[[quoteright:350:http://static.tvtropes.org/pmwiki/pub/images/The_Binding_of_Isaac_game_224.jpg]]

->''"The Binding of Isaac is a [[NauseaFuel sick]], [[SurrealHorror disturbing]], [[CrapsackWorld dark]], [[ReligiousHorror blasphemous]], [[CrossesTheLineTwice offensive]] game... And {{holy shit|Quotient}}, do [[ArsonMurderAndAdmiration I LOVE it!]] (...) Its [[BlackComedy dark, twisted sense of humor]] and hardcore gameplay won't appeal to everyone, but [[DareToBeBadass it WILL appeal to people who are AWESOME!]]"''
-->-- '''[[http://www.youtube.com/watch?v=Eh7IecOXE34 Full Spectrum Reviews]]'''

''The Binding of Isaac'' is a hybrid {{Roguelike}}/[[VideoGame/TheLegendOfZelda Zelda-like]] Top-Down Shooter made by Creator/{{Edmund McMillen}} and Florian Himsl, released in September 2011.

The story follows [[MeaningfulName Isaac, a young boy]] who finds himself in possibly the worst situation imaginable. After his [[TheFundamentalist fanatical Christian]] mother begins [[HearingVoices hearing the voice]] of {{God}}, she removes all of his worldly possessions and [[NakedOnArrival 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 [[DungeonCrawling the basement]], where, still naked, he goes on the run as he fights off nightmares both physical and mental (using [[AbnormalAmmo his own tears]], no less), discovers secrets about his mother's past, and ultimately confronts her in an effort to survive.

An expansion, ''Wrath of the Lamb,'' came out in 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. [[http://www.youtube.com/watch?v=OXSRKzAzaQI It has a trailer.]] Be afraid.

An UpdatedRerelease, ''The Binding of Isaac: Rebirth'', which overhauls almost every aspect of game, has since been released on PC, Vita and PS4 on November 4, 2014. It has a rather creepy [[https://www.youtube.com/watch?v=OLtZW3xIix0 trailer]] (and an even [[UpToEleven creepier]] one [[https://www.youtube.com/watch?v=1aBb0M8HRCQ 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, and Xbox One consoles were also confirmed for a 2015 release.

''Rebirth'' is planned to get its own expansion later, known as ''Afterbirth''. Not only is it planned to come with the usual stuff (such as new items, bosses, and a new character), but it also features a brand-new game mode and even more alternative floors. Some of this new content are ideas [[AscendedFanon suggested by the community]]; this includes items, challenges, and other features.

There's also "The Binding of Isaac: Community Remix", a massive overhaul GameMod that adds multiple new items to the game (some from ''Rebirth'' leaks) as well as fixing descriptions, adding new music, and other such things.
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!!Tropes found throughout the series include:

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[[folder:#-F]]
* OneUp:
** The [[ExactlyWhatItSaysOnTheTin 1-Up]] gives an extra life without drawbacks.
** The Ankh gives you one extra life, but revives you as the LethalJokeCharacter.
** The Dead Cat gives you [[CatsHaveNineLives 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' Rags gives you the same ability as its namesake character, to immediately resurrect upon death in the same room with one heart, allowing you to potentially finish off whatever killed you instead of having to start the room over again. You also gain the Anemic item for the rest of the game. Resurrecting with Lazarus' Rags will also turn your character into Lazarus when it comes to completion and achievements.
* HundredPercentCompletion: 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".
* TwentyPercentMoreAwesome: The video announcement for the Halloween update states that the game is now '20% more evil'.
* AbnormalAmmo: Not just his tears, through upgrades Isaac can shoot: flies, blood, teeth, urine, chocolate milk, ''ghost'' tears, explosive ''vomit'' and more.
* AllJustADream: [[spoiler:The first time you defeat Mom, the victory narration is revealed to be just a story Isaac is drawing. She's still coming.]]
* AnAdventurerIsYou: Each of the characters is based on a stock character class.
* AndYourRewardIsClothes: Did you beat [[spoiler:Mom's heart]]? Congratulations, have a fez! ...What, you were expecting an ending?
* AntiFrustrationFeatures:
** 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. Unfortunately, this is not the case if the player gets Curse of the Labyrinth, which combines the first and second floors into one. Here, both treasure rooms will be locked. ''Rebirth'' fixes this by unlocking both, which makes that curse a benefit on the first floor.
** The NoDamageRun 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 go back 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.
** ''Rebirth'' is all about making things more clear/less annoying, particularly for items which were previously poorly explained (most of the time) and just telling you how to unlock the majority of the characters, etc. 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.
* ArtificialStupidity: The Meat Boy familiar will eat your enemies for breakfast... if only it didn't occasionally get stuck chasing after and ineffectually biting enemies that are invincible (for example, the Mask part of the Mask-Heart monster).
* AscendedExtra: Isaac's pet Guppy is only seen in Isaac's will and some items. If you get three of said items, [[spoiler:you turn into Guppy (giving you flight and the same fly-spawning ability as The Mulligan, if said ability was on steroids).]]
* AsteroidsMonster: Envy, Blastocyst, Fistula and Teratoma, and to a lesser extent the double flies.
* AttackDrone: Some of the of the powerups.
* AwesomeButImpractical:
** The [[VideoGame/MeatBoy Dr. Fetus]] powerup. Good luck trying to kill [[DemonicSpiders hosts]] with it. The Ipecac Syrup, added in the expansion, [[UpToEleven is even more Awesome But Impractical]] -- it launches the same green spit bombs as the green enemies do. It can blow open doors and poisons enemies, but on the other hand, it travels in an arc that can make it very difficult to work with, especially on faster enemies. It also explodes when traveling over a rock, [[DepthPerplexion even when it's visibly meters above it]], so it ''will'' blow up in your face. Of course, just like Brimstone, mastery of such items can make them DifficultButAwesome.
** Several rare items, particularly in combination with each other, would be awesome... except finding them all in the same playthrough is unlikely due to the randomized nature of the game.
** 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 he has 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. Normally, this wouldn't be a problem, but [[spoiler:since the Cathedral's boss fires a shot every time you hit it, you'll face BulletHell 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 IF: 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 [[NoSell does no damage]] against [[spoiler:Isaac and Blue Baby]].
** The Crying Detonator combo. Using Sad Bombs, Bob's Brain, Anarchic's Cookbook (or Tammy's Head), Rubber Cement and Ipecac leads to utter destruction. While it is far more powerful with the Anarchic's Cookbook, The mere fact that there are exploding bouncing tears everywhere, even [[HoistByHisOwnPetard you]] can get killed very easily. [[DifficultButAwesome Master this however]], and not even [[spoiler:[[TrueFinalBoss Mega Satan]]]] can stop you.
** [[Franchise/{{Pokemon}} The MissingNo Item]].
* AuthorAvatar: In an odd sort of way, Ultra Pride can be for both Edmund and Florian.
* TheBadGuyWins: [[spoiler:In the epilogue, Isaac didn't kill Mom. She's still coming.]]
** [[spoiler: In Rebirth, one interpretation of the secret 16th ending is that Isaac was the Antichrist all along, Mega-Satan successfully corrupted him, and Mom really was hearing the voice of God.]]
* BattleInTheCenterOfTheMind: [[spoiler:Every character in the game is Isaac. Try and work that one out.]]
* BiblicalMotifs: And a storyline, and a ''ton'' of references. Plus, Literature/TheBible itself appears as a collectible item, with the Book of Revelations as a separate item.
* BigBad: Who it is is really, really, really convoluted. Is it God, ordering Mom to kill Isaac? Is it Mom, who's just imagining things? Is it [[spoiler:Satan/Mega Satan, who's trying to awaken Isaac as the Anti-Christ and using Mom as his pawn?]] Is it some combination of the three?
* BlackComedy: The entire game is front-to-back, blacker-than-black Dead Baby Comedy. Power-ups include the severed body parts of beloved pets (which act as damage-ups, given that the game [[AbnormalAmmo weaponises tears]]), a wire coat hangar (a home-made abortion tool which amps up Isaac's tears), and various ([[MindScrew possibly]]) aborted or otherwise dead siblings of Isaac's including Brother Bobby, Sister Maggy, and [[NightmareFuel Harlequin Baby]] (who act as [[{{Familiar}} combat pets]]). Hell, there's even a SecretCharacter [[spoiler:in the form of a cyanotic baby corpse]].
* TheBlank: The angel statue that appears in Angel Rooms as an alternative to deals with the Devil. [[spoiler:And the real Angels contained within.]]
* BloodstainedGlassWindows: [[spoiler:The alternative final floor is a cathedral; be prepared for some intense fights in it.]]
* BloodyBowelsOfHell: The game leans heavily towards this over FireAndBrimstoneHell.
* BloodyMurder: Almost all enemies use weaponized blood to attack. With the correct upgrades, Isaac can too.
* BonusLevelOfHeaven: [[spoiler:The Cathedral.]]
