Characters / The Binding of Isaac

A list of characters who appear in The Binding of Isaac. Currently under construction, please contribute!

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Plot-relevant characters

    Isaac 

Isaac

http://static.tvtropes.org/pmwiki/pub/images/isaacopening.png
A little boy who flees into the basement of his house to escape from a mother who wants to kill him, and must fight for his life against a never-ending assortment of demons and mutants. Beating the game in certain ways or collecting certain items unlocks variations of Isaac with their own unique attributes.

For tropes related to Isaac and his alter-egos in gameplay, scroll down to the section on player characters. For tropes related to Isaac as the boss enemy of the Cathedral, scroll down to the section on major bosses.
  • Break the Cutie: The entire premise of the game is one for him.
  • Children Forced to Kill: Isaac has to fight and kill plenty of enemies just to survive, up to and including his own religiously psychotic mother. Or so it seems.
  • Create Your Own Villain: Well, antagonist. Isaac would never have been forced to kill his own mother if she hadn't snapped and tried to sacrifice him in the first place. This gains some very interesting implications at the true ending of Rebirth, where it's revealed that Isaac becomes a demon or the Anti Christ inside the Chest. Did Isaac have darkness inside him all along, or did he decide to become evil after all the horror that he went through?
  • Crossdresser: A few between-level cutscenes show him wearing a dress or a wig, not to mention the fact that all female "player characters" are him crossdressing.
  • Dark and Troubled Past: The horrible memories of things that Isaac suffered through are relived in level loading screens. In one ending, the photograph montage of Isaac's past leaves a lot of interpretation as to what happened to his family, namely what happened to his father (who likely left, went missing, or died).
  • Dead All Along: After beating It Lives for the tenth time and unlocking Ending 10, Isaac comes across his corpse stuffed into a chest. At least one interpretation of this involves Isaac having been dead the whole time. The next three endings explore the events leading up to his death in reverse chronological order, first showing him willingly locking himself in the chest, then showing his self-loathing which led to this, and finally showing his tragic backstory through a heartbreaking Photo Montage. After beating Hush in Afterbirth, if you end your run there, not going on to Cathedral or Sheol, you get Ending 17, which outright confirms it, as Mom opens the Chest to find Isaac's skeletal remains.
    • Ending 20, the true ending to the game, reveals that he has long since been Driven to Suicide and that none of the game is even happening.
  • Driven to Suicide: Isaac hangs himself upon finding Transcendence (a noose) in Ending 2. This is Played for Drama in the final few endings: After we see that Isaac has been Dead All Along in the tenth ending, we are shown him willingly entering the chest himself in Endings 12 and 13, and the twentieth ending reveals that it was due to his self-loathing as he sees himself as a demon after his family breaks apart.
  • Dying Dream: Ending 20 reveals that the whole game is this, including the thought of his mother attempting to kill him.
  • Enemy Without: There are four variations of Isaac that serve as bosses: It Lives, the boss of Womb II confirmed to be Isaac as an embryo; normal Issac, the boss of Cathedral; Hush, the boss of the Blue Womb and a suffocating version of Isaac; and finally ???, boss of the Chest and Isaac after suffocating to death.
  • Full-Frontal Assault: Unless an item is picked up, Isaac is battling the monsters of the basement stark naked.
  • Hilariously Abusive Childhood: Many of the level intermission vignettes have Isaac getting picked on by kids, being dressed in drag, getting shit as a birthday gift, realizing he has no toilet paper while he's on the toilet, and the list goes on. And that's not even speaking of some of the items — the Wooden Spoon gives you scars, implying Isaac was beaten with it by his mom, the Belt is self-explanatory, and many of the food items aren't exactly fit for human consumption.
  • It's All My Fault: The ending you get for beating the Chest seems to imply that the cause of Isaac's self-loathing is because he blames himself for his father leaving. His mother seems to blame him as well. Ending 20 eventually reveals that he does blame himself, and this is what ultimately causes him to be Driven to Suicide.
  • Kindhearted Cat Lover: Whenever the player gets a Game Over, Isaac usually states in the diary that he will give what he collected to his pet cat, Guppy.
  • Madness Mantra: BAD BAD BAD BAD I'M THE DEVIL...
  • Sanity Slippage: Isaac is slowly going insane the longer he is in the chest, as shown by the final level and boss being amalgams of previous levels and bosses.
  • Then Let Me Be Evil: It is strongly implied that at the ending of Rebirth, Isaac becomes a demon or The Antichrist in order to deal with all the horror he went through, up to and including his mother.
  • Troubled Fetal Position: Isaac's signature pose, which all but cements the kid as The Woobie.
  • Walking Spoiler: Due to the endings revealing more and more of what is actually going on between him and his family, it's only a given.

    Mom 

Mom

http://static.tvtropes.org/pmwiki/pub/images/momopening.png
Debut: The Binding of Isaac

Isaac's mother. A devoutly religious woman who, in a fit of rage/insanity/fanaticism/whatever you think it is, she tries to kill Isaac, forcing him to flee to the basement.

For tropes relating to Mom as a boss enemy, scroll down to the section on major bosses.
  • Abusive Parent: Takes away all her son's stuff, locks him in his room, and then tries to kill him. Convinced by God to do so or not, that's all the evidence you need. Due to the true ending revealing that this turns out to be a possible subversion, depending on whether you see the between-level cutscenes are canon or not.
  • The Bad Guy Wins: It's revealed that Isaac suffocated to death while inside the chest. She wanted Isaac to die because of an (alleged) order from God, do the math. However, subsequent endings reveal that Isaac locked himself in the Chest before the events of the game, and Afterbirth+ reveals that she was never intending to kill him, and that Isaac saw himself as a demon and the cause of his family breaking apart, making this a cruel subversion
  • Big Bad: Ultimately subverted.
  • The Extremist Was Right: She's a religious fanatic that appears to think that she's on a mission from God, which is clearly not the case. At least until the end of Rebirth, strongly implying that Isaac either was or now is a demon. If you subscribe to the "just batshit insane" theory, then she's Right for the Wrong Reasons.
    • Subverted as of Ending 20: Isaac made up the story, possibly to explain why she was acting so abusive without taking the blame off of himself.
  • The Fundamentalist: She's heavily Christian, and her hearing the voice of God is what kicks off the plot.
  • Hidden Depths:
    • Ending 13 note  reveals that she used to be completely sane, and the caring and loving mother of a fully-functional family. However, as the family fell apart, so did her sanity.
    • Endings 15 and 17 suggest that in the end, she might be at least somewhat concerned about her son's well-being.
    • The final ending, 20, confirms that Mom is nowhere near the psycho that Isaac's brain made her out to be, and never intended to kill him at all. She's seen weeping in front of the television after his father leaves the family.
  • Knife Nut: She's holding her knife in almost every cutscene she appears in, and you can unlock said knife as a usable weapon in-game.
  • Maybe Magic, Maybe Mundane: Is she really hearing the voice of God, or is she just batshit crazy? The best we get from McMillen is him stating:
    “It was never implied that [Mom's] insane. And if you’re going by the Bible and you believe all the stories, then why wouldn’t you think that it’s possible now for that stuff to happen?
  • No Name Given: Unlike Isaac, even her first name is unknown.
  • Offing the Offspring: Her ultimate goal in Isaac's Dying Dream.
  • One Bad Mother: Bonus points for actually being the main character's mother.

