button cannot get rid of the Tick trinket, which can only be removed by swapping it with the Matchstick (or by absorbing it via Gulp! or Smelter).
  • Subdued Section: Nativitate has an eerie and soft intro in a style unknown to the rest of the soundtrack. Then it stops for a few seconds, and starts up again with a much heavier sound.
  • Suddenly Voiced: For most of the game's lifespan, Isaac was only able to make pained grunts when getting hit, while Mom's dialogue was limited to her angrily calling Isaac's name. Come the release of Repentance and its new story content, however, both of them are given fully-voiced lines.
  • Super Prototype:
    • Technology 0.5 is a far superior laser when compared to Technology and Technology 2. It doesn't replace your regular tears so is a direct firepower upgrade when you pick it up, doesn't have a damage multiplier reduction built into its stats and perfectly synergizes with your power-ups. It also randomly adds power-up effects. Its only downside is that it occasionally misfires, but you'll barely even notice it.
    • And then there's Technology 0, added in one of the first Booster Packs for Afterbirth †. Like Tech 0.5, it does not replace your tears with a laser, instead giving the effect of having all Tears generate a chain of electricity between each other. Coupled with spread shots and/or a high range stat, and it can pierce through most enemies in the game without a challenge.
  • Surprisingly Happy Ending: After a whole decade of Isaac ending up in progressively worse situations across the two games, culminating in an original final ending that confirms the entire game was hallucinated by him while suffocating inside a chest, Repentance switches gears by giving Isaac a definite victory by going back home to destroy a manifestation of Christian fundamentalism and subsequently the strongest demon in the game, the Beast. The new final ending reveals that the whole game was really a father-son bedtime story, and implies that Isaac goes on to make a much happier one. The ending is ambiguous (Edmund said after the expansion's release that it wasn't completely straightforward), but overall much more lighthearted than the previous final cutscene.
  • Suspicious Video-Game Generosity: The item pool got buffed again from Wrath of the Lamb by introducing items that confer benefits worth two separate items from "Vanilla" Isaac. You'll need those buffs for sure...
  • Talking Poo: The Dingle boss, and his minions (Squirts). Afterbirth adds Dangle and Brownie, and Repentance adds the Dross bosses: Clog, Turdlet and Colostomia.
  • Theme Naming: The Disc One Final Bosses and Rebirth's True Final Boss all have boss themes that end in cide. (Matricide, Ventricide, Hericide, and Infanticide. Averted with the fake True Final Bosses, who have uniquely named themes (Ascension and The Fallen Angel)
  • Thief Bag: In Afterbirth, money bags will occasionally spawn that drop a few bombs, coins, or keys when grabbed.
  • Too Awesome to Use:
    • The Friendly Ball allows you to "capture" an enemy and release it to fight with you until it dies, persisting between rooms and floors. However, since the item has a 4-room recharge, it can take at least eight rooms in order to successfully release an enemy you captured, and even then your new companion will most likely die after a couple rooms.
    • Mega Blast is an activated item which fires a massive Brimstone laser which does insane damage and persists between rooms and floors, lasting for fifteen seconds. However, not only does it have a 12-room recharge (the only item in the game to require that many rooms), but the battery pickup will not fully recharge the item for another go. Because of this, Mega Blast tends to be saved for endgame floors/bosses that players want to wipe out quickly, such as the insanely tanky Bonus Boss.
    • The Chaos Card can One-Hit Kill literally anything in the game. The catch, of course, is that it's a card and can therefore only be used once. Like Mega Blast above, the Chaos Card tends to be saved for endgame bosses such as said insanely tanky Bonus Boss. Unless, of course, the player gets a Blank Card…
    • Diplopia is an activated item that doubles every pickup and collectible item in the room. With the number of items in the game that stack with themselves, this can create some insane synergies. However, it's only good once, and you can't cheat by using the ? Card to duplicate its effect because doing so consumes Diplopia anyway for no additional benefit. It often comes down to a decision regarding whether to use it on the decent item you have in front of you, or to hold onto it in hopes of getting a better item to duplicate later on.