* BodyHorror: 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, a walking CDC nightmare, cry blood, prop your eyes open with toothpicks, jam implants into your eyes, and so many more.
* BoringButPractical:
** Flight. It doesn't directly help you kill your enemies, but being airborne suddenly means that ''many'' dangers are a thing of the past, like chasms, spikes, creep, etc. It even has a combat benefit in many rooms: being able to [[OutsideTheBoxTactic fly above a rock or a hole that enemies can't reach means]] that you're effectively untouchable by melee-only enemies, [[CombatPragmatist 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.
** 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 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 Hemogoblins, can regenerate back from the point after they count as a kill for the purpose of this item. Like the above, 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 is more or less this. On one hand, its only effect is to give you soul hearts (and make the Harbingers appear). On the other hand, ''its effect gives you soul hearts'' and it gives you more chances of getting Meat Boy. Depending on how many levels you play through, you can easily get more soul hearts than you have room to display, provided you don't take too much damage.
** The Candle from ''Wrath of the Lamb'' is also more or less this. It shoots a small flame forward a relatively short distance, doing very decent damage to anything it passes through, enough to OneHitKill most normal enemies. It also has the shortest non-zero recharge rate of any activateable item in the game, i.e. a few seconds. You don't even have to clear the room first. This makes it EXTREMELY useful against anything large, armored, or slow/immobile. Oh, and it can damage [[spoiler:Satan's]] second form while it's still in [[spoiler:statue]] mode while you're fighting his first form. Not as impressive as the Doctor's Remote or the Necronomicon, but still extremely useful.
** The Bloody Penny trinket. It 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 [[HeartContainer eternal heart]] at the start of each floor) or The Polaroid (gives you an emergency protective forcefield as well as letting you into the VeryDefinitelyFinalDungeon), 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; Slot Machines sometimes give out bombs, keys, money, and hearts in exchange for money)
** Things don't get more BoringButPractical than the Yum Heart. 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 OneHitPointWonder desperate to find more hearts somewhere.
** Depending on your situation, the Box can be an absolute godsend. All it does is give you one of each consumable, a pill, a trinket, and a tarot card. 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 PowerupLetdown, in certain runs where the RandomNumberGod 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. If you were fine on consumables and/or needed that damage/hp upgrade because you're starting to fall behind, however... Hope you brought your D6.
** 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 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 then 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, the blue rocks that can produce Soul Hearts or other items...you get the picture. 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 CastFromMoney type objects, such as [[SmartBomb 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 [[CastFromHitPoints exchange life for money]], with a small chance at other prizes) and a Slot Machine (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, 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... except that the process is very, very tedious.
** Several effects, like [[ArmorPiercingAttack Piercing shots from the Cupid's Arrow]] and [[SpreadShot Triple shots from using Inner Eye]] are boring, but useful in most situations. the boring part leaving when [[MixAndMatchWeapon you get both]].
** How to Jump in ''Rebirth''. 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 BulletHell). And 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 in the game, 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, sometimes 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.
* BossBonanza: The [[TheVeryDefinitelyFinalDungeon Womb]] randomly has previous end-of-level bosses spawn in normal rooms. [[PlanetHeck Sheol]] and the [[BonusLevelOfHeaven Cathedral]] have a much higher chance of these encounters. The [[BrutalBonusLevel Chest]] has a boss in ''every single room''. And the same is true of its Rebirth-exclusive counterpart, the Dark Room.
* BossRush:
** Many of the bosses will also make encore appearances as {{Degraded Boss}}es; sometimes two at once. The challenge rooms become this in later levels.
** Taken up a notch with ''Wrath of the Lamb'''s "Curse of the Labyrinth." This RandomEvent basically ''combines'' two floors, meaning you'll have to fight two bosses one after the other. There's also special challenge rooms that only spawn bosses, even at earlier levels (and they spawn ''harder'' bosses in later levels); these rooms are normally accessible only if the player has one or half a heart left.
** Pushed even further with [[spoiler:The Chest, TheVeryDefinitelyFinalDungeon. To elaborate, this floor is made up entirely of rooms containing two bosses that must be fought at the same time, which definitely helps to solidify this game's NintendoHard status.]]
** And up a notch further in Rebirth: [[spoiler: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. Take heed though, pick up one and you have to fight almost every boss in the game coming in pairs with no escape but victory or death. Succeed and you get another item. There's also the Dark Room, which is just like the Chest.]]
* BreathWeapon: If enemies literally spitting flies at you doesn't count, then the SHOOP DA WHOOP item does. True to the meme it's named after, it allows Isaac to fire a single giant laser every two rooms.
* BrutalBonusLevel: [[spoiler:Sheol.]] ''Wrath of the Lamb'' also adds [[spoiler:The Cathedral]], and, if you [[spoiler:beat The Cathedral while carrying The Polaroid]], then [[spoiler:The Chest as well]]. ''Rebirth'' adds [[spoiler:the Dark Room, accessed by beating Sheol while carrying the Negative.]]
* BulletHell: Things can get quite ugly with several shooting enemies on the screen.
** In Rebirth, [[spoiler:Mom's heart]] has a firing pattern very reminiscent of traditional ''danmaku'' bosses, with slow projectiles that have wildly confusing trajectories. Then there's [[spoiler: Mega Satan, especially in his second form.]]
* TheCameo:
** One of the available powerups is a chunk of meat which, after collecting several, can be used to create a VideoGame/MeatBoy 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 ''VideoGame/TimeFcuk'').
** 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, [[spoiler:It Lives and ???]].
* CastFromHitPoints:
** The Devil Rooms, which exchange Max HP (or Soul Hearts) for items. There are also several items (such as Blood Rights and The Razor) that damage the player when used.
** Eve, who runs on this and CriticalStatusBuff.
** ''Wrath of the Lamb'' further builds upon this trope with spiked doors that lead to special red treasure chests, as well as sacrifice rooms and demonic beggars that reward the player for damaging himself.
** Several items, like the Dead Bird and Red Patch, will only activate if Isaac takes damage.
** And of course, the Kamikaze item. You can blow anything up, at any time...however, the bomb you use is ''stuck to you'', forcing you to take damage in the explosion.
* CartoonBomb: With a skull (or a trollface) on it.
* ChargedAttack: The chocolate milk enables a Hold-type attack. Brimstone is also hold-type. If you have both at the same time, they interact... oddly.
* CheckpointStarvation: Played straight in Vanilla, averted in Rebirth. In vanilla, 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.
* TheComputerIsALyingBastard: The item description for the item SMB Superfan reads "All stats up!". While the item does increase all of your other stats, it decreases speed.
** That is actually a bug; SMB Superfan was intended to bring all stats up. There are other items that do lie about what they do, though. For example, the toothpicks item says "tears up" when it's actually a "shot speed up" item. ''Rebirth'' fixes all of these items, or [[AscendedGlitch changes the description to fit the effect]].
* ColorCodedForYourConvenience: Savvy players know to avoid anything that is green. Not keeping your distance will result in taking an explosive projectile to the face.
* CompetitiveBalance: Each character is designed with this is mind.
** Isaac/The Warrior aka, JackOfAllStats: 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 [[spoiler:Mom's Heart]] as the secret character, Isaac obtains [[spoiler:The D6]] as a starting item. The item [[spoiler:allows Isaac to transform an item into another one,]] meaning Isaac can have a selection of ''4 items per floor.''
** Cain/The Thief aka, FragileSpeedster: 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, TheMedic/StoneWall: 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 SquishyWizard: Judas has low HP and decent 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 CriticalHitClass: Eve starts with low HP, but has Whore of Babylon, an item that boosts her speed, firing rate, and damage when she is at low health. Eve relies on this to be effective, and if she has a chance to obtain lots of spirit hearts and lower her red hearts, Eve can be a powerhouse that can trounce even the toughest of enemies.
** Samson[=/=]TheBerserker aka GlassCannon: 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. He's been buffed significantly in Rebirth, now that Bloody Lust has been changed to damage ''taken'', not dealt, along with giving him three regular hearts. This makes him far more viable now.
** [[spoiler:???/Blue Baby]]/JokeCharacter/LethalJokeCharacter: [[spoiler:???]] has a distinct disadvantage to anyone else: he has no Heart Containers, only spirit 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), [[spoiler:???]] can absolutely decimate any boss in the game, and sweep enemies to the side. Besides this, Devil Rooms cost 3 spirit hearts for everything, and HP Up's only give you more spirit hearts. ''Rebirth'' also gives him slightly more damage and speed than Isaac (but not to the extent of Cain or Judas).
** Azazel[=/=]The Demon aka GlassCannon / FragileSpeedster: 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 MasterOfNone: Rebirth character. Starting with two abilities that don't work well with each other, Lazarus is a rather niche character. His pill can be a huge saving grace early in the run, but his ability to come back to life once, Nine Lives style with only one heart, can mess you up ''bad.'' It's worth noting that he respawns ''inside the room he died'', oftentimes allowing him to defeat what killed him. Furthermore, upon reviving, his stats go up a bit, giving him more of a fighting chance.