Playable Characters

    In General 
  • Acrofatic: Several items make your character fatter, and most don't negatively impact speed. Combine these with items that increase your speed and you have this trope.
  • Amazing Technicolor Population: Stat-boosting pills change the character's color (Green for Luck, red for Health, etc), as do a few of the items. Most demon-related items turn the character into a black-skinned demonic hybrid, and Anti-Gravity turns them blue, for two examples.
  • Badass Adorable: He's still a child at the end of the day.
  • Berserker Tears: Unlike most examples, he fights by crying.
  • Blessed with Suck: Guppy's Tail makes you find golden treasure chests more often. This comes at the expense of keys needed to open those chests, though if you're lucky, the chests cough them up. Works wonders with a Skeleton Key or a Flat Penny. This is made potentially worse or better with The Left Hand item that changes all chests to Red Chests. They don't require a key to open, but half or more of the things that come out of a red chest are not something you want (including enemies and troll bombs).
  • Blonde, Brunette, Redhead: The three female playable characters: Magdalene (blonde), Eve (brunette), and Lilith (redhead).
  • Body Horror: Most of the powerups mutilate the character in some horrific way.
  • Bring My Brown Pants: Entering a room with half a heart makes the player leave yellow urine stains on the floor (unless playing as Eve, whose stains are red instead).
  • Bullet Hell: It's possible to get a character that shoots a four-way bullet spread at the speed of a machine gun with additional single shots from the other three cardinal directions.
  • Cartoon Bomb: The bombs used by playable characters are round, have a fuse, and are marked with a skull.
  • Collision Damage: Isaac takes damage from pretty much everything that moves. The player can cause this if they use My Little Unicorn (or the Chariot tarot card) or the Virus powerup. The former is temporary and provides invincibility as well, while the latter is permanent but doesn't prevent damage in return.
  • Dark Is Not Evil: An Isaac with a lot of Devil items is still just looking to survive.
  • Deal with the Devil: The player can trade heart containers to Satan for demonic items.
  • Detachment Combat:
    • When using The Shears or Transcendence (the latter is permanent), Isaac floats around as a severed head. The former also turns his body into a secondary attacker, but only works for the current room.
    • The Peeper pops out one of your eyes, which bounces around the room, hurting everything it touches.
  • Did You Just Punch Out Cthulhu?: Isaac can tear apart any number of bosses over the course of the game, including Satan.
  • Empowered Badass Normal: No matter which version you choose, you'll inevitably turn him into a supernatural being in the process of beating the game.
  • Eye Scream: Some items cause this.
  • Fantasy Character Classes: The characters are based on archetypal RPG classes (with the exception of ??? and Lazarus).
    • The Hero/Balanced/Fighter: Isaac.
    • The Cleric: Maggie.
    • The Druid: Eve.
    • The Thief: Cain.
    • The Mage: Judas.
    • The Berserker: Samson.
    • The Beastmaster: Lilith.
  • Faustian Rebellion: The items Satan sells you at the cost of your maximum health often end up making beating him a lot easier.
  • Flash of Pain: His sprite blinks when he takes damage.
  • Flight: Plenty of items, both permanent and temporary. You can become an angel, a demon, an angelic ghost, a demonic ghost, a floating head, and a cat (which somehow flies). All of these let you pass through rocks and over pits.
    • Becoming possessed by Guppy (the flying cat form) allows you to fly because why couldn't a possibly demonic cat possession also allow you to fly. Also, it's a kitty cat.
  • Full-Frontal Assault: Before the events of the game, his clothes were taken away from him as part of Mom's attempts to remove the evil around him. In most playthroughs, he fights his way through monsters naked.
  • Good Wings, Evil Wings: Both are possible — Fate or Holy Grail for the "good" variant, Lord of the Pit for the "evil" one.
  • Hammerspace: It's not specified where all the coins, keys, and bombs are stored.
  • He Who Fights Monsters: As he fights through deformed monsters and demons, he will end up getting a lot of things that make him just like those deformed monsters and demons.
  • Improbable Weapon User: Mainly his own tears. With appropriate items, he can upgrade to chocolate milk, urine, and explosive vomit, among other things.
  • Ironic Name: "Isaac" means "he laughed" or "he who laughs." Not much laughing occurs. Quite the opposite, really. invoked
  • Item Get!: Holds up new items — even in battle. You're not invincible when that happens, either.
  • Kindhearted Cat Lover:
    • Isaac seems to adore Guppy the cat the most out of his (deceased) pets.
    • He seemed to also like Max (Cricket in Rebirth) second most out of the pets, or possibly equally to Guppy, because Max's head makes him cry hardest of all items.
    • There is also Tammy, who, although she doesn't evoke as strong a response as Guppy, will still make Isaac cry more than normal, translating into a 360 shot.
  • Meaningful Name: The names of all of the playable characters relate to Bible stories, with Isaac's name relating to the Binding of Isaac story the game is based on.
  • No Name Given: His surname is never known.
  • Ocular Gushers: Most playable characters have a constant stream of tears running down their faces. After all, they are Isaac's main weapon.
  • Onion Tears: Once you find the Sad Onion powerup.
  • Paper-Thin Disguise: It is strongly implied in-game (and confirmed by Word of God) that all the characters are just Isaac putting on a wig, eyepatch, hat, etc.
  • Secret Character: There are numerous transformations available, which are activated by collecting three items of a particular set (flies, cats, etc.), after which the transformation effects are permanent for that run. The three items only need to have been held once, even if replaced (for spacebar items) or rerolled (for passive ones). The original had only Guppy, while Rebirth added Lord of the Flies. Afterbirth added eight more. Afterbirth+ added a few more.
    • Guppy, activated by picking up three Guppy items, transforms the player character into Isaac's pet cat. Doing so unlocks a new item (the first time), gives you flight, and applies a buffed version of the Mulligan that spawns a fly upon every hit.
    • Lord of the Flies/Beelzebub, activated by picking up three fly-related items, turns the player into a humanoid fly, grants flight, and turns all regular flies into kamikaze flies.
    • Seraphim, activated by picking up three holy items, gives Isaac angel wings for flight and three soul hearts.
    • Bob, activated by picking up three Bob items, turns Isaac green and causes him to release a trail of green creep where he walks.
    • Leviathan, activated by picking up three satanic items, transforms Isaac into a mini-Cthulhu, granting flight and three black hearts.
    • Spun, activated by picking up three syringe items, makes Isaac buff, gives a damage increase, and spawns a random pill.
    • Yes Mother, activated by collecting three Mom items, dresses Isaac as Mom and creates a copy of Mom's knife which trails behind the player (not as a familiar, but as an inverted copy of the knife controlled by movement).
    • Fun Guy, activated by picking up three mushroom items, gives Isaac a mushroom hat and adds a heart container.
    • Oh Crap, activated by picking up three poop-related items, turns Isaac into a walking poop and heals him by half a heart every time a poop is destroyed.
    • Super Bum, activated by picking up the three Bums, combines the three Bums into a single Bum that grabs every pickup in the room and drops twice as many in return.
    • Conjoined, activated by picking up three of a certain set of familiars (Brother Bobby, Harlequin Baby, Headless Baby, Little Steven, Mongo Baby, Rotten Baby, and Sister Maggy), causes Isaac to grow two tumors on either side of his face, giving him permanent diagonal tears similar to The Wiz.
  • Swiss Army Tears: As a weapon! With the appropriate items, Isaac can get poison tears, homing tears, fear-inducing tears, spectral tears, magnetic tears, petrifying tears, exploding fire tears, tears that summon beams of light from Heaven, or any combination thereof. And that's not even a complete list.
  • Tears of Fear: Pretty understandable given the situation he's in.
  • Tears of Blood: Except this time it isn't a bad thing. It means your tears will do more damage.
  • Throw Down the Bomblet: The bombs he can carry around are pretty good for killing things as well.
  • Took a Level in Badass: Throughout the game, if the right items are acquired. Isaac starts the game as a crying child and can potentially end it as a flying demon that can demolish bosses in seconds with giant lasers.
  • Troubled Fetal Position: In the floor transitions and at the start of each floor.
  • Urine Trouble:
    • The aptly named Number One upgrade replaces his tears for a urine attack instead.
    • Also Lemon Mishap, which leaves a puddle that damages enemies.
  • Winged Humanoid: Possible with certain items.

    Isaac 

Isaac

http://static.tvtropes.org/pmwiki/pub/images/isaac.png
Debut: The Binding of Isaac
Named after the biblical son of Abraham, is playable from the start and has no defining accessories. According to Edmund McMillen, he represents the fighter class of traditional games and begins with no items.
  • Badass Normal: He's the only playable character that starts off without any sort of supernatural assistance. Even the D6 is merely a six-sided die.
  • Bald of Awesome: Being a young child, he has no hair normally. It doesn't hinder him at all.
  • Boring, but Practical: In both the Flash and Rebirth versions, he starts with a single bomb. It seems like nothing, but it guarantees you'll be able to blow up a marked rock (and get DMG up and/or Soul Hearts) should you encounter one, or take a shot at finding a secret room.
  • Heart Is an Awesome Power: His unlockable starting item is merely a six-sided die. Many players agree that it is among the best starting items in the game regardless.
  • He Who Fights Monsters: Isaac himself is an alternate True Final Boss Disc Three Final Boss that can be fought instead of Satan. More details below.
  • Jack-of-All-Stats: Average in all stats except for having 3 starting hearts.
  • Luck Manipulation Mechanic: The all-mighty D6 dice! An activated item capable of transmuting any pedestal item into a different one.
  • Magikarp Power: At first, he'll be outclassed by almost every other character you unlock, as they are more durable, faster, more powerful, and/or more useful than him. But if you take your time and persevere in order to unlock his starting item, he'll become the most versatile and flexible of them all.
  • Master of None: His stats are middle of the road, and he gets no starting item to work with, unlike the others. ...Until you unlock the D6.

    Magdalene 

Magdalene

http://static.tvtropes.org/pmwiki/pub/images/magdalene.png
Debut: The Binding of Isaac
Named after a friend of Mary, the mother of Jesus, she is distinguished from Isaac by her blonde hair and bow. Maggy represents the cleric class from traditional games and starts with the Yum Heart: an activated item that restores one heart upon use.
  • Boring, but Practical:
    • Her gimmick is somewhat bland compared to the others, but having four red hearts to work with allows for Devil Room deals early on without severe consequences. Additionally, the Yum Heart restores a red heart every 4 rooms (without any items that accelerate charging), which keeps her healthy and can be a godsend during the later chapters.
    • In Afterbirth+, you can unlock a starting Speed Up pill for her through the challenges. Not flashy, but useful since she starts out with reduced speed.
  • Combat Medic: Called "the cleric" in the art booklet.
  • Dead Person Impersonation: The death of the real Maggy seems to be what caused Isaac's family to fall apart.
  • Everyone Calls Him "Barkeep": Usually referred to as Maggy.
  • Tertiary Sexual Characteristics: Longish hair and a bow.
  • Useless Useful Spell: Yum Heart with certain playstyles can become this. If you need to use it (read: took red heart damage), your chance at getting powerful Devil Deal items got severely cut.

    Cain 

Cain

http://static.tvtropes.org/pmwiki/pub/images/cain15.png
Debut: The Binding of Isaac
Named after the biblical son of Eve and first murderer, he is distinguished from Isaac by an eyepatch. He represents the Thief class from traditional Role-Playing Games and starts the game with the Luck Foot: a passive item that improves random events in various ways. In Afterbirth, donating 67 coins to the donation machine at the end of Greed Mode allows him to start with the Paper Clip, a trinket that allows him to open chests without using keys.
  • Bald of Awesome: Just like standard Isaac.
  • Born Lucky: Cain's starting item gives him increased luck in certain situations.note 
  • Eyepatch of Power: He has an eyepatch, and also starts with higher attack than most other characters.
  • Fragile Speedster: Starts with great speed and attack and an average rate of fire, but suffers from low range and health. Outright called "the thief" by the art booklet. Being "the thief" could be considered Fridge Brilliance, considering he "stole" his own brother's life in the Biblical story of Cain and Abel.
  • Glass Cannon: Starts with great attack and a good rate of fire, but low health.
  • Heart Is an Awesome Power: After the release of Rebirth, Cain was widely considered the worst (non-joke) character with his sub-par stats and his newly nerfed items. His only saving grace was that when it came to money, there was no one who could reliably get and keep as much as him. Come Afterbirth, Cain became widely considered the absolute best character for the new Greed Mode, which is all about money (his new general stat buffs certainly didn't hurt either).
  • Master of Unlocking: In Afterbirth, where you can unlock the Paper Clip trinket as an starting item for him, allowing him to unlock golden chests without using keys.
  • Nerf: The mechanics of Lucky Foot changing in Rebirth indirectly hurt him REALLY bad. Afterbirth compensates a bit, as stated above.
  • Unexpectedly Realistic Gameplay: Downplayed and it doesn't cause any real puzzles, but his eyepatch does factor into gameplay. For most characters, Technology 2 stacks the beam on top of their tears. Cain's the exception, his tears being shut off due to his eyepatch. Chemical Peel is changed, too; while with other characters, every other tear will have increased damage due to one half of the face being scarred, but with Cain, every tear has a 50% chance of dealing increased damage.