  • Took a Level in Badass: A number of characters, playable and otherwise:
    • Mom's Heart/It Lives was a rather straightforward Flunky Boss in the original game. This time around... let's just say that Bullet Hell veterans will feel right at home.
    • Isaac/??? likewise get a bullet hell buff and new attacks.
      • Isaac's third form is considerably easier in Rebirth — his light attacks are easy enough to avoid as you can guess the pattern of the safe zones, plus his bullet hell attack has a much shorter range, is easier to dodge, and is not homing. And he doesn't spawn angels in this phase.
    • Eve has more health and Whore of Babylon kicks in at one full heart instead of a half, allowing her to take advantage of the serious damage buff while still being able to endure a blow from most early game enemies, giving her a chance to build up a buffer of soul hearts.
    • Samson has much more health, increases damage done for every hit he takes, has much larger range, and keeps accrued bonuses across the entire floor instead of just the room.
    • Most characters receive an upgrade to their starting load out once certain conditions are met. Such as Cain starting with the Paper Clip (unlocks Gold Chest for free) or Eve starting with the Razor Blade (boost damage for the room at the cost of a heart, letter her trigger Whore of Babylon at will).
  • True Final Boss: Attempts always end after a certain boss, though unlocking more stuff and meeting certain requirements can result in the bar being raised on who is considered the final boss. It gets to the point where the very first playthrough (with Mom as the Final Boss) can almost be considered the game's tutorial.
    • However, with a bit of Sequence Breaking using a Devil Room that spawns after the Mom's Heart fight, which always leads to Sheol even if you haven't yet unlocked It Lives!, one can theoretically reduce this to eight runsexp.(spoilers!) . Still quite a few runs though.
    • In Afterbirth †, there is the True True Final Boss in the Truly Absolutely Final Dungeon, Delirium.
    • And in Repentance, The Beast takes the place as the True True TRUE Final Boss.
  • Unexpected Gameplay Change: Some challenges take away your tears completely and force you to fight with either attack familiars or a recharging spacebar item and maxed bombs.
  • Unique Enemy:
    • There's a blue-skinned variation of the Conjoined Fatty monster that has a very slight chance of appearing in the I AM ERROR room and the Chest. Despite what its appearance may imply, it's completely distinct from a champion variant and attacks with vicious homing Ipecac shots.
    • The bosses Hush and Ultra Greed spawn Gapers that are reskinned to fit the respective boss's motif. Hush also spawns uniquely-skinned Ring Flies that form more complex formations than the normal variety and reskinned Boils with a different attack pattern. All of them can be found at an incredibly rare rate within normal rooms.
    • Two Stone Grimace varieties are extremely rare and only appear in one floor. One is the Broken Gaping Maw, which alternates between trying to suck the player character in with more force than the normal Gaping Maws and not doing anything, found only in the Womb and neither of its alternate floors. The other is the Brimstone Grimace, which fires brimstone lasers and can only be found in Sheol. It is possible to play the game for a long time through several runs without even seeing either of them, until Afterbirth made the Brimstone Grimace more common and Booster Pack #5 of Afterbirth+ made the Broken Gaping Maw show up far more frequently.
    • The Cross Stoney, a Stoney that also shoots constantly in all directions, is exclusive to the April's Fool challenge in Afterbirth+.
  • Unwinnable by Mistake: The Blue Bomber challenge turns off your tears but gives you the Kamikaze spacebar item and Pyromaniac (immunity to bombs). Some enemies, however, are immune to Kamikaze, so you need bombs or another source of damage to kill them. If you have neither, you have to reset. Mercifully, a patch added Brother Bobby to the player's arsenal, which prevents the situation from being completely unwinnable.
  • Up to Eleven: Afterbirth has the XXXXXXXXL challenge, which is the "XL" level effect on steroids. All levels have a size which is roughly equal to an XL Womb level.
  • Variable Mix: If the room has a lot of enemies or there's a miniboss, an electric guitar harmony fades into the music.