** Eden[=/=]DittoFighter: Rebirth character, the genderless Eden starts with randomly assigned stats and a couple of items.
** [[spoiler:The Lost]]/HarderThanHard: Rebirth character. If you discover the complex sequence of events needed to unlock this, you deserve [[OneHitPointWonder your punishment]].
* ConfusionFu:
** This is the basic niche of the Dead Sea Scrolls. It gives the effect of a random spacebar item per charge.
** The 3 Dollar Bill gives your tears a random effect that changes in every room you visit.
** 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.
** The Rainbow Baby has a random effect for every individual tear.
** Eden in ''Rebirth'' starts with randomly generated stats and items.
* CrapsaccharineWorld: Cutesy ThickLineAnimation, adorable characters... and the [[NightmareFuel horrific monsters they have to beat to avoid becoming a red smear on the basement floor.]] WordOfGod says he [[IntendedTrope did this intentionally]] to [[http://www.eurogamer.net/articles/2012-06-29-the-binding-of-edmund-mcmillen make the game more palatable]].
* CreepyBasement: Hoo, boy.
* CriticalAnnoyance: 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 ([[CombatSadomasochist 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.
* CriticalExistenceFailure: Isaac is perfectly fine when at half a heart, besides [[PottyFailure peeing himself]] when entering a room. Dying causes him to ''explode into a mist of blood.''
* CriticalStatusBuff: The Whore of Babylon item gives a major buff when you're reduced to half a heart or have nothing but soul hearts.
* CursedWithAwesome:
** Literally with the Whore of Babylon item, noted above, which has the quote "What a whorrible night for 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.
* CyberneticsEatYourSoul: One of the items you can buy at the cost of heart containers in the Devil Room is Technology.
* CycleOfHurting: 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 ItemGet pose.
* DamnYouMuscleMemory: 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; E is "use item" or "interact with surroundings" most of the time, ''not'' [[OhCrap drop a bomb where you're standing]]. The main problem with all this is that the 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. Wait, why didn't my Book of Belial trigger? [[OhCrap Oh cra- BOOOOOOOM!]]
* DarkerAndEdgier: Compared to ''VideoGame/MeatBoy''. And that's [[FromBadToWorse saying something.]]
* DarkIsNotEvil: When you collect the Dark Bum, you get an ominous message saying that "he wants to take your life," which tends to have the player bracing themselves for a crippling PowerUpLetdown. When he starts stealing red hearts from the ground if you don't pick them up fast enough, it's enough to make you wail in frustration... until you realize that for every couple of red hearts he picks up, he spits out a Soul Heart. This is almost always an excellent trade, and it's enough to make you love Dark Bum more than Thomas Jefferson.
* DeadCharacterWalking: It was patched out quickly, but there was a glitch in 1.3 that could do this. if you had only half a heart left [[spoiler:aside from ???, who has no natural hearts]], and take 1 heart of damage, rather than dying like normal, you would instead act as if you had constant mercy invincibility. It was as if you had book of shadows on constantly; 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.
* DealWithTheDevil: 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.
* DeathOfAThousandCuts:
** 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 StandardStatusEffects 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 can be this, as it 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% damage, but you can literally fire dozens of tears per second.
* DegradedBoss: Many bosses can reappear somewhere as a "normal" enemy.
* DenialOfDiagonalAttack: While it is possible to influence the angle of your tears by moving strategically, other than that you'll have to make do with just the basic cardinal directions. Whipping shots becomes impossible with Technology and Spoon Bender, and similar items have no effect on it.
* DesperationAttack:
** The Whore of Babylon item, which makes the player much stronger when having only half a heart. Eve starts the game with this item. [[spoiler: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... [[TeleportingKeycardSquad but they're also challenge rooms that spawn bosses when the item is taken]]. Again, [[spoiler:??? and The Lost can always enter these rooms]]. Unfortunately, these rooms were massively {{Nerf}}ed 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.
* TheDevTeamThinksOfEverything:
** Cain wears an eye patch. He shoots tears only from one eye, and when he acquires an item that would go over his patched eye, it's replaced with an exclusive-to-him version.
** The Common Cold replaces your tears with poisonous mucus. If you find something that replaces your tears with something else (e.g. lasers), your attacks can still poison enemies, and most such items get an additional effect. Mom's Knife, for example, drips with green poison, and most lasers turn green.
* DifficultButAwesome:
** The Fetus in a Jar 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, and enemies (especially bosses) die REALLY easily. This is taken UpToEleven with the Ipecac Syrup in Wrath of the Lamb: Does more damage than bombs and poisons on hit, but its arcing path is very difficult to work with.
** Eve has shades of this with her OneWingedAngel form accessible with one of her starter items (see FragileSpeedster[=/=]GlassCannon below for more details).
** Distant Admiration fly. With proper skills, it can kill a boss relatively quickly.
** Mom's Knife. Incredible attack power, terrible range unless you charge it (and there's no indicator of how much you've charged it, so that's basically guesswork).
** The Brimstone item. It upgrades your damage, can hit multiple times per shot, and ignores all terrain. On the other hand, 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 [[ChargedAttack 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.
*** The 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 [[FinalBoss 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 AscendedGlitch, though requiring slightly different timing.
* DiscOneFinalDungeon: [[spoiler:The Depths 2. The Disc Two Final Dungeon, the [[WombLevel Womb 2]], was the original VeryDefinitelyFinalDungeon. A Halloween patch added Sheol, the new Disc Three Final Dungeon. ''Wrath of the Lamb'' introduces the Necropolis 2 as an alternate Disc One Final Dungeon, and Utero 2 as an alternate Disc Two Final Dungeon. It also introduces the XL versions of these levels, which combine stages 1 and 2. A later patch turns the Cathedral into an alternate Disc Three Final Dungeon. Finally, with the Wrath of the Lamb version 1.3 patch, [[BrutalBonusLevel The Chest]] gets added as the [[RunningGag Disc Four Final Dungeon]]. Rebirth firmly turns it UpToEleven with both a new, post-Sheol floor Chest equivalent called The Dark Room, with a new final boss, "The Lamb", as well as a secret final boss, "Mega-Satan".]]
* DoesThisRemindYouOfAnything: The whole game is one for Religion. It asks some very serious questions regarding Religion itself, and is partly based off of Edmund's fears of Religion when he was a child.
* DoubleUnlock: 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!
* DownerEnding: [[spoiler:The first ending has Isaac about to be abused by his mother again. Most subsequent endings imply Isaac was DrivenToSuicide after his family fell apart and his mother started abusing him.]]
* DownTheRabbitHole: The trapdoor in Isaac's bedroom floor leads down to much more than a simple basement.
* DungeonBypass: The [[Film/{{Inception}} "We Need to Go Deeper"]] shovel allows you to dig down to the next dungeon level without finding and beating the boss [[ShortCutsMakeLongDelays (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 [[spoiler:Sheol]] early.
* DummiedOut: 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. In the v1.5 retail version the pill is back.
* DynamicDifficulty: 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 4 Horsemen of the Apocalypse 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 [[ClassicVideoGameScrewYous 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.
* EarlyBirdBoss: 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 2 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 DeathOfAThousandCuts. If it shows up later, you're probably powerful enough to deal with it handily.
** In ''Rebirth'', The Fallen's HP is actually decreased on earlier floors, which makes things more fair.
* EarnYourBadEnding: The vanilla ending is pretty happy [[spoiler:until it's revealed that it was AllJustADream, and Mom's still coming for you]]. Most of the endings after that pretty much go downhill from there.
* EarnYourFun: More harder levels and gimmicky characters become unlocked and many of the more [[GameBreaker gamebreaking]] items in the game become available in the item pool once you beat the game in several ways, most ways being [[NintendoHard stupidly hard]]. Such as beating all the bosses on hard mode with only ''[[OneHitPointWonder one hit point]]''.
* EleventhHourSuperpower:
** Level 4 Meat Boy, who can only be obtained without bug-exploitation just before the start of the last level of the game. [[spoiler:The last level for your first ten playthroughs, at least...]] He will crush almost everything on that level with ease if you can stay out of the way for more than a few seconds.
** You're guaranteed some of these in the the very last bonus level, [[spoiler: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.
* EliteMooks: Each enemy type has a tougher, tinted / colored version that will almost always drop an item when killed. Different colors have different meanings, which are NOT consistent between enemies. An orange Knight won't act like Orange Chub. Bosses can also be colored as well.
** An [[https://www.youtube.com/watch?v=EGHrNaRzKKo upcoming update]] to the original game will feature even tougher white-colored enemies/bosses, which have several unique tweaks to their behavior that make them far more dangerous than usual. The upgraded Hollow shown in the first few seconds of the video should give you a good idea on how powerful these new foes will be. Fortunately, these enemies are said to only appear in a new "hard mode" the update also introduces.
* EnemiesWithDeath: You get to fight Death, one of the Riders.
* TheEndOrIsIt: [[spoiler: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'...]]