    Eve 

Eve

http://static.tvtropes.org/pmwiki/pub/images/eve.png
Debut: The Binding of Isaac
Named after the first woman and original sinner, Eve is distinguished from Isaac by long dark hair and eyeshadow. Representing the druid class from traditional games, she starts with The Whore of Babylon, which turns her into a demon at low health, and the Dead Bird, which fights for her when she is damaged.
  • Attack Animal: The Dead Bird.
  • Awesome, but Impractical: In the normal game, her Razor Blade is this. It damages the player and gives an attack boost for the current room, which is fairly useless over multiple rooms and only good for getting the Whore of Babylon to activate on the first floor. In Greed Mode, however, it's a lot more useful since Angel/Devil Rooms are guaranteed by beating the final wave.
  • Cast from Hit Points: The Dead Bird is activated by taking damage in a room and works as a faster level three Meat Boy for the room in which it activates.
  • Combat Sadomasochist: Both of her starting items (and the items she can unlock) are Cast from Hit Points, and she is the only character who doesn't leave a puddle of urine upon entering a room with half a red heart; instead, she leaves blood. Afterbirth ups the ante if you donate enough coins in Greed Mode, by giving you the Razor Blade.
  • Critical Status Buff: When she's down to half a heart, her Whore of Babylon powerup grants her greatly increased attack. Rebirth buffed this by allowing it to activate at one full heart.
  • Cursed with Awesome: The "curse" that Eve starts the game with.
  • Difficult but Awesome: Starts with terrible attack stats, but getting the most out of Eve's Critical Status Buff makes her a One-Hit-Point Wonder with higher starting attack than most other characters.
  • Four Is Death: Eve is the fourth playable character, with a risky playing style that actually benefits from being close to the brink of death. In Ending 11, the character montage showed Eve being present at the game's fourth level, the Womb. And considering how Mom is...
  • Fragile Speedster: High speed, low health, low attack unless reduced to critical health.
  • Glass Cannon: Becomes one when hurt due to her Critical Status Buff.
  • Looks Like Cesare: Has eyes encircled with black as well as black hair (though not scraggly).
  • Tertiary Sexual Characteristics: The Dead Bird in her hair looks like a bow.

    Judas 

Judas

http://static.tvtropes.org/pmwiki/pub/images/judas.png
Debut: The Binding of Isaac
Named after the disciple who betrayed Jesus, he has a fez to distinguish himself from Isaac. He represents the mage class from RPGs and starts the game with the Book of Belial, an activated item that increases attack power for one room. In the original game, he unlocks after beating the Womb for the first time. In Rebirth, he unlocks after beating Chapter 5 for the first time.
  • Bald of Awesome: Just like Isaac.
  • Glass Cannon: More so than Cain. Starts with average rate of fire, range, speed, and high attack power, further bolstered by his Book of Belial. In exchange, he starts with only 1 heart.
  • Deal with the Devil: While holding the Book of Belial, the chances of a Devil Room appearing are increased. Unfortunately, Judas' starting 1 heart makes it impossible to make deals unless he gets more heart containers or soul hearts, or buys an item that grants him black hearts.
  • Difficult but Awesome: Starts off with the most consistent damage of all the playable characters (Eve and Samson's powers are conditional), but with only one starting heart and average speed for dodging, you don't have much leeway to screw up.
  • Nice Hat: Edmund based Judas' Fez on Phil Fish, due to him voting against Edmund and his comrades at the Independent Games Festival... although Edmund later stated that said explanation was bullshit, but he went along with it for a while 'cause he found it funny.
  • Secret Character: There's a form of Judas in Rebirth only attainable by dying with the Judas' Shadow item. For the rest of the run, you will play as a character known as Black Judas in-game, who has a 2.0 damage multiplier but starts only with two Black Hearts. Finishing a run as Black Judas counts as a Judas run for unlocks and achievements.
  • Squishy Wizard: The artbook refers to him as the "mage", fitting in the other RPG archetypes. It fits since Judas has high starting damage (especially with the Book of Belial active) but very low starting HP.

    Samson 

Samson

http://static.tvtropes.org/pmwiki/pub/images/samson.png
Debut: Wrath of the Lamb
Introduced in Wrath of the Lamb and named after the biblical strongman, he has long, brown hair and Big Ol' Eyebrows. He also starts with a red headband called Blood Lust. This is fitting with the fact that he represents the Berserker class. An extremely important part of this character is how he was substantially changed. In the original, Samson started with one heart container and gained a damage boost for every enemy he kills in a room. In Rebirth, he starts with three heart containers and gains a damage boost every time he takes damage, which lasts for an entire floor.
  • Blood Knight: His trademark power-up is called Blood Lust, after all!
  • Conservation of Ninjutsu: Invoked by his special hair-band. The more enemies you face in a room, the more powerful you'll get! But single-target rooms won't trigger any attack-boost. This is changed in Rebirth.
  • Critical Status Buff: In Rebirth, Bloody Lust makes Samson deal more damage as he takes damage, with him reaching near maximum damage for having taken several hits in a row. In this case, he literally Turns Red to reflect Bloody Lust activating.
  • Difficult but Awesome: Bloody Lust makes Samson powerful... if you manage to survive the hordes of enemies that give him his strength. In Rebirth, this mechanic is changed, though the original mechanic returns as a separate item, Lusty Blood, in Afterbirth.
  • Glass Cannon: With just 1 starting heart container, he's as fragile as (and under some circumstances, even more than) Judas. However, his special gear makes him extremely effective against large groups of enemies and/or mook makers. This is changed in Rebirth, which gives him much more starting health; however, in order to max out his damage output, he has to deliberately take a lot of damage.
  • Martial Arts Headband: It's the source of his Blood Lust passive ability.
  • Power-Up Letdown: He's probably the most difficult character to unlock in the original (with ??? possibly being more tedious than difficult), as it requires you to pass on 2 free items on a regular run... and its capacity is dubious to say the least. After months of complaints, Edmund promised a buff to Samson. The buff was starting with a soul heart on top of his single regular heart. He became much more useful in Rebirth, as he now starts with three regular hearts and his buffs from killing enemies and taking damage remain until you leave the floor. On the other hand, now you have to clear two floors in a single run without taking any damage to unlock him, which is arguably even more difficult.
  • Took a Level in Badass: In Rebirth, Samson starts with a much more forgiving 3 hearts and gets to keep his buffs until he leaves the floor. This is a major improvement from his previous incarnation.

    ??? 

???

http://static.tvtropes.org/pmwiki/pub/images/bluebaby.png
Debut: The Binding of Isaac

Unlocked after defeating Mom's Heart/It Lives ten times, ??? is a shout-out to Edmund McMillen's Newgrounds account (specifically his "Dead Baby Dressup" game). Starts with The Poop, which creates a pile of dung once per room. Whilst his official name is ???, his nickname is "Blue Baby", which is how he signs his death note on the Game Over screen (and how he's listed on the PSN Trophies list for Rebirth).

Despite being a Joke Character gameplay-wise, he is possibly one of the most plot-relevant characters in the game. His reveal indicates that the vignette of Isaac suffocating in the chest is canon, and not only that, but he somehow died of asphyxiation whilst there. At this point, it is implied that Isaac has been Dead All Along and Endings 11 and 12 note  seem to indicate that Isaac actually locked himself in the chest after being Driven to Suicide.
  • Amazing Technicolor Population: Justified, since he's dead and cyanotic.
  • Back from the Dead: Well, he's still dead, but he's moving around, and acts normally despite being a corpse.
  • Boring, but Practical: You'd be surprised how situationally useful The Poop can be. It can range from simply blocking enemies to overriding other floor hazards.
  • Crouching Moron, Hidden Badass: If ??? gets The Whore of Babylon, he automatically gets its Critical Status Buff. Permanently.
  • Jack-of-All-Stats: No outstanding stats or notable starting item, with the exception of his -1 HP, meaning that he depends on soul hearts just to live. However, he is actually slightly faster and stronger than Isaac.
  • Joke Character: He is incapable of gaining red heart containers. Any situation that would give him red hearts instead converts them to soul hearts.
  • Joke Item: The Poop.
  • Lethal Joke Item: A pile of poop spawned by the aforementioned item can be used to create temporary cover. It is affected by the Petrified Poop trinket, meaning you can get a coin every room or two if you have it, and can be bombed like a rock to cross single-tile-wide gaps. In Afterbirth, using the Poop deflects incoming projectiles a'la buffed Butter Bean.
  • Lethal Joke Character: Since his health is entirely soul hearts, he can exploit certain health-based mechanics (such as the already-mentioned Whore of Babylon), leading to a few potential Game Breaker combos.
  • My Name Is ???: A unique example in that it's never revealed what the question marks are obscuring, unless you go with the theory that he's just a postmortem Isaac.
  • No Dead Body Poops: Oh yes they do. Which is the whole point of his Joke Item.
  • Toilet Humor: You can't get much closer to this than being able to poop on command.
  • Undead Child: He's the corpse of a child that's somehow still animate.
  • The Unpronouncable: With a name like ???, his Fan Nickname (Blue Baby) was inevitable.
  • Wingding Eyes: Crossed-out eyes.