  • Video Game Cruelty Punishment: As of Repentance Devil Beggars will always release hostile spiders if destroyed with a bomb. If that doesn't sound so bad, try bombing a Bomb Bum and see what happens.
  • Violation of Common Sense: With a hard cap of 12 hearts compared to the original having no cap at all, it's actually to your benefit to only have a few red hearts. Having full health means you will be unable to pick up soul hearts or the new black hearts (which deal full room damage when depleted, similar to the Death tarot card). This means runs in which a low number of health containers are found may actually be more viable. It also seems that soul hearts are a bit more common, and black hearts are more likely to pop up than you might expect for something so useful.
  • Western Zodiac: The 12 signs of the zodiac appear as upgrades for Isaac.
    • Aries increases speed and lets you deal damage to enemies by running at them.
    • Taurus slows the player down, but when in a room full of enemies you build up speed over time, eventually getting the My Little Unicorn effect.
    • Gemini gives Isaac a little buddy attached to him that attacks enemies when they get close.
    • Cancer gives 3 soul hearts and makes all damage reduced to 1/2 a heart when he takes damage in a room.
    • Leo allows the player to break rocks, mushrooms, skulls, etc. by stepping on them.
    • Virgo has a chance to make the player invincible after taking damage and makes all negative pills positive.
    • Libra balances the player's stats and gives 6 coins, bombs, and keys.
    • Scorpio gives the player's tears poison effects.
    • Sagittarius makes the player's tears penetrate enemies and increases speed.
    • Capricorn increases all stats.
    • Aquarius leaves a trail of tears behind the player, damaging enemies that step in it.
    • Pisces increases tear fire rate and the amount of knockback enemies take from tears.
    • Finally, the Zodiac item takes the effect of a random zodiac sign every floor.
  • Wham Episode:
    • The 15th and 16th ending. Respectively, Isaac is shown to have gone missing, with his mom searching for him, and Isaac turns into a demon!
    • Ending 17 and 18. The first confirms Isaac died trapped inside the chest. The latter is another take on his death: he's crushed by debris while trapped in some cave and his corpse becomes Greed.
    • Ending 20. His mother trying to kill him was imagined. Isaac killed himself by locking himself up in the chest because he blamed himself for his family falling apart, thinking that he was corrupted by sin.
  • Wham Line: "Are you sure this how you want this story to end, Issac? You're the one writing it, it doesn't have to end this way."
  • Wham Shot: Isaac's face when picking up the Torn Photo. This is the one item where Isaac's change in appearance is not Played for Laughs, and becomes particularly horrible considering what the Afterbirth endings reveal.
  • Winged Soul Flies Off at Death: The new death animation. And it's a top secret character, called "The Lost"!
  • A Winner Is You: Inserting coins into the Greed Donation Machine grants you a few unlocks for the first hundred or so. Originally, it stopped at around 109, but a patch allowed the player to set the donations towards 999, which does nothing but praise the player with fireworks at the end and award an achievement. But something major happens if you donate one more coin...
  • When Life Gives You Lemons...: The name of Challenge #12.
  • World of Symbolism: Isaac explores the same themes as most of Edmund McMillen's older games, such as Time Fcuk, but uses very different imagery to do so.
  • Wrap Around: The Continuum item grants this effect to your tears, causing them to pass through walls and emerge from the opposite side.
  • You Kill It, You Bought It: Implied to happen in the 16th ending, when Isaac kills Mega Satan, only to become a demon as satanic as him!
  • You Shouldn't Know This Already: After beating ??? or The Lamb for the first time, you unlock the Golden Door, which can be opened by bombing the Angel statues in Angel Rooms. However, if you try bombing one of those statues before beating the game, nothing happens.

Today I died. I was killed by this page on some wiki. (ugly)
I leave all that I own to my pet cat Guppy. (Pretty cool stuff!)
-XOXO Troper

Alternative Title(s): The Binding Of Isaac Rebirth

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