* EmptyRoomUntilTheTrap: 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 [[DegradedBoss degraded bosses]] in a row.
* EverythingTryingToKillYou
* EvolvingWeapon: The Cube of Meat, an item that only drops from The Harbinger bosses. Every cube you get increases how effective the powerup is. It also reminds one of [[VideoGame/MeatBoy a certain skinless boy]] after the second cube. ''Rebirth'' add the Ball of Bandages.
* EvolvingTitleScreen: The title screen changes as you reach various milestones in the game, like killing the FinalBoss or [[OneHundredPercentCompletion collecting every item]].
* ExplodingBarrels: Wooden barrel variety.
* FauxHorrific: 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.
* FeedItABomb: A useful strategy for dealing with Chub is to get it to charge you then drop a bomb in its path, causing it to eat it.
* FliesEqualsEvil: Flies are a very common enemy, and several other enemies ''vomit'' flies.
* FlipScreenScrolling: Not surprising for a game that takes cues from ''The Legend of Zelda''. Although ''Rebirth'' adds some bigger rooms that use traditional scrolling.
* FlunkyBoss: 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.
* {{Foreshadowing}}: While they usually just feature Isaac getting humilated in some way, one of Isaac's between-level nightmares shows him suffocating inside of a chest, [[spoiler:foreshadowing the Sheol ending and ???'s possible origins.]]
* FreezeFrameBonus: 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 [[spoiler:"GET IN THE BOX!" (possibly a ShoutOut to ''VideoGame/TimeFcuk'')]].
* FromBadToWorse: One of the ''rewards'' is making the game more difficult after beating it a certain number of times.
[[/folder]]

[[folder:G-L]]
* GainaxEnding: At first. All endings after the "Epilogue" except the tenth, Endings 1-9, are little more than item/character/achievement unlocks with no plot-relevance, and make no sense within the story. However, Ending 10 is a huge WhamEpisode, revealing that [[spoiler:Isaac has been DeadAllAlong, as he actually suffocated to death in the Chest as seen in one of the vignettes]], and Endings 11, 12, and 13 then explain the events leading up to that point, wrapping up the story.
** Rebirth's Ending 16 shows what happens to Isaac inside the chest. [[DownerEnding It isn't pretty.]] [[NightmareFuel Quite the opposite, really.]]
%% Please place all GameBreaker examples in the YMMV page.
* GameBreakingBug:
** One of the items introduced in the Halloween update, Brimstone, causes Isaac to shoot enormous streams of blood from his mouth. Sounds awesome? Depending on which upgrades you get or currently have, you may end up ''being unable to attack at all''. The boss of the level added at the same time as Brimstone was also initially immune to it due to a bug.
** Another glitch from 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 [[spoiler:and you miss out on fighting the TrueFinalBoss.]]
** A similar glitch to the above used to happen in ''Wrath of the Lamb''. The RandomEvent "Curse of the Labyrinth" combines two floors from the same tier into an "XL" version, causing the player to have to fight [[BossRush two bosses, one after the other.]] It is possible for the FinalBoss to be the first boss fought in the Depths XL (or its alternate, the [[spoiler:Necropolis]] XL). Normally, when the FinalBoss 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 he can use a teleportation item. A similar thing could happen with [[spoiler:Mom's Heart in the Womb/Utero]]. Thankfully, this bug was fixed a few days later.
** Not exactly game breaking, but there's the whole mess with ''Wrath of the Lamb'''s [[spoiler:[[TrueFinalBoss True True Final Boss.]]]] The boss screen shows [[spoiler:???'s image with the name "Daddy Long Legs".]] Previously, it was [[spoiler:an alternate version of Daddy Long Legs, labeled in the game's files as Triachnid (a MythologyGag to a previous game by the programmer)]]. Currently, it's [[spoiler:Blue Baby]]. This created a lot of EpilepticTrees.
** The v1.3 update 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 NoDamageRun 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 cannot 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.
* GetBackHereBoss: The second stage of The Mask Of Infamy (can only be hurt in the back, and it turns and zigzags very quickly). Also inverted with Lust and Super Lust, where ''you'' have to run away as fast as you can to avoid taking CollisionDamage.
* GhostButler: 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. [[SubvertedTrope Unless you have bombs, in this case you can blow doors open to force your way out.]] Although leaving rooms this way without eliminating every monster will respawn them all to their original numbers if you re-enter the room.
* GlowingEyes: Some powerups, such as Sacred Heart, cause this.
* GodIsEvil: [[spoiler:Subverted. Guess Isaac's Mom was just a nutjob after all. Or manipulated by Satan.]] Or [[spoiler:just like the biblical Binding of Isaac, it was a test of some sort. [[WildMassGuessing The question is who the test was for, but it's highly likely it was failed. Whether or not this was because Isaac's Mom was a nutjob or Isaac was either the anti-Christ or corrupted (as suggested by the ''Wrath of the Lamb'' ending) is anyone's guess.]]]]
** [[spoiler:Though if Ending 16 in ''Rebirth'' is any indication, Mom may have been making the right call...]]
* GoodWingsEvilWings: 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.
* [[GrossoutShow Grossout Game]]: This one runs the full gamut: piss, poop, vomit, blood, BodyHorror, fetuses, and so on...
* GuideDangIt: 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, detailed in that section.
* GuiltBasedGaming: If you try to quit the game, Isaac will ask you "Are you sure you want me to die?"
* HaveANiceDeath
--> Dear Diary, [[TheManyDeathsOfYou Today I died.]] I was killed by this thing [picture of monster] in some basement/cave/dark[[spoiler:/warm/cold]] place. I leave all that I own [items] to my pet cat Guppy. Goodbye cruel world -XOXO Isaac/Maggy/Cain/Judas/Samson/Eve[[spoiler:/Blue Baby aka ???]]/Azazel/Lazarus/[[spoiler:The Lost]]
** Although you can [[spoiler:transform into Guppy, said character]] does not sign the death will.
* HeartContainer: These can be found here and there, although only one of them resembles a heart. 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).
* HeartsAreHealth: Your typical straight example, more or less copying ''Franchise/TheLegendOfZelda'' exactly. Notably, there's also the <3 item, which is a HeartContainer that resembles an anatomical heart rather than a HeartSymbol.
* HitboxDissonance:
** The flies have a larger hitbox than their visible size. One one hand, this makes it easier to hit them. On the other hand, 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, however, 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 said over obstacles or enemies. Knowing this makes the Suicide King challenge in ''Rebirth'' somewhat easier.
* HoistByHisOwnPetard: Several bosses have attacks that end up screwing themselves over, especially those with explosives. Bear in mind that ''no'' enemy is immune to their own blasts and you can take full advantage of that.
* HolyHalo: With three different varieties! There's The Halo (which is exactly what it sounds like: 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, it's a glowing light behind Isaac's head, but it's a "spikier" light. That is, it's a many-pointed star of light instead of a bright haze). Also inverted, in that there's three varieties of demon horns as well: Loki's Horns (tiny black horns Isaac wears on his head), the Pentagram (larger horns that literally grow out of his head), and Brimstone (giant, curled black ram's horns that sprout out the sides of his head).
* InexplicableTreasureChests:
** Treasure chests are everywhere in the basement of the house. Even more inexplicably in [[spoiler: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''!
* InfinityPlusOneSword:
** 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 JokeCharacter, which is nothing short of an [[NintendoHard extremely painful ordeal.]] But if you manage to accomplish this mission, your reward will be: [[spoiler:[[MundaneMadeAwesome The all-mighty D6 dice!]] [[LuckManipulationMechanic An activated item capable of transmuting any collection related item(s) into other random item(s)]].]]
** Another one shows up in the Halloween Update. [[spoiler:Mom's Knife, which is absurdly powerful.]]
** The [[spoiler:D6]] was arguably topped by the [[spoiler:D20, which rerolls every type of item except for collection items]] ([[http://www.youtube.com/watch?v=0TzaRcJco0k&feature=em-uploademail as Northernlion could attest to]].)
*** ''Rebirth'' tops this with the [[spoiler:D100]]. Will the chain ever end?
** ''Rebirth'' adds plenty more unlockable, powerful items. But the big one has to be [[spoiler: Godhead]], which increases all tear-related stats, makes your tears homing, and gives them a damaging halo. It's unlocked by [[spoiler: beating all the end-game bosses (except Mega Satan) and Boss Rush on Hard with The Lost.]]
*** Beating [[spoiler: Mom's Heart]] on Hard with [[spoiler: The Lost]] also unlocks the Infinity Plus One [[CoOpMultiplayer 2-Player Familiar]]. [[spoiler: [[NighInvulnerability It's completely invincible.]]]]
*** Another example is the [[BulletTime 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.
* IncrediblyLamePun: "[[VideoGame/CastlevaniaIISimonsQuest What a WHORRIBLE night for a curse!]]"
* InterchangeableAntimatterKeys: In classic Zelda style, any key fits all locks, but disappears once used.
* InterfaceScrew: 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.