    Lazarus 

Lazarus

http://static.tvtropes.org/pmwiki/pub/images/lazarus.png
Debut: Rebirth
Named after the one that was brought back from the dead. Unlocked in Rebirth by having four or more soul hearts at once. His gameplay is unique in that to gain a fairly powerful item and stat boosts, the game encourages you to kill yourself.
  • Back from the Dead: Starts play with one extra life.
  • Bloody Murder: After he uses his extra life, he gets the Anemic item, which lets him leave a damaging trail of blood for the duration of the room upon taking damage.
  • Crutch Character: One of the first characters available to unlock, he has a higher tolerance for damage than Isaac, with the only apparent downside being his negative luck. After unlocking the D6, however, his advantage seems rather insignificant. Without using his extra life very tactically, he ultimately plays like a less lucky Cain or a less powerful Judas.
  • Jack-of-All-Stats: Apart from his resurrection ability, he essentially plays like Isaac without a D6.
  • Meaningful Name: Who else but Lazarus would start with the ability to rise from the dead?
  • Not Quite Dead: His gimmick is being able to resurrect instantly, rather than outside the room he died (forcing him to restart it).
  • Power-Up Letdown: His extra life only resurrects you with exactly one heart container, and only gives you a blood trail for the current room if you get hit once. On the other hand, it does resurrect you on the spot (unlike the other extra lives that return you to the previous room), possibly allowing you to finish off whatever killed him. Lazarus also receives a considerable boost in Luck, Range, Damage, and Speed, but still, that one heart container makes it a tricky run.
  • Took a Level in Badass: Completing Challenge 31 in Afterbith+ allows him to use Anemic right from the start, without losing his extra life first. Additionally, it is activated permanently if Lazarus dies and is resurrected (or another character resurrects as him), with no need to take damage.

    Azazel 

Azazel

http://static.tvtropes.org/pmwiki/pub/images/azazel1.png
Debut: Rebirth
A demonic-looking character whose name is associated with evil. Unlocked in Rebirth by making three deals with the devil in one run. Starts with the ability to fly and a short ranged version of Brimstone.
  • Action Bomb: He starts with only black hearts, which trigger the Death effect when depleted.
  • Close-Range Combatant: The closest thing that Rebirth has to one, since his starting weaponry forces him to get relatively close to enemies. He compensates by possessing above-average speed and flight.
  • Dark Is Not Evil: He's a demon and starts with Black/Evil Hearts, but he's still a playable character just trying to escape the basement dungeon.
  • Disc One Nuke: His flight and Brimstone are a major advantage in the early game, and flight remains an important bonus throughout. However, his limited range will really begin to catch up to him when it comes time to face endgame bosses.
  • Dungeon Bypass: Possessing flight lets him ignore spikes, rocks, and other terrain right from the start. Rather useful.
  • Fragile Speedster: Fast, possesses flight, and has high starting damage, but at the cost of poor range that cannot be increased and having to charge Brimstone for attacks. His black hearts damage everything if one gets depleted and he can eventually gain red heart containers through Health Up items, but he is overall much frailer than characters who can pick up red hearts from the get-go.
  • Frickin' Laser Beams: His starting attack is a limited-range Brimstone.
  • Glass Cannon: High damage, but starts with no red hearts.
  • Hit-and-Run Tactics: Unlike others who depend on kiting with tears, Azazel often has to fly in, blast several enemies with brimstone, then get out of there before the rest retaliate.
  • Horned Humanoid: One of his horns is broken.
  • The Last of These Is Not Like the Others: Azazel is by far the most different from the other playable characters, since he starts with miniature brimstone instead of tears, has evil hearts (but can gain red ones, unlike ???), and has flight (which only The Lost shares). The result is a rather different playstyle, including item selection, since his smaller brimstone makes many items useless/redundant but others extremely powerful.
  • Necessary Drawback: Though Azazel can increase his Brimstone's range with upgrades, the benefit is almost entirely negligible unless you pick up several of them, which is unlikely without the use of items and consumables which can re-roll items. In addition, his short range negates any homing effects.
  • Required Secondary Powers: A melee character would've been an interesting addition to the roster, but an actual melee attack would make multiple rooms Unwinnable by Design. Azazel being equipped with the piercing brimstone and flight fixes both of these problems.
  • Winged Humanoid: Starts with flight.

    Eden 

Eden

http://static.tvtropes.org/pmwiki/pub/images/eden.png
Debut: Rebirth
Named after the biblical garden, Eden's stats and starting items are randomly generated every time you play as them. Their gender is undefined, and to play as them requires the use of one "Eden token", earned by beating Mom's Heart. Unlocks in Rebirth after beating the Womb for the first time.
  • Anime Hair: The hairstyle is random, and many options look like this. The color is always white.
  • Confusion Fu: Always starts with a random set of items and stats.
  • Monochromatic Eyes: In the ending that unlocks the character.
  • Mystical White Hair: The mystical part coming from their random stats and starting items.
  • Necessary Drawback: As Eden's random stats can potentially give you either very good or very bad combinations, each Eden run consumes an Eden token, and you only earn one each time Mom's Heart/It Lives/Ultra Greed is beaten. This is to discourage reset spamming for a good/game-breaking item combination.
  • Otherworldly and Sexually Ambiguous: Word of God is that Eden lacks gender. As described by Edmund McMillen:
    Eden isnt a man, nor are they a woman.. they just are.

    The Lost 

The Lost

http://static.tvtropes.org/pmwiki/pub/images/thelost.png
Debut: Rebirth
A ghost character, and one of the only characters not based off of any sort of religious contextnote , next to ???. It is the secret character of Rebirth, and had a very complicated unlock process... until Afterbirth came along.
  • Chekhov's Gunman: Actually, it appears several times prior to being unlockable, albeit so briefly that few players would actually notice. Specifically, it is the ghost that departs from Isaac every time he dies.
  • Glass Cannon: And how! It has zero health and can't gain any type of hearts, period. The only upside to this is that it takes Devil Room and Black Market upgrades for free, possibly resulting in a One-Hit-Point Wonder powerhouse early on.
  • Guide Dang It:
    • Unlocking it is a bit...convoluted. First, you need to let yourself get killed by a Mulliboom in the basement as Isaac. Next, you need to blow yourself up with one of your own bombs as Maggie in the caves, then let yourself get killed by Mom's foot as Judas. Finally, you need to let yourself be stomped to death by Satan's hoof as Azazel. And you need to do all of this in a row, without winning or dying in any other way. Mess up once and you have to start all over again. Not very easy, to say the least, although the at the very least you get (cryptic) hints as to the next step once you've completed one, and you can use seeds to make it easier, in contrast to almost every other unlock. The "guide" was at least hidden in-game, though it would have been hard for one person to piece together. Dying in a sacrifice room while holding the Missing Poster trinket shows a puzzle piece as a cause of death. Putting the puzzle pieces together show the four game over screens required to unlock The Lost.
    • Afterbirth vastly simplified the unlock process; you just need to die in a Sacrifice Room while holding the Missing Poster. Of course, nowhere in the game does it tell you this, and the fact that the former method is much better known doesn't help.
  • Joke Character: Starts with the ability to fly, with the downside of having absolutely no HP.
  • Lethal Joke Character:
    • Its lack of HP means that it can take Devil items (some of the most powerful items in the game) for no cost at all. Normally, it would cost one or two heart containers for one of those items. In Afterbirth this is cranked up hugely due to the Holy Mantle being unlockable as a default item. This means both that it can take an extra hit per room (so you both don't have to be absolutely perfect and effectively heal 1 hit of damage per room) and you have access to curse rooms at no cost. Together it begins to go from being a joke character into a feesable Glass Cannon if you get multiple Devil deals in a run.
  • One-Hit-Point Wonder: Dies in one hit from anything. Earlier versions had him die from continuing a run. In Afterbirth, however, it is possible to earn the Holy Mantle shield as a starting item for him.
  • Our Ghosts are Different: This one gets blown away by a stiff breeze.
  • Power Up Letdown: Some thought that Samson and ??? weren't worth the effort it took to unlock them, but The Lost is quite possibly even worse in this regard, especially since unlocking it involves a dose of Guide Dang It too. Afterbirth mitigates this by having The Lost start with the D4 (which allows it to reroll any items it is carrying) and Spectral Tears (which lets its attacks pass through objects) as well as the Holy Mantle (lets him take one hit per room without dying) once you donate enough coins to the Greed Machine. Most notably, the powerful items you unlock by beating the game with are why people get him. As soon as that's over with, many players will choose to forget he even exists.
  • Took a Level in Badass: Afterbirth gave him the potential to start with the D4 and the Holy Mantle. Plus, he gained spectral tears.
  • Useless Useful Spell: Once you unlock the Holy Mantle for the Lost, the D4 becomes this, simply because the ability to actually take a hit per room outweighs just about anything it can potentially be rerolled into. A large portion of the items in the game also fall into this category, namely any item that gives only health and anything that activates on taking damage.
  • Walking Spoiler: Naturally, being the secret character of Rebirth.
  • Wham Shot: Afterbirth adds a new, interesting loading screen. That voice that told Isaac's Mom he was evil? It was the Lost, speaking from the attic.