* InvincibilityPowerUp: The My Little Unicorn and the single-use Chariot tarot card both protect from and cause CollisionDamage. The Gamekid powerup turns the game into a short round of ''VideoGame/PacMan''. The Book of Shadows prevents the player from taking damage.
* IronicNurseryTune: After [[spoiler: beating "It Lives!" for the first time]], a distorted version of "Jesus Loves Me" replaces the normal credits music.
** [[spoiler: It's even creepier in the trailer for Rebirth]]
* ItemGet: 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...
* JackOfAllStats: Isaac starts as this.
** MasterOfNone: Before you unlock his item. See InfinityPlusOneSword.
* JokeCharacter: ???, unlocked by beating the true form of the game's TrueFinalBoss 10 times. He starts with the same stats as Isaac (not even Cain or Judas's improved offensive stats) except [[spoiler:all his hearts are soul hearts, which can't be healed like regular hearts can. He can't earn regular hearts, either; any health upgrades are converted into soul hearts.]] He also starts the game with [[spoiler:The Poop, which creates a pile of poop. His lack of heart containers prevent many items from doing anything.]]
** LethalJokeCharacter: [[spoiler:The soul heart mechanic of the character]] does lend itself to some amusing exploits, though.
*** Devil/Angel rooms are guaranteed if you don't take red heart damage on a floor unless one recently appeared, in which case the probabilty is reduced. That means [[spoiler:??? always gets the maximum Devil/Angel rooms lady lucky is willing to give, no matter the amount of damage it took. The powerful items found in these rooms give ??? a significant edge.]]
*** [[spoiler:Since Challenge Rooms check against regular hearts, he can enter them with any amount of health. The same goes for items which check against minimum health, meaning the Polaroid (temporary shield) and Whore of Babylon (CriticalStatusBuff) either activate every single time or are always on.]]
*** [[spoiler:Items that grant a temporary health bonus instead grant a permanent soul heart (the Liberty Cap can thus be tediously abused for functionally limitless health)]].
*** [[spoiler:The Wafer item reduces the cost of all Devil Room items to 1/2 a soul heart.]]
** In Rebirth, there's an even larger Joke Character in [[spoiler:The Lost]]. While playing as [[spoiler:The Lost]], you [[spoiler:start with no health and cannot pick up any health, meaning that you will always die in one hit, meaning you'll need a no damage run to win. You don't even start with anything particularly unique - you can fly, but so can the other character with Brimstone and actual health.]] The only thing he has going for him is [[spoiler:He gets items in devil deals for free.]]
* JokeItem: [[spoiler:The Poop, which just drops a pile of dung where you're standing and can be used once per room.]] Maybe it can be used for cover and maybe it can be used to block an enemy chasing you, but virtually any other item is more useful in any situation.
* KaizoTrap: 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.
* KingMook: Chub is the standout, but as of Wrath of the Lamb, the Mask of Infamy is for the Mask-Heart enemies.
* TheKrampus: Added as a boss in the Christmas update. The Krampus will randomly appear in the Devil Room (10% chance at first and 40% at later visits) and drop one of two unique items when defeated.
* LastDitchMove:
** Some enemies explode, while some others spawn bullets when killed. For bosses, Steven has a last-ditch move.
** [[spoiler:The Daddy Long Legs tries to crush you with its body as it dies.]]
** Due to a glitch, sometimes the effect of final bosses ''[[KaizoTrap dying]]'' can actually hurt you.
* LastLousyPoint: 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 has been removed from every item pool, meaning it's only attainable as a rare boss drop from Steven himself and cannot be rerolled from Little Steven.
* TheLegionsOfHell
* LethalJokeItem:
** The [[spoiler: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 [[TrueFinalBoss True Final Bosses]] in one use. Don't try to use it on any of the True Final Bosses added after the initial release, though. '''[[PressXToDie Just don't]]'''.
** By extension, the [[spoiler:Rosary]] has a fairly disappointing obvious effect (It just adds 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 [[spoiler:Bible]] more likely in shops, guaranteeing an easy final boss if you get it. Plus, when playing as [[spoiler:???]], it's a ''godsend.''
** The Dead Cat item makes it so you [[CatsHaveNineLives 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, or ''especially'' [[spoiler:??? and The Lost]]), it's basically free lives.
* LightIsNotGood: [[spoiler: 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.
* LivingApart: In later levels, enemies are encountered that are just living brains, rolling guts, or disembodied hearts.
* LogicBomb: In Ending 6, [[spoiler: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.]]
* LosingYourHead:
** 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. One of the better items.
** The Shears and the "The Hanged Man" Tarot card also do this, but only for one room. Transcendence, a noose, does it permanently. If Isaac has items that make him look demonic, on the other hand, [[WingedHumanoid he gets wings instead]].
** The Guillotine from ''Rebirth'' turns Isaac's head into a familiar. If you can get past the InterfaceScrew, the head not only protects the body from bullets but the item is also a stat boost.
* LuckBasedMission:
** 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. This includes 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. Basically, 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.
** [[UpToEleven Acquiring 4 Cubes of Meat!]] It 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. Also, it's not even a complete guarantee - even with the Book of Revelations, Conquest can replace Death on the Womb/Utero and give you the White Pony instead of a Cube of Meat. In Rebirth, it's even worse - the Harbingers now have an equal chance of dropping either a Cube or a Ball of Bandages, ''and'' there's a separate achievement and reward for getting four Balls!
** Bosses also contribute to the [[LuckBasedMission 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, etc.) and some an endless source of frustration (Gurdy for a slow-moving character, Monstro II for people who dislike close-range combat). And 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, [[UnstableEquilibrium the difficulty snowballs depending on how much]] the RandomNumberGod loves (or hates) you.
* LuckManipulationMechanic:
** 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'' adds the ability to reroll enemies, the player's own items, and everything at once.
** 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 also ups the chance of the player encountering one of the Harbingers (and acquiring a Cube of Meat 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, though.
* LuckStat: It starts at 0 and only affects two things: the chance 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). Somewhat ironically, negative luck actually increases the former in most cases. While the Lucky Foot item and Lucky Toe trinket both increase the chance of getting a drop after clearing a room, they do not actually increase the Luck stat.
** Luck has been fixed in ''Rebirth''. Lucky Foot and Lucky Toe actually affect the Luck stat, and Luck now affects certain items and slot machines.
* LudicrousGibs: The fate of every single enemy you destroy is being turned into these.
[[/folder]]

[[folder:M-R]]
* {{Macrogame}}: Each playthrough is different than the previous one, but as achievements are unlocked, more and better items begin appearing.
* MagikarpPower:
** 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, 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 spirit hearts. However, using it later in the game is benefecial 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. [[spoiler:Additionally, doing so will give you all benefits of the LethalJokeCharacter.]]
** Believe it or not, [[spoiler:???]]. Mom always drops The Polaroid when killed. Besides being required to access the [[DiscOneFinalDungeon Disc Two Final Dungeon]], the Polaroid casts a protective shield wherever you have 1/2 heart or less. [[spoiler:???]] ''always'' has 1/2 heart or less. That's right folks, the LethalJokeCharacter 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 NoDamageRun 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 VideoGame/MeatBoy (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.
* MalevolentArchitecture: 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.
* MarathonBoss: 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 [[spoiler:Isaac]] and [[spoiler:???]]. And then there's [[spoiler:Mega Satan]] of ''Rebirth'', who takes forever without the Chaos Card due to being a BossRush as well as a regular boss.
* MeaningfulName: The Wrath of the Lamb expansion's name foreshadows two of the bosses introduced in the game ([[spoiler:i.e., a MessianicArchetype version of our main character, and one of his alter egos]]).
** The titular Lamb itself [[spoiler: 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]]
* MementoMacGuffin: The Polaroid, which depicts Isaac's family before the madness started. Getting knocked to half a heart with it on gives Isaac [[ThePowerOfLove a protective shield.]] [[spoiler:Defeating the Cathedral's TrueFinalBoss while holding it takes you to a new level.]] ''Rebirth'' adds it's counterpart, an unrecognizable picture of Isaac's family called The Negative. Instead of a shield, [[CounterAttack it deals damage to all enemies when Isaac is brought to half a heart]], and upon beating [[spoiler: Sheol]] it brings Isaac [[spoiler:down to the Dark Room to fight the Lamb]].
* MoodDissonance: 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.
* MookMaker: Several of the bosses, though not all of them. Also one of the enemy types. [[spoiler:It Lives]] is a [[MookMaker Degraded Boss Maker]].
* MoneyForNothing: Once you reach [[spoiler:the Womb]], money has very little use.
* MythologyGag: Several of the bosses and items are references to other works by Edmund. Namely, [[VideoGame/MeatBoy the Cubes of Meat, Dr. Fetus and Mega Fetus, the Super Bandage, Larry Jr., C.H.A.D.]], [[VideoGame/TimeFcuk Steven and Li'l Steven]], blobs that look like VideoGame/{{Gish}}, Gish himself as a boss, and [[spoiler:Triachnid]] in ''Wrath of the Lamb''.
* MultipleEndings:
** Depths Ending: [[spoiler: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 a kitchen knife.]]
** Womb Endings: [[spoiler:Isaac opens a chest to find various items inside, culminating with what appears to be his own rotting corpse.]]
** Cathedral Ending: [[spoiler:Isaac looks in the mirror and realizes he is corrupted with sin. He then looks at his toy chest.]]
** Sheol Ending: [[spoiler:Isaac's [[SplitPersonality 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: [[spoiler:The collapse of Isaac's family is shown through a series of Polaroid photos. It is implied Isaac may have killed his sister by locking her in his toy chest, causing his father to leave and mother to start abusing him.]]
** Dark Room Ending: [[spoiler:Isaac's mother is shown wandering outside, searching for the now missing Isaac.]]
** Golden Door Ending: [[spoiler: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.]]
* MundaneMadeAwesome: After everything that just happened, Isaac gets rather surprisingly excited over the items in the endgame chest, including [[spoiler:a hat, a nail, a quarter, a fetus in a jar, a wafer, and some bling]].
* MyNameIsQuestionMarks: [[spoiler:The last unlockable character is simply called "???"]]
* NameOfCain: 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 EyepatchOfPower and his abilities make him resemble a "rogue" class.
* NightOfTheLivingMooks: There are various undead enemies Isaac must defeat. See also NonHumanUndead, below.
* NintendoHard: 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.\\
\\
And ''Wrath of the Lamb'' takes this [[UpToEleven even further!]] If you thought the vanilla version was merciless, you won't survive the new horrors you'll face here.
* NoCutsceneInventoryInertia: 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. [[spoiler:Although, considering the achievements imply most of the characters are Isaac wearing a PaperThinDisguise of some sort, it may be fitting after all.]]
* NoDamageRun:
** 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 [[spoiler:The Lost, who literally has no health and dies if hit by anything]].
* NoEnding: 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.
* NoOntologicalInertia: Averted; any flunkies spawned by a boss will not die when the boss does (except in the case of [[FinalBoss Mom]]) and must be killed in order to clear the boss room.
* NonHumanUndead: Remember those horrific monsters Isaac must defeat? They come back from the dead later.
* NonIndicativeDifficulty: 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.
* ObviousBeta:
** Let's be honest: [[http://bindingofisaac.wikia.com/wiki/Wrath_of_the_Lamb_Bugs the expansion was as glitchy as hell on release.]] Even the patches can create further bugs; version 1.15 prevented several players from unlocking Challenge 10 or Ending 12.
** [[http://www.gamasutra.com/view/feature/182380/postmortem_mcmillen_and_himsls_.php?page=3 Word of God]] says the release of the original game was this.
* OffingTheOffspring: Unsurprisingly, since the game was named after the original (the Choice of Abraham kind).
** [[spoiler:[[SubvertedTrope Subverted]] in the epilogue.]]
** [[SubvertedTrope Subverted]] and [[InvertedTrope inverted]] every time you beat Mom. Played straight if Mom beats you.
* OldMediaAreEvil: The plot and imagery are inspired from inflammatory Christian propaganda from TheEighties, such as the Jack Chick comics.
* OminousLatinChanting:
** "Enmity of the Dark Lord", the song that plays when you fight [[spoiler:Satan]] and [[http://www.youtube.com/watch?v=ba5vrK6xYNE "Lament of the Angel"]], the [[spoiler:Cathedral's]] background music generate this impression using gibberish. "[[https://www.youtube.com/watch?v=AdL1rF7_hfM Everlasting Hymn"]], ''Rebirth's'' Cathedral music, has real Latin.
* OneHitPointWonder:
** 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.
** [[spoiler:The Lost]] lives off this trope, [[spoiler:literally starting with no health and incapable of increasing it]]. Hope you brought your Holy Mantle.
* OtherworldlyAndSexuallyAmbiguous: The multiform Eden is neither male nor female.
* OuijaBoard: Serves as a power-up that gives Isaac ghost tears that move through objects.
* PaletteSwap: Enemies with a different color palette. They're stronger but drop items upon death.
* PaperThinDisguise: It's ''heavily'' implied that [[spoiler:the unlockable characters are just Isaac wearing these.]]
** WordOfGod angrily [[http://edmundm.com/post/97038438355/how-old-is-isaac-supposed-to-be-and-are-the-other confirms]].
* PhotoMontage: The 13th Ending shows various Polaroids of Isaac and his family's life before the events of the game.
* PlungerDetonator: The Remote Detonator item.
* PottyFailure: 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.
* {{Pun}}:
** For example, a certain pill makes Isaac shit bombs [[note]][[DontExplainTheJoke explosive diarrhea]][[/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 [[AnArmAndALeg Lucky Foot]] item.
* PowerupLetdown:
** 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 [[spoiler: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 UnwinnableByDesign. 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 (''LoC tears get stronger the longer they're in the air. Faster tears hit a wall, an enemy, or the ground faster, thus limiting the amount of damage they can build''). 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 [[spoiler:???]], 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.
** Sacrifice Rooms usually aren'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 [[spoiler: 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 BulletHell tactics, so having that extra orbital to block shots with might be more important.
* PurposefullyOverpowered: 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]]Some of them, for example, combine the effects of two already powerful pick-ups, in a single item![[/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.
** [[TheBerserker Samson]] in ''Wrath of the Lamb'', who is basically Judas [[MovesetClone with a slightly lower base attack.]] (He is skilled at [[SituationalSword fighting mobs of enemies]], but that's not much consolation if you have to fight one of the few bosses that doesn't [[FlunkyBoss 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''.
* PowerupMagnet: There's a [[Franchise/{{X-Men}} "Magneto"]] powerup which attracts powerups even through rocks and over gaps.
* ThePowerOfLove: [[spoiler:The Polaroid]], a picture of [[spoiler:what seems to be Isaac and family]], shields Isaac on his last hit point.
** ThePowerOfHate: ''Rebirth'' adds [[spoiler: The Negative, which is The Polaroid's dark counterpart.]] It damages all enemies when Isaac reaches his last hit point.
* PsychopathicManchild: Several bosses give this impression due to the goofy, yet deranged smiles they make when attacking.
* QuirkyMinibossSquad: The SevenDeadlySins are this, each one a personification of its namesake.
* RandomEvent: The Curses introduced in ''Wrath of the Lamb'', which are randomly activated at the beginning of a level. Curse of Darkness [[InterfaceScrew 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 [[BossRush two bosses]].
** ''Rebirth'' changes the Curses around and adds some new ones.
* RecursiveAmmo: The Parasite gives Isaac the ability to split his shots when they hit something.
* ResetButton: Forget Me Now, an item unlocked by [[spoiler: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.
* ReviveKillsZombie: [[spoiler:The Bible]], while normally dealing no damage, kills Mom [[spoiler:and Mom's Heart]] instantly. Inverted with [[spoiler:Satan, Isaac, and ???; using the Bible will instantly kill '''you''' instead]].
* RewardingVandalism: 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 [[spoiler:The Bean]], and blowing up enough rocks unlocks [[spoiler: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.
* RummageSaleReject: 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 BodyHorror.
[[/folder]]

[[folder:S-Z]]
* SaveScumming: Because the game is randomized, you can instead do this on startup to get preferential powerups instead of garbage (the latter being more likely).
* ScareChord: Played on certain endings and when you enter the Devil Room.
* SecretCharacter: [[spoiler:???, who is actually Dead Baby from one of Edmund Mcmillen's earlier works]].
* SelfDeprecation: The creators of the game appear as a rare variant of one of the Seven Deadly Sins. [[spoiler:Namely, the the butt-ugly Ultra Pride, due to a feeling of pride and satisfaction at finishing their game.]]
* SelfImposedChallenge: Pretty much the only reason why anyone would pick up the nine lives item (which reduces your hearts to only one and resets them to one every time you die) willingly outside of HundredPercentCompletion, the challenge of the same name, or [[spoiler:turning one of the characters into Guppy.]]
** The Challenge Runs in ''Rebirth'' are now even more challenging! Well, some of them. "Computer Savvy" gives you Technology 1 ''and'' Technology 2 right at the beginning (but no Item Rooms), so that's fairly easy, but challenges like "Beans!" or "Solar System" prevent you from firing any tears at all (rendering some pills, some tarot cards, and almost half of the game's items completely useless), forcing you to kill everything solely with specially-powered poop-bombs or orbiting attack-fly familiars.
* {{Sequential Boss}}es: Particularly [[spoiler:the [[TrueFinalBoss True Final Bosses]]. Special mention to Satan, the only boss with multiple health bars.]] ''Rebirth'' tops [[spoiler:Satan]] with [[spoiler:Mega Satan]], who, including all of the {{Degraded Boss}}es he summons, has ''twelve separate phases.''
* SequenceBreaking:
** It's possible to reach [[spoiler:Sheol]] without beating the game ten times either by using the "We Need To Go Deeper" item or taking the hatch in the ([[LuckBasedMission possible]]) Devil Room in the supposedly last stage. ''Rebirth'' allows you to use Angel Rooms to reach [[spoiler: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 [[spoiler:The Ankh]], an item that gives you an extra life, at the cost of [[spoiler: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.