    Lilith 

Lilith

http://static.tvtropes.org/pmwiki/pub/images/lilith.png
Debut: Afterbirth
Named after Adam's first wife who defected from Heaven with Samael. She's also known as the "The Mother of Demons." Unlocked in Afterbirth after clearing Greed Mode with Azazel. Starts out blindfolded, meaning she is unable to shoot tears, but also starts with the Incubus familiar whose tears have the same properties as hers, the Box of Friends active item which duplicates her familiars for one room, and the Cambion Conception passive item which permanently grants a new familiar upon taking a certain amount of damage.
  • Eye Scream: Implied by the bloodstains on her blindfold.
  • Glass Cannon: Box of Friends effectively doubles her damage output for a room, allowing her to deal significant amounts of damage, but with only one red heart, she struggles at taking hits.
  • Lethal Joke Character: While incapable of shooting tears herself, she starts with the item 'Box of Friends', which adds a copy of any familiars she has for a single room, and takes four rooms to recharge. This makes her the best character by far in Greed Mode, as the battles on each floor take place in a single room, and she recharges the item every four waves of enemies. At a minimum, you can quadruple your friends by the end of the floor.
  • Horned Humanoid: Has demon horns.
  • Mechanically Unusual Class:
    • Lilith has a blindfold over her eyes, similar to challenge runs where you can't shoot any tears. She compensates for this by starting out with the Incubus, a powerful familiar that copies (for the most part) the player's tear stats.
    • Lilith's Incubus is part of being Lilith. It will not reroll even with a full run re-roll; you can pick up a second Incubus normally; and if you somehow change from Lilith to another character, you lose the Incubus.
  • Nerf: Her active item, Box of Friends, was nerfed in a patch about a week after Afterbirth's release, making it a 4-room charge instead of 2. This brought Lilith down from "Amazing character that can cut through the game with ease with her army of minions" to "Good, but not great."
  • The Minion Master: Starts with a familiar as her only way to deal damage. Her Box of Friends active item creates a copy of each familiar she possesses when used (copied familiars don't count, so only the originals are duplicated), and her Cambion Conception gives her another (permanent) familiar if she is damaged a certain number of times (which, as stated under the Glass Cannon entry above, can be tricky due to her low starting health).

    Keeper 

Keeper

http://static.tvtropes.org/pmwiki/pub/images/keeper2.png
Debut: Afterbirth
A character unlocked by the efforts of the community in the Afterbirth ARG, Keeper is the corpse of Isaac that kills himself by hanging. His name refers both to the shopkeepers he resembles and to Cain's words in the book of Genesis, "Am I my brother's Keeper?" He starts with triple shot, is healed by pennies, and cannot pick up hearts.
  • Back from the Dead: Like ???, he is an undead Isaac. In this case, one who paid for his greed and became one of the greyed corpses you see running the shops.
  • Chekhov's Gunman: He appears in Ending 18 as the corpse of Isaac that shrivels up to a shopkeeper.
  • Double Unlock: Although the ARG "unlocked" him by adding him to the game, he's only playable after donating one thousand coins to the Greed Machine.
  • Good Counterpart: He's basically Greed as a playable character.
  • Guide Dang It: Not necessarily his unlock, but to unlock his unlock. A long series of cryptic hints led to a treasure hunt in Santa Cruz, CA, culminating in the discovery of a small buried Keeper doll. This led to the patch being released on Steam that added Keeper to the game.
  • Lethal Joke Character:
    • Get items that drop coins in battle and he can practically never die.
    • Against all forms of Greed, Keeper is basically immortal as long as he has coins. Since they make you drop coins on contact, you just keep getting health back. This goes double for Ultra Greed's second form, which generates coins during the fight and thus ensures you'll never run out short of gross incompetence.
  • Joke Character: Keeper can only have a maximum of two hit points at any given time, meaning that Curse Rooms (without Holy Mantle) and two-heart Devil deals will straight-up kill him, while one-heart Devil deals leave him as a One-Hit-Point Wonder. He also can't take soul or black hearts, as they get converted into flies.
  • The Last of These Is Not Like the Others: Competes with Azazel for most unconventional playstyle. He has only two hit points, but those are coins instead of hearts. Any heart pickups that appear are replaced by friendly flies.
  • Magikarp Power: Downplayed. He's a difficult character to play as, but by defeating certain bosses as Keeper, you can get some useful starting items for him (and a single dumb penny), making him... still pretty difficult to play, but at least better. He's a little bit more useful in Greed Mode once he gets the Wooden Nickle, which can allow him to rapidly build up coins and allow more item purchases.
  • Undead Child: Distinct from ??? in that his cause of death is from hanging, not suffocation.
  • Walking Spoiler: Like the Lost, he is a heavily hidden character. The developers even took care for him to not be discovered by hackers before his ARG had been concluded.

    Apollyon 
http://static.tvtropes.org/pmwiki/pub/images/apollyon_5.png
Debut: Afterbirth+
A mysterious new character who debuted in Afterbirth+ and is unlocked by default (but not in the Nintendo Switch version, in which they are unlocked by killing Mega Satan for the first time). He starts with the new Void item, which destroys and copies the effects of activated items while turning passive ones into random stat boosts. Apollyon is the Greek name of (and he therefore represents) Abaddon, the angel of destruction and the abyss in Revelations, the last book of the Bible.
  • The Assimilator: Void destroys passive items and turns them into stat boosts, and gains the effects of any active items it destroys. The latter can stack indefinitely, so Void can become a pretty ludicrous item by the end of the game.
  • Black Eyes of Evil: His eyes are completely black. On the loading screen between floors, they look like hollow holes.
  • Fallen Angel: Mentioned to be one of his character traits in the blog post. In addition, his name is Greek for the Hebrew word Abaddon, which means he could represent the angel of destruction and has a relation to Sheol.
  • Good Wings, Evil Wings: He has a pair of feathery wings. He is one of the playable characters, so he's good.
  • Horned Humanoid: He has two small black horns.
  • Magikarp Power: Starts with 2 hearts and average~low stats, and his starting item Void does nothing initially. However, Void can convert unwanted passive items into random stat boosts and absorb the effects of activated items permanently. This allows you to turn useless items into damage or other benefits, and Void's ability to stack effects allows for insane combos that can shatter the game if you luck out and find the right items.
  • Meaningful Name: His name is the Greek word for "destroyer". Sure enough, his main ability is to destroy items to gain their abilities or stat bonuses.
  • The Swarm: Not in game, but most of Apollyon's unlocks are locust trinkets of some kind, as a reference to the biblical Apollyon's status as the king of an army of locusts.
  • Wings Do Nothing: Unlike Azazel, he doesn't use his wings for flight. They're just decoration.

Monsters

    Flies 

Flies

In General

  • Airborne Mook: Making them immune to floor hazards, and allowing them to pass over obstacles.
  • Big Creepy-Crawlies: Even the weakest variety of fly (that you can actually fight) is still the size of one of Isaac's eyes.
  • The Goomba: Though these ones are killed by shooting at them with tears instead of jumping on their heads.

Black Fly/Attack Fly/Moter/Ring Fly/Swarm Fly/Dart Fly

http://static.tvtropes.org/pmwiki/pub/images/fly3.png
Debut: The Binding of Isaac (Black Fly, Attack Fly, Moter)
Rebirth (Ring Fly, Swarm Fly, Dart Fly)
  • Asteroids Monster: Moters split into two Attack Flies when killed.
  • Bulletproof Human Shield: As the Halo Of Flies and Pretty Fly powerups. The Forever Alone and Distant Admiration powerups spawn one capable of dealing good damage.
  • Everything's Better with Spinning: Ring Flies converge to form a spinning ring which tracks you as a group. As they're destroyed, they keep up the circle with wider gaps.
  • Mascot Mook: The black variety is featured prominently in the artwork, and easily has the most related items.
  • Mook Promotion: If allowed to approach a pile of poop, a black fly consumes it and turns into a Pooter.
  • Zerg Rush: Swarm Flies join into a clump and then rush you.

Level 2 Fly/Full Fly

http://static.tvtropes.org/pmwiki/pub/images/level2fly.png
Debut: Rebirth
  • Elite Mooks: As the name suggests, they are standard Flies, but larger and stronger.
  • Kaizo Trap: Full Flies release six bullets on death.

Pooter/Fat Fly (Super Pooter)

Debut: The Binding of Isaac (Pooter)
Wrath of the Lamb (Fat Fly/Super Pooter)
http://static.tvtropes.org/pmwiki/pub/images/pooter.png
  • High-Pressure Blood: One of the first enemies you'll see that can fire blood shots.
  • Mook: Not any more dangerous than the normal flies.
  • Retcon: The Fat Fly was renamed "Super Pooter" in Afterbirth+.

Boom Fly/Red Boom Fly/Drowned Boom Fly

http://static.tvtropes.org/pmwiki/pub/images/boomfly.png
Debut: The Binding of Isaac (Boom Fly, Red Boom Fly)
Afterbirth (Drowned Boom Fly)
  • Action Bomb: With the exception of the Red Boom Fly, they explode when killed.
  • Disaster Dominoes: An efficient way to clear a Boom Fly room is to kill one of them while they're all flying in a cluster. Doesn't work with Red Boom Flies, which shoot blood shots in 6 directions on death instead of exploding.

Sucker/Spit

http://static.tvtropes.org/pmwiki/pub/images/sucker.png
Debut: The Binding of Isaac (Sucker)
Wrath of the Lamb (Spit)
  • Action Bomb: Spits.
  • Disaster Dominoes: In a room with several Spits, killing one while standing in the right direction will often cause a chain reaction of vomit bombs that kill the rest of them.
  • Taking You with Me: They release hard-to-dodge ranged attacks for Suckers or vomit bombs for Spits when killed.

Eternal Fly

http://static.tvtropes.org/pmwiki/pub/images/eternalfly.png
Debut: Wrath of the Lamb
  • Invincible Minor Minion: Until the monster or poop they orbit is destroyed, at least.
  • Meaningful Name: As mentioned above, the Eternal Fly is, well, eternal, until the poop or monster they orbit is destroyed.

    Humanoid 

Humanoid

Pacer/Gusher

http://static.tvtropes.org/pmwiki/pub/images/pacer.png
Debut: The Binding of Isaac
  • Elite Mooks: Gushers to Pacers.
  • High-Pressure Blood: Gushers shoot out a fountain of blood from their necks, but only the occasional blood shot can actually deal damage.
  • Overdrawn at the Blood Bank: Gushers, as the name implies, have a constant stream of blood flowing from their necks.

Gaper/Frowning Gaper/Flaming Gaper/Cyclopia

http://static.tvtropes.org/pmwiki/pub/images/gaper.png
Debut: The Binding of Isaac (Gaper, Frowning Gaper)
Afterbirth (Flaming Gaper, Cyclopia)

Gurgle/Splasher

http://static.tvtropes.org/pmwiki/pub/images/gurgle.png
Debut: Wrath of the Lamb (Gurgle)
Rebirth (Splasher)
  • Super Spit: They hack up explosive Ipecac shots rather than the usual damaging blood.

Null

http://static.tvtropes.org/pmwiki/pub/images/null.png
Debut: Rebirth

Horf/Psychic Horf

http://static.tvtropes.org/pmwiki/pub/images/horf.png
Debut: The Binding of Isaac (Horf)
Rebirth (Psychic Horf)

Hopper/Flaming Hopper/Leaper

http://static.tvtropes.org/pmwiki/pub/images/hopper.png
Debut: Binding of Isaac (Hopper, Flaming Hopper)
Wrath of the Lamb (Leaper)
  • Bullet Hell: Leapers shoot bullets in all four cardinal directions when they jump. If five end up in a room, be prepared to dodge.
  • Elite Mooks: Fire Hoppers and Leapers, the former being stronger and the latter shooting bullets.
  • Infernal Retaliation: Hoppers move twice as fast, jump twice as far, and inflict more damage when they're on fire.
  • Mook Promotion: A Hopper that touches fire becomes a Flaming Hopper.
  • Violation of Common Sense: Not only does every other enemy (or character, even) take damage themselves from fire, but Isaac's main combat strategy is attacking enemies with water. Despite that, they seem to have double health.

Maw/Red Maw/Psychic Maw

http://static.tvtropes.org/pmwiki/pub/images/maw.png
Debut: The Binding of Isaac (Maw, Red Maw)
Wrath of the Lamb (Psychic Maw)
  • Elite Mooks: Red Maws, with their lips stitched, can't shoot, but chase you pretty fast. They also get knocked away farther when hit, offsetting their speed in terms of how easily you can keep them at bay (especially if you have a high rate of fire), but making it difficult to line up multiple shots against them.
    • Psychic Maws also have their eyes stitched. They have a Third Eye which shoots homing tears, and a Eternal Fly circling them.
    • Maws themselves are basically Horfs, only with the ability to move, which eliminates the tactic of staying just outside of their range and shooting.
  • Tears of Blood: The Psychic Maw has this. The other Maws may have this, but it looks more like their eyes may have exploded out of their sockets instead.

Mr. Maw/Swinger

http://static.tvtropes.org/pmwiki/pub/images/mrmaw.png
Debut: Wrath of the Lamb (Mr. Maw)
Rebirth (Swinger)
  • Asteroids Monster: Depending on whether you kill it by shooting it in the head or "body", it will spawn a Maw or Gusher/Globin respectively.
  • Detachment Combat: Mr. Maws will swing their heads out at you when you get close. Swingers are perpetually swinging, as their name suggests. Mr. Maw can also voluntarily separate into a Maw and a Gusher.
  • Epic Flail: Their heads can be swung around like these.

Skinny/Rotty

http://static.tvtropes.org/pmwiki/pub/images/skinny.png
Debut: Rebirth
  • Body Horror: It has a more deformed appearance than most humanoid enemies.
  • Punny Name: "Skinny" not only refers to the folds of skin hanging off their faces, but also sets them up as a Fat and Skinny duo with the Fatty.
  • Stripped to the Bone: Non-fatal variant; killing a Rotty will sometimes cause it to become a Bony.

Bony/Black Bony

http://static.tvtropes.org/pmwiki/pub/images/bony.png
Debut: Rebirth (Bony)
Afterbirth (Black Bony)

Fatty/Flaming Fatty/Pale Fatty/Fat Sack/Blubber/Half Sack/Conjoined Fatty/Blue Conjoined Fatty/Stoney

http://static.tvtropes.org/pmwiki/pub/images/fatty_5.png
Debut: Rebirth (Fatty, Pale Fatty, Fat Sack, Blubber, Half Sack)
Afterbirth (Flaming Fatty, Conjoined Fatty, Blue Conjoined Fatty)
Afterbirth+ (Stoney)
  • Acrofatic: While they're normally slow and bumbling, Fat Sacks have an attack where they jump high in the air and slam down.
  • Amazing Technicolor Population: Blue Conjoined Fatties, a rare enemy with a rather outlandish skintone.
  • Dead Weight: Fat Sacks are fat zombies.
  • Gasshole: Fatties can fart to push things away. Pale Fatties shoot blood when they do this.
  • Infernal Retaliation: Flaming Fatties are faster than normal.
  • Invincible Minor Minion: Stonies behave just like Fatties but only die when every other enemy is killed.
  • Kevlard: They're decently durable, and that probably comes from their immense fat.
  • Multiple Head Case: Conjoined Fatties have two heads. "Killing" it only destroys one head, reducing it to a regular Fatty.
  • Nightmare Face: The Fat Sack's face, with the empty eye sockets and Tears of Blood.

Mulligan/Mulligoon/Mulliboom/Nest/Hive/Drowned Hive/Swarmer

http://static.tvtropes.org/pmwiki/pub/images/mulligan18.png
Debut: The Binding of Isaac (Mulligan, Mulligoon, Hive)
Wrath of the Lamb (Mulliboom, Swarmer)
Rebirth (Nest)
Afterbirth (Drowned Hive)
  • Action Bomb:
    • Normal Mulligans puff up and explode into flies if they survive long enough and get cornered.
    • Mulligoons replace the flies with a 4-way shot plus a troll bomb.
    • Mullibooms come with bombs implanted in them and will charge and blow up on you.
    • The Swarmer spawns a Bomb Fly upon death.
  • Body Horror: The tumor-headed Mulligan grows more decayed as the flies inside it transform it into the Hive. From there, the Hive becomes a Swarmer, a rotten head filled with flies.
  • Incurable Cough of Death: Has a very distinctive-sounding one when it spits up flies to attack you.
  • Mook Maker: Hives occasionally cough up flies and spit them at you, and Swarmers constantly spawn small flies as well as Pooters and Fat Flies when damaged.
  • Taking You with Me: Unlike his cousins, the Mulliboom (the one with a bomb stuck in his head) will rush straight at you and try to suicide-bomb you.

Globin/Gazing Globin/Dank Globin/Black Globin

http://static.tvtropes.org/pmwiki/pub/images/globin.png
Debut: The Binding of Isaac (Globin, Gazing Globin)
Afterbirth (Dank Globin, Black Globin)

Knight/Selfless Knight/Floating Knight/Bone Knight

http://static.tvtropes.org/pmwiki/pub/images/knight.png
Debut: The Binding of Isaac (Knight, Selfless Knight)
Rebirth (Floating Knight)
Afterbirth (Bone Knight)
  • Airborne Mook: Floating Knight are flying Knight heads, ignoring pits and barriers.
  • Attack Its Weak Point: The front of them is solid bone or petrified flesh. The only way to actually damage it is to hit it in the back of its head, the exposed brain.
  • Bloody Murder: Bone Knights trail damaging creep behind them, making it more difficult to target the exposed brain.
  • Elite Mooks: Its second variety chases the player instead of wandering aimlessly. They have a crying Isaac face instead of a brain on their back. Bone Knights are even more elite, leaving a trail of creep as they walk.
  • Shield-Bearing Mook: As customary, the solution is to flank it where its shield can't protect it.

Vis/Double Vis/Scarred Double Vis/Chubber

http://static.tvtropes.org/pmwiki/pub/images/vis.png
Debut: The Binding of Isaac (Vis)
Wrath of the Lamb (Double Vis, Chubber)
''Rebirth (Scarred Double Vis)
  • Body Horror: They look like headless humans, only with a gaping hole on their chests that shoots blood beams of Brimstone.
  • Bloody Murder: Fires what appears to be blood in a wide beam across the room it's in.
  • The Computer Is a Cheating Bastard: A variation of them fires Lil' Chubby instead of a beam. Their Lil' Chubby can go through rocks, and deal damage on the return trip, but yours can't do either.
  • Elite Mooks: As of the Wrath of the Lamb DLC, Vis has a second form which fires Brimstone both in front of and behind it. Afterbirth adds the Scarred Double Vis, which shoots in all four cardinal directions.
  • High-Pressure Blood: They shoot huge streams of blood that pass through all obstacles. In fact, they're probably the first enemy you'll see that can use Brimstone.

Baby/Angelic Baby

http://static.tvtropes.org/pmwiki/pub/images/baby.png
Debut: The Binding of Isaac (Baby)
Wrath of the Lamb (Angelic Baby)
  • Body Horror: Flying, blood-covered fetuses.
  • Elite Mooks: Has an angelic version that shoots a three-way spread shot instead of one bullet.
  • Fetus Terrible: Think of the familiar babies you can get, only shrieking and bloody.
  • Light Is Not Good: The friendly and less harmful looking Angelic Baby is actually more dangerous due to it having a spread attack.
  • Teleport Spam: Will frequently disappear into the nether before reappearing elsewhere a moment later.

Dople/Evil Twin

http://static.tvtropes.org/pmwiki/pub/images/dople.png
Debut: The Binding of Isaac
  • Doppelgänger: The Evil Knockoff variety.
  • Elite Mooks: The Evil Twin. Instead of looking like a meat puppet, it looks like a demon that can: fly, shoot a three spread, and do more damage. Thankfully, its bullets travel slower, giving you time to dodge and shoot.
  • Enemy Without: They mirror the player's movements, including shooting, but most rooms come with a mechanism with which to destroy them. There are also bombs, and praying that whipped shots will work.
  • Flaying Alive: Dople has no skin.
  • Foreshadowing: The appearance of these foreshadows the nature of Isaac the boss.
  • Mirror Match: They can perfectly mimic Isaac's movements and attack at the same time, though they cannot replicate certain tear properties.

Homunculus/Begotten

http://static.tvtropes.org/pmwiki/pub/images/homunculus25.png
Debut: Rebirth
  • Blackout Basement: Begottens darken the room if they break free.
  • Breaking the Bonds: They become significantly more dangerous when they break their chains and can pursue you.
  • Expy: They start out trying to get at the player while chained but can break free and chase them, not unlike the Chain Chomp.
  • Invisible Monsters: Begotten becomes this once it breaks loose, similar to the Null. Fortunately, it also has the same Invisibility Flicker.

    Worms and Grubs 

Worms and Grubs

Maggot

http://static.tvtropes.org/pmwiki/pub/images/maggot.png
  • The Goomba: Has no attacks, and just wanders around the room randomly.

Charger/Drowned Charger

http://static.tvtropes.org/pmwiki/pub/images/charger.png
  • Deadly Lunge: If they spot Isaac, they will open their mouths wide and speed towards him... exactly like Chub does.
  • High-Pressure Blood: What sets Drowned Chargers apart is that they burst into shots of blood when killed.

Spitty/Conjoined Spitty

http://static.tvtropes.org/pmwiki/pub/images/spitty.png
  • Multiple Head Case: The conjoined version has a head on both ends, letting it spit blood in both directions simultaneously.

Grub

http://static.tvtropes.org/pmwiki/pub/images/grub.png
  • Asteroids Monster: Each of its segments becomes a lesser variety of maggot on death.
  • Boss in Mook Clothing: Grubs are basically a stronger version of Larry Jr. In some boss fights against the latter, one of them is replaced with a Grub.

Round Worm/Nightcrawler/Roundy/Ulcer

http://static.tvtropes.org/pmwiki/pub/images/roundworm.png
  • Fast Tunnelling: All of them can submerge and re-emerge anywhere in the room quickly.
  • Mook Maker: Ulcers will spawn Dart Flies if left alone.

Para-Bite/Scarred Para-Bite/Cod Worm

http://static.tvtropes.org/pmwiki/pub/images/parabite7.png
  • Shield-Bearing Mook: Cod Worms are entrenched in hard shells which they can retreat into at will.
  • Zerg Rush: Para-Bites will bum-rush you endlessly, only briefly backing away to repeat the process.

    Spiders 

Spiders

Spider/Big Spider

http://static.tvtropes.org/pmwiki/pub/images/spider.png
  • Asteroids Monster: Big Spiders split into two after death.
  • Demonic Spiders: Literally. Gameplay-wise, they're slightly harder to kill than flies thanks to their erratic movements and greater speed. invoked

Level 2 Spider/Ticking Spider

http://static.tvtropes.org/pmwiki/pub/images/level2spider.png
  • Action Bomb: Ticking Spiders explode when killed.
  • Elite Mooks: Like the Level 2 Fly family, they are improved versions of standard spiders.

Baby Long Legs/Small Baby Long Legs/Crazy Long Legs/Baby Crazy Long Legs

http://static.tvtropes.org/pmwiki/pub/images/babylonglegs.png
  • Mook Maker: Baby Long Legs make Spiders, while their Small counterparts create Sacks.
  • Weaponized Offspring: Both versions of the Baby Long Legs spawn spiders to attack Isaac with, though they don't seem too happy about it.

Wall Creep/Blind Creep/Rage Creep/The Thing

http://static.tvtropes.org/pmwiki/pub/images/wallcreep_2.png

Trite/Ragling/Blister

http://static.tvtropes.org/pmwiki/pub/images/trite_5.png

    Other 

Other

Dip/Corn Dip/Squirt/Dank Squirt/Dinga

http://static.tvtropes.org/pmwiki/pub/images/dip.png
  • Asteroids Monster: Dingas split into two Squirts when killed, while Squirts split into two Dips. Dank Squirts split into two Clots instead.
  • Stealth Pun: Dips. (Which are made out of...)
  • Talking Poo: Actually, its more like yelling poo.
  • Toilet Humor: Regular poop lying everywhere not good enough for you? Now you can fight the poop!

Clot/Clotty/Mega Clotty/I. Blob/Ministro

http://static.tvtropes.org/pmwiki/pub/images/clot_0.png
  • All Your Powers Combined: The I. Blob's projectiles, which are a combination of the other two.
  • Asteroids Monster: Mega Clotty will split apart into two Clotties on death.
  • Blob Monster: Meat blob, tar blob, and... whatever the I. Blob is made of.
  • Bullet Hell: They tend to shoot haphazardly, and very often, in every direction.
  • The Cameo: Clot is very similar to Gish.
  • Elite Mooks: The I. Blob variant shoots projectiles in all eight directions.

Host/Red Host/Walking Host/Walking Red Host

http://static.tvtropes.org/pmwiki/pub/images/host.png
  • Attack Its Weak Point: Only in the grey variation. To hit them, you must wait for them to expose the flesh under the skull. Moving them over spikes will kill them.
  • The Cameo: From a multiplayer flash game with the same name.
  • No-Sell: Regular Hosts are protected by their skull when they aren't firing. Trying to hit them does nothing. Trying to bomb them just causes them to make a troll face.
  • Shield-Bearing Mook: The skulls the normal variety cover themselves with are impervious to pretty much any normal damage source.

Meatball

http://static.tvtropes.org/pmwiki/pub/images/meatball.png
  • Body Horror: Its a sentient lump of mangled meat with an exposed skull that becomes a Host when damaged enough.

Brain/Poisoned Mind/MemBrain/Guts/Mama Guts

http://static.tvtropes.org/pmwiki/pub/images/brain_0.png
  • Asteroids Monster: MemBrain and Mama Guts splits into two Brains and Guts respectively when killed.
  • Body Horror: Does a sliding brain or rolling ball of organs sound pleasant to you?
  • Organ Autonomy: Quite literally, they're organs that move on their own.

Poky/Slide/Wall Hugger

http://static.tvtropes.org/pmwiki/pub/images/poky.png

Stone Grimace/Vomit Grimace/Gaping Maw/Broken Gaping Maw/Turret Grimace/Stone Eye/Brimstone Grimace

http://static.tvtropes.org/pmwiki/pub/images/grimace.png

Boil/Gut/Sack/Walking Boil/Walking Gut/Walking Sack

http://static.tvtropes.org/pmwiki/pub/images/boil.png
  • Elite Mooks: As of Wrath of the Lamb, there is the Gut, which shoots green bombs, much like Sloth. As well as mobile versions of each.
  • Healing Factor: The non-mobile versions slowly regenerate over time when damaged. However, they spawn with less-than-full HP and can't attack until they've reached their maximum health. Their health is directly proportional to their size.
  • Mook Maker: Sacks are made of spider silk, which spits out spiders. It, too, has a mobile version.

One Tooth/Fat Bat

http://static.tvtropes.org/pmwiki/pub/images/onetooth.png
  • Artificial Brilliance: The Fat Bat actively tries to avoid your shots as it approaches you. However, it's not quite fast enough to dodge effectively.
  • Bat Out of Hell: Doesn't look very menacing in comparison to most examples, but it attacks all the same.
  • Incredibly Durable Enemies: The Fat Bat's slow speed is offset by its abnormally large health pool and erratic movement, especially for an enemy that appears starting from the Caves.

Dukie

http://static.tvtropes.org/pmwiki/pub/images/dukie.png
  • Mook Maker: Spawns Dart Flies as it bounces around the room.

Butt Licker

http://static.tvtropes.org/pmwiki/pub/images/buttlicker.png
  • Asteroids Monster: The segments split and act independently after one of them dies.
  • Toilet Humor: Besides constantly tossing each other's salads, they'll eat poop. Doesn't get much more toilet-humorous than that.

Eye/Bloodshot Eye

http://static.tvtropes.org/pmwiki/pub/images/eye21.png
  • Elite Mooks: A second version which looks like a white eyeball and nothing else. It shoots blood beams instead of electrical beams.
  • Eye Beam: Their entire schtick.
  • Eye Scream: Being giant eyes, it's inevitable.
  • Faceless Eye: They're giant eyes that are found just lying on the ground.
  • High-Pressure Blood: Bloodshot Eyes fire thick blood beams.

Leech/Kamikaze Leech/Holy Leech

http://static.tvtropes.org/pmwiki/pub/images/leech.png

Mask + Heart

http://static.tvtropes.org/pmwiki/pub/images/maskplusheart.png
  • Attack Its Weak Point: The Mask is completely immune to all damage. The Heart isn't.
  • Organ Autonomy: The hearts are not defenseless: they will attempt to run away from Isaac and will defend themselves with a four-way blood burst if he gets too close.

Keeper/Hanger

http://static.tvtropes.org/pmwiki/pub/images/keeper.png
  • Bandit Mook: When they hit you, you lose three cents. Only two of those cents fall onto the ground to be picked up again.
  • Orbiting Particle Shield: Each Hangar has an Eternal Fly defending it.

Death's Head/Dank Death's Head/Flesh Death's Head

http://static.tvtropes.org/pmwiki/pub/images/deathshead.png
  • High-Pressure Blood: Flesh Death's Heads will burst into shots of blood when they die, either from manual shooting or from the room being otherwise cleared.
  • Invincible Minor Mook: The standard variety keep bouncing around the room until every kill-able enemy in it is dead. They aren't immune to the Chaos Card, but why would you waste a Chaos Card on one? Dank and Flesh varieties can be killed like any normal enemy.

Lil' Haunt/Wizoob/Red Ghost

http://static.tvtropes.org/pmwiki/pub/images/lilhaunt.png

Imp

http://static.tvtropes.org/pmwiki/pub/images/imp.png
  • Artificial Brilliance: The only enemy in the game to make an active effort of anticipating incoming shots and dodging out of the way.
  • Teleport Spam: Teleports to a random spot in the room after firing 3 shots.
  • Underground Monkey: A Sheol variant of Baby enemies.

Mom's Hand/Mom's Dead Hand

http://static.tvtropes.org/pmwiki/pub/images/momshand.png
  • Evil Laugh: Mom's laugh is the cue that they're in the current room.
  • Mook Bouncer: In addition to dealing damage, they also pull you back to the starting room of the floor if they grab you. This can actually be helpful for a quick trip back to the starting room on a dead end, if you have the hearts to spare.
  • Wall Master: They hide on the ceiling of rooms, eventually diving down on Isaac.

Tumor/Psy Tumor/Camillo Jr.

http://static.tvtropes.org/pmwiki/pub/images/tumor.png
  • Shout-Out: Camillo Jrs. have their name based off of Kracko Jr. However, they look more like regular Kracko, so the Jr. is likely there to make the inspiration clearer.

Nerve Ending/Nerve Ending II

http://static.tvtropes.org/pmwiki/pub/images/nerveending_0.png
  • Body Horror: It's a GIANT DISEMBODIED NERVE ENDING!
  • Combat Tentacles: Nerve Ending II is one.
  • Expy: The Nerve Ending II's color scheme and method of attack bear more than a passing resemblance to a Spine Crawler.
  • Joke Character: Nerve Ending doesn't attack, it just blocks bullets. By the time you're far enough into a run to see these guys, that's already not a problem. Nerve Ending II actually does attack though (it swats at you), which may take some players by surprise the first time they get near them.

Fred

http://static.tvtropes.org/pmwiki/pub/images/mrfred.png
  • Body Horror: It's based on a baby with Harlequin-type ichthyosis.
  • Whack-a-Monster: They pop up in random areas of the room and attack either by firing an explosive shot or shooting a spray of bullets while diving through the air, before going underground and repeating the process.

Lump

http://static.tvtropes.org/pmwiki/pub/images/lump.png
  • Fast Tunnelling: They can move around "underground" very quickly.
  • Whack-a-Monster: When approached, Lumps will fire in the player's general direction to distract them, then go underground and pop up in another part of the room.

Oob/Black Maw

http://static.tvtropes.org/pmwiki/pub/images/oob.png
  • Action Bomb: Black Maw explodes when killed and can spawn Kamikaze Leeches, which also explode.
  • Enemy Summoner: Both are capable of spawning more enemies: Oob summons Brains, while Black Maw spawns Leeches.
  • Shout-Out: They are directly taken from Super Meat Boy, another Edmund McMillen game. The former was an enemy, the latter was an obstacle.

Portal

http://static.tvtropes.org/pmwiki/pub/images/portal_6.png
  • Mook Maker: Their only purpose is to spawn random enemies until destroyed.

Mushroom

http://static.tvtropes.org/pmwiki/pub/images/mushroom.png
  • Incredibly Durable Enemies: They have a very large health pool, especially for an enemy that can show up as early as the Caves.
  • Mushroom Man: It's a mushroom with a face, though that's hardly unusual compared to some of the other enemies present.

Bosses

    Basement, Cellar, Burning Basement 

Monstro

http://static.tvtropes.org/pmwiki/pub/images/monstro.png
Debut: The Binding of Isaac
  • Asteroids Monster: His Eternal form in the Eternal Edition splits as it takes damage. The smaller forms shoot less bullets, however.
  • Body Horror: He's some kind of deformed head, lacking a nose and most of his teeth, and features a prominent harelip.
  • Clipped-Wing Angel: The gray champion version is bigger with more health, but it can't use the leap attack of the normal version, making it far easier to circle around and shoot with no risk to yourself. Even in pairs, it's much less of a threat.
  • Degraded Boss: After the game is beaten six times, he'll start showing up in non-boss rooms occasionally.
  • Dual Boss: The red Champion version is two smaller Monstros.
  • Easter Egg: One spacebar item, Monstro's Tooth, calls in the aforementioned boss to body-slam enemies. On the chance that you end up getting it as one of your first items, Guess which boss you can't use it against.
  • Heel–Face Turn: The Monstro's Tooth item lets you summon him to crush your enemies.
  • Warm-Up Boss: One of the less complicated bosses and one of the most likely to run into first. Many prefer getting zero-damage boss battles on Monstro to increase likelihood of Devil Rooms spawning.

Larry Jr.

http://static.tvtropes.org/pmwiki/pub/images/larryjr.png
Debut: The Binding of Isaac
  • Asteroids Monster:
    • Not so much as other bosses, but if the middle section is killed first in a 5-segment one, it will split into two more versions with just two segments.
    • Much easier to happen with his Eternal counterpart in the Eternal Edition, where a broken-off segment can last as just a head, and can regenerate segments.
  • The Cameo/Mythology Gag: A smaller version of Larrie's Lament.
  • Degraded Boss: The most common boss enemy to find in The Womb.
  • Dual Boss: Easily the most common one; his standard fight in the Basement is always two of him. Afterbirth occasionally mixes things up with a single very long Larry Jr., but he'll likely be splitting into multiple Larries over the course of the fight anyway because of how the boss works.
  • King Mook: As of Wrath of the Lamb, smaller enemies similar to it appear.
  • Warm-Up Boss: Shares this with Monstro. It can't even shoot at you (except for the green champion version), but its movement patterns can be erratic thanks to the poop it leaves behind.

The Duke of Flies

Debut: The Binding of Isaac
http://static.tvtropes.org/pmwiki/pub/images/dukeofflies.png
  • Body Horror: It's safe to assume that the only reason he can move around is because of the flies that have infested his bloated body. He also lacks eyes of any kind.
  • Degraded Boss: After unlocking "Everything's Terrible!"
  • Flunky Boss: "Attacks" by creating tons of flies.
  • Mook Maker: As its name implies, it creates flies.
  • King Mook: To the Dukies.
  • Taking the Bullet: Spawned flies circle the Duke for a bit before coming after you, letting them soak up your shots.

Gemini

http://static.tvtropes.org/pmwiki/pub/images/gemini.png
Debut: The Binding of Isaac
  • Brainsand Brawn: The little one is the Brains while the big one is the Brawn.
  • Bullfight Boss: The big one can tire out after a few moments of running. If the big one is killed first, then the small one will turn red, give chase, and never tire out.
  • Dual Boss: One big version that charges the player and one small version that shoots bullets.
  • Heel–Face Turn: The Gemini zodiac sign gives you a friendly copy of the smaller twin who attacks your enemies.
  • Polar Opposite Twins: The big one is aggressive and chases you; the little one looks calm and makes ranged attacks until you kill the big one, at which point it charges at you.
  • Turns Red: What the small one does when the big one dies.

Steven

http://static.tvtropes.org/pmwiki/pub/images/steven.png
Debut: The Binding of Isaac
  • Bonus Boss: After being unlocked, occasionally replaces Gemini.
  • The Cameo / Mythology Gag: Steven is from Time Fcuk.
  • Heel–Face Turn: the Little Steven item summons a friendly version of Steven who shoots at enemies, and is almost always dropped by this boss (except when it drops the "Steven" item instead, which is just a damage upgrade).
  • Last Ditch Move: Both parts of Steven shoot bullets in all directions when they die.
  • Turns Red: When their aggregate health reaches 1/3, the small one detaches and turns red just like Gemini's.

Fistula

http://static.tvtropes.org/pmwiki/pub/images/fistula.png
Debut: The Binding of Isaac
  • Asteroids Monster: It has three distinct sizes. The final part either spawns a Charger or a Boom Fly, depending on the version fought.
  • Body Horror: It's named for a real-life chunk of Body Horror in which organs try to absorb each other.
  • Degraded Boss:
    • Appears as this in later stages. Most of the time, though, it will already be broken down to its second or third stage.
    • In Rebirth, the boss is now a Chapter One boss.

Widow

http://static.tvtropes.org/pmwiki/pub/images/widow.png
Debut: Wrath of the Lamb
  • Body Horror: She's made out of a head with four eyes, leg bones, and toes.
  • Flunky Boss: Not only does she spawn spiders directly, but she can also spawn Mook Maker spider silk Boils! And that's not even getting into the ones that can spawn bigger Asteroids Monster spiders or other kinds of Boils.
  • Giant Spider: She's a larger version of the Trites. Considering that those are already the size of Isaac, it makes for an intimidating boss fight!
  • Glass Cannon: Relatively low HP for a boss, but moves unpredictably and spits out the equally unpredictable spiders, making her harder to beat without taking any damage than most bosses early on.
  • King Mook: To the spiders.
  • Rush Boss: She's a fragile boss, but if you don't kill her quickly, you'll get overwhelmed by the spiders she summons and the speed she moves at.

Pin

http://static.tvtropes.org/pmwiki/pub/images/pin.png
Debut: Wrath of the Lamb
  • Tactical Suicide Boss: Is not immune to its green exploding projectiles, and due to a quirk in hit detection, can actually damage itself with them when it begins to bury underground. The dark grey one averts it, replacing the bombs with bullets.
  • Warm-Up Boss: Competing with Larry Jr. and Monstro for easiest boss in the game. Relatively low health, a large hitbox, extremely predictable and avoidable attacks, and he's not immune to his own projectiles. Many players feel silly if they take any damage to this boss at all, let alone die to him.

Gurdy Jr