* SerialEscalation: This game has issues with one-upping its own final dungeons every couple of patches.
* SetBonus: As noted in AscendedExtra, acquiring 3 out of 4 specific cat-related items[[note]]these being Guppy's Tail, Guppy's Head, Guppy's Paw and Dead Cat.[[/note]] in a single playthrough will [[spoiler:transform Isaac into his deceased cat, Guppy]]. Not only does it make you look [[LethalJokeCharacter hilarious]] / [[BadassAdorable adorable]], but it grants you [[spoiler:the abilities to [[IBelieveICanFly fly]] and [[MookMaker 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. [[PowerUpLetdown It's a bit of a letdown, though.]]
* SevenDeadlySins: 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''.
* SmartBomb: The Death tarot card, the Necronomicon, and one of the functions of the Sun tarot card.
* ShoutOut: See ShoutOut.TheBindingOfIsaac.
* SkeletonKey: Gives [[{{Cap}} 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 InterchangeableAntimatterKeys, 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 [[spoiler:Mega Satan]] in ''Rebirth''.
* SortingAlgorithmOfWeaponEffectiveness: For the most part, items unlocked later aren't any more powerful than those 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.
* SpikesOfDoom: These appear in later levels in some of the rooms.
* SpitefulAI: 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 [[TemptingFate 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.
* StealthPun:
** 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.
** 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 [[spoiler: Sheol]] takes you to the [[spoiler: 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!
* StandardStatusEffects: Paralysis, slowdown, poison. Interesting case, since only Isaac gets to inflict them on enemies (with the exception of the Death boss).
* TheStinger: [[spoiler:"Get in the [[VideoGame/TimeFcuk box]]!"]]
* StuckItems:
** 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; [[StealthPun 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 [[spoiler: the Polaroid, a trinket needed to access the VeryDefinitelyFinalDungeon]].
*** ''Rebirth'' finally allows the player to drop Trinkets...except for the Tick.
* SummonBiggerFish: Monstro's Tooth causes the boss Monstro to fall and stomp an enemy on screen for massive damage. [[CaptainObvious 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''. [[FridgeBrilliance Maybe because mom has]] [[CaptainObvious 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.
* SurrealHorror
* SuspiciousVideoGameGenerosity: The [[SerialEscalation truly, verily,]] most definite Final Dungeon as of [[RunningGag 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...
* TheSwarm: ''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..
* TacticalSuicideBoss: 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 [[spoiler:Mom's foot]] step on another part of [[spoiler:her body]], dealing a large amount of damage.
** [[spoiler: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.
* TakeThat:
** A big one against conservative Christianity, and [[OldMediaAreEvil its television and print propaganda.]]
** Subverted in the case of Judas' fez. People believed it was because Phil Fish, the creator of ''{{Fez}}'', traitorously voted against Edmund [=McMillen=] in the Independent Games Festival. [=McMillen=] went with this idea because he found it funny, but [[http://edmundm.com/post/85254517860/wasnt-judas-fez-because-of-phil-fish he actually just needed an iconic hat.]]
* TakeThatAudience: [[http://www.bindingofisaac.wikia.com/wiki/File:STOP_PLAYING.png The title screen]], after clearing the game [[AntiPoopSocking 200 times]].
* TakeYourTime: There is no time limit of any kind, and you can backtrack as much as you want within a floor.
* TearsOfFear: Isaac's default mode, given his crapsack childhood of abuse, harassment, and attempted filicide. When Isaac picks up a demonic upgrade, these frequently become BerserkerTears. Other upgrades or items give him TearsOfBlood. And when he gets the 3 Dollar Bill, he gets TearsOfJoy.
* TeethFlying: The [[http://bindingofisaac.wikia.com/wiki/Items#Tough_Love Tough Love]] item, appearing visually as [[AbusiveParents 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.
* TeleportingKeycardSquad: Taking an item or opening a chest in Challenge rooms triggers three waves of enemies.
* ThereIsNoKillLikeOverkill: 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 Breaker}}s in this series is so numerous [[GameBreaker/TheBindingOfIsaac it warranted its own page]].
* ToiletHumor
* TooAwesomeToUse: 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.
* TurnsRed: All four of the Harbingers, Gemini, Peep to a minor extent, Carrion Queen, and [[spoiler:Mom's Heart]]. Some are actually less difficult after turning red.
* TreasureIsBiggerInFiction: Coins in this game are pretty large.
* TrueFinalBoss: [[spoiler:Mom's Heart/It Lives]] in the original release, [[spoiler:Satan]] as of the Halloween update. [[spoiler:Isaac]][[note]]As an alternative fight to [[spoiler:Satan]][[/note]] in the original version of ''Wrath of the Lamb'', [[spoiler:???]][[note]]Fought after [[spoiler:Isaac]][[/note]] in the updated version. In ''Rebirth'', [[spoiler:The Lamb]], the boss of [[spoiler:the Dark Room]] is this at first, until the ''very definitively truest final boss of the game'', [[spoiler:Mega Satan]].
** Said to be one-upped in the new Rebirth expansion, Afterbirth.
* UndergroundMonkey: In both new model and palette swap flavor. Even a few bosses come in variants, some of which being {{Shout Out}}s to other games.
* UnstableEquilibrium: 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 spirit 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 spirit hearts either, since the rewards are rarely worth them. Overall, they're still a balancing factor, since spirit hearts are bound to pop up even on below average runs.
** Blood donation machines gives 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 also will replenishes 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.
* UnwinnableByMistake:
** 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 [[GameBreakingBug 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 SequentialBoss fight.
** ''Wrath Of The Lamb'' also adds a [[SelfImposedChallenge 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 [[UnwinnableByInsanity 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 [[spoiler: 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 [[spoiler: The Chest or Dark Room]] has a hole which resets the stage.
* TheVeryDefinitelyFinalDungeon: [[spoiler: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. It's boss is The Lamb.]]
* VirtualPaperDoll: Almost all of the upgrades Isaac can pick up will change the look of his sprite.
* TurnsRed: [[spoiler: Boss!Isaac (and Blue Baby)]] on their 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.
* VersusCharacterSplash: Happens before boss fights.
* ViolationOfCommonSense: Want God to notice your good deeds and reward you? ''Murder a beggar.'' [[note]]This is because Angel Rooms and Devil Rooms use the same formula to determine if they appear, and blowing up [=NPCs=] gives you a boost to your Devil Room chance. If there is an Angel Room on the floor, it will contribute to that chance instead.[[/note]]
* WholePlotReference: To [[Literature/TheBible Genesis chapter 22]].
* WombLevel: Appropriately titled The Womb, and its alternate, Utero.
* WorldOfSymbolism: 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).
** [[spoiler:The Polaroid]] might imply that [[spoiler:[[HiddenDepths Isaac's mom might not always have been the psychopath we know her as.]]]]
* XMeetsY: ''VideoGame/TheLegendOfZelda'' meets ''Super VideoGame/MeatBoy'' meets ''Literature/TheBible''... in {{Roguelike}} style!
[[/folder]]

!!Tropes exclusive to the remake include:

[[foldercontrol]]

[[folder:A-M]]
* AmbiguousGender: A new character, Eden, is gender-neutral.
* AnimationBump: The game is a slightly pixellated {{Retraux}} style. The endings, however, are very fluidly animated.
* AntiFrustrationFeatures:
** When you [[ItemGet 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 [[DamnYouMuscleMemory 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 BrutalBonusLevel run.
* TheArtifact: The starting room still displays the Move, Attack, Bomb, Item diagrams, but not the keybindings, which can now be changed on the menu.
* AscendedGlitch:
** 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.
* AscendedMeme: 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 [[ARareSentence other improbable phrases]] that only make sense in ''The Binding of Isaac''.
* BackFromTheDead: ''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.
* BittersweetEnding: [[spoiler: The "Missing" ending can be interpreted as this. While Isaac is without a home, he survives his mom's murder attempt, and, according to WordOfGod, he eventually becomes the protagonist of ''VideoGame/TimeFcuk'']].
* BlackMarket: 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).
* BlackSpeech: The incomprehensible, guttural chanting in the Sheol theme could count.
* BondageIsBad: 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.
* BossRush:
** 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).
** [[spoiler:Mega Satan summons all the Horseman, all the Super Sins and the Angels during the battle.]]
* BraggingRightsReward:
** There is no practical reason to beat [[spoiler:Mega Satan]] other than seeing the final ending. There's no unlocks for doing so, no reward for doing so, not even a mark on that character's boss kill list. All you get is a new picture for your save file. But if you can do it, you're definitely good (or very lucky).
** With the exception of [[spoiler: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 Level}}s is purely for sport.
* BulletHell: The battles against [[spoiler:Mom's Heart/It Lives, Isaac, ???, and Mega Satan]] offer (a mild example of) this.
* CastFromMoney: 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.
* ClassicCheatCode: Some seeds have interesting effects, such as BRWN SNKE ([[ToiletHumor causes Isaac to leave a trail of poop wherever he goes]]) or [=B00BT00B=], described below.
* ColorCodedForYourConvenience: Champion enemies have traits based on their suit of color, and all of them do one heart of damage rather than half. Grey means an enemy [[MadeOfExplodium explodes]] at death and drops a bomb, red means [[DamageSpongeBoss massive HP]] and possible regeneration with a heart drop, black drops a key, blue means it explodes into bullets on death, green leaves a trail of green creep and drops a pill, and pink generally means "less dangerous, but can still hurt you".
* CombinatorialExplosion: 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''...''[[RuleOfThree Brimstone]]''? (if combined with Tammy's Head).
* CoOpMultiplayer: 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 [[spoiler: Mom's Heart]] on Hard Mode.
* CounterAttack: Evil (black) hearts inflict the Necronomicon effect on all enemies in the room when a full black heart is depleted. In fact, ''Rebirth'' includes many items that damage enemies when Isaac takes damage. An example is The Negative, the new counterpart of The Polaroid.
* CursedWithAwesome: 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.
* DamnYouMuscleMemory: 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 HitboxDissonance.
* DarkerAndEdgier: 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 BlackComedy) atmosphere of the first.
* DarkIsNotEvil: 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 EnsembleDarkHorse, complete with [[https://twitter.com/darkbum Twitter page]].
* DeathDealer: Completely literally with the Chaos Card, which instantly kills ''anything'' it hits when thrown. It can even one-shot bosses!
* TheDevTeamThinksOfEverything:
** 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're at half a heart left and you enter a new room, [[PottyFailure 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.
* DifficultButAwesome:
** 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 UpToEleven 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 LeadTheTarget 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.
* DownerEnding: [[spoiler: Either Isaac goes missing, or is transformed into a demon while dying.]]
* EscortMission: 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.
* ExactWords: The Suicide King card's description is "A true ending." FromACertainPointOfView, it does just that, since [[spoiler: it drops several items then kills you.]]
* GameBreakingBug: There seems to be a game crashing bug that crashes the game when enough damage is done to [[spoiler: Mega Satan]] that they die without changing forms. It's rare, but it appears to be very consistent.
* GameplayAndStoryIntegration: 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. [[spoiler: 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.]]
* GuideDangIt: Unlocking [[spoiler: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 [[AlternateRealityGame ARG]]-type response with the community working and speculating to solve it. and 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 [[SelfImposedChallenge half]] [[MoonLogicPuzzle of]] [[NintendoHard it.]]
** 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'').
** 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.
** Lazarus' entire playstyle is bizarre. He's the only character to start 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 (The Lazarus Rags) give him a damage bonus when he revives.
* HitboxDissonance: 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.
* InterestingSituationDuel: 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 JokeItem) 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).
* InterfaceScrew: 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.
* JackOfAllStats: Invoked by the Libra item, which actively balances your stats once you collect it. Can turn the player into a MasterOfNone or MasterOfAll, depending on what items it is mixed with.
* JumpScare: [[spoiler:Ending 3 a.k.a. The Rubber Cement ending.]]
* KaizoTrap: [[spoiler: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.]]
* LastLousyPoint: ''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.
* LethalJokeWeapon: 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 [[AllYourPowersCombined 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 or ???'s Only Friend.
* LuckBasedMission:
** 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 SaveScumming a set of good items.
** "Dice Rooms" are rare rooms with [[ShapedLikeItself 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.
* TheLoad: 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.
* MagikarpPower: 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.
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[[folder: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.
* NintendoHard: A "hard" difficulty level has been added, which significantly dials up the amount of enemies and [[EliteMook champions]] while giving you less powerful items.
* NoFairCheating: Level seeds allow you to repeat a powerful run, but block any achievements. Similarly, challenge maps only allow you to unlock the associated achievement.
* NotTheIntendedUse: The [[spoiler:key pieces from the Angels]] are supposed to be used to unlock the TrueFinalBoss. 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.
** Also, 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 one-heart 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.
* OneHitKill: The Chaos Card kills almost anything in one hit. The only things that can survive it are [[spoiler:the first form of Satan, the first form of the TrueFinalBoss, Mega Satan]] and, likely due to a bug, Scolex.
* PowerUpLetdown:
** 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 SplashDamage. 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 [[spoiler:???]] wasn't worth the effort in the original? The crappy reward-to-effort ratio for [[spoiler: The Lost]], a genuine JokeCharacter, is even ''worse''.
** 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.
* PromotedToPlayable: 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.
* RecurringBoss:
** 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.
* {{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.
* SaveScumming: 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.
* SchmuckBait: There's an unlockable tarot card called [[spoiler:Suicide King.]] Upside, it spawns a bunch of cool pickups. Downside? [[spoiler:[[ExactlyWhatItSaysOnTheTin You die]] ]].
* SecretCharacter: A new one for this version. [[spoiler:"The Lost", a OneHitPointWonder incapable of gaining any hearts and starts with flight. You get him by dying to Mullibooms as Isaac on the basement, to your own bombs as Maggie in the caves, to Mom as Judas, and to Satan stage 2 as Azazel, with no deaths in-between and in that order.]]
* SequelDifficultySpike: 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 having been given new moves and BulletHell abilities which make the fights twice as hard as they were in the original game.
* SillinessSwitch: 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.
* SpeedRun:
** Now actively encouraged, since many features were added to help out the dedicated SpeedRun community. All runs have an internal timer attached to them (viewable by pulling up the map, which also displays the current level for screenshot purposes) and the game keeps track of your best times for fresh runs as each character (i.e. randomly generated, not from a seed). The main purpose of seeds was to allow for many people to play the same run and compare completion times, since without seeds the times were too often influenced by the RandomNumberGod. There are now leaderboards for the quickest runs for a particular daily seed and [[SeriousBusiness competitive speed-running]] may very well arise.
** Additionally, beating Mom in under 20 minutes grants access to the special BossRush 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.
* SpreadShot: Where to start? Several enemies use these, and there are ''many'' items that allow the player to fire in spreads.
* StandardStatusEffects: 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.
* StealthPun: The Latchkey gives you [[LockAndKeyPuzzle Luck and Keys]].
* {{Stonewall}}: One of the new items, Punching Bag, gives you a friendly Mulligan that acts as a tank for you.
* StuckItems: 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 is the one truly irremovable item in the game.
* TalkingPoo: The Dingle boss, and his minions (Squirts).
* ThemeNaming: The {{Disc One Final Boss}}es [[spoiler:and the True Final Boss]] all have boss themes that end in ''cide''. (''Matricide'', [[spoiler:''Ventricide'', ''Hericide'', and ''Infanticide''.]] Averted with the [[spoiler:fake]] {{True Final Boss}}es, who have uniquely named themes ([[spoiler:''Ascension'' and ''The Fallen Angel'']])
* SuspiciousVideoGameGenerosity: 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...
* TookALevelInBadass: A number of characters, playable and otherwise:
** Mom's Heart/It Lives was a rather straightforward FlunkyBoss in the original game. This time around... let's just say that ''danmaku'' BulletHell veterans will feel right at home.
** [[spoiler:Isaac/???]] likewise get a bullet hell buff and new attacks.
** 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 spirit 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.
* TrueFinalBoss: 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; for example, the very first run ends with Mom at stage six, though beating her a second time spawns the door to the next stage, where players have to fight the new final boss, [[spoiler:Mom's Heart]]. Assuming everything is done perfectly, you have to reach the credits ''eighteen'' times to unlock TheVeryDefinitelyFinalDungeon and face the [[spoiler:8]]th final boss.[[labelnote:exp.(spoilers!)]]First run ends at Mom, unlocks Womb; eleven runs to unlock It Lives! and Cathedral/Sheol access; five runs to kill Isaac/Satan to unlock The Chest/The Darkroom access; one run to kill either ???/The Lamb and spawn the Golden Gate.[[/labelnote]]
* UnexpectedGameplayChange: Some challenges take away your tears completely and force you to fight with either attack familiars or a recharging spacebar item and maxed bombs.
* VariableMix: In Rebirth, if the room has a lot of enemies or there's a miniboss, an electric guitar harmony fades into the music.
* ViolationOfCommonSense: 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 spirit 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 spirit hearts are a bit more common, and black hearts are more likely to pop up than you might expect for something so useful.
* WhamEpisode: The 15th and 16th ending. [[spoiler: Respectively, Isaac is shown to have gone missing, with his mom searching for him, and Isaac turns into a demon!]]
* WingedSoulFliesOffAtDeath: The new death animation. [[spoiler: And is a top secret character, called "The Lost"!]]
* WhenLifeGivesYouLemons: The name of Challenge #12.
* WorldOfSymbolism: *Isaac* explores the same themes as most of Edmund [=McMillen=]'s older games, such as ''VideoGame/TimeFcuk'', but uses very different imagery to do so.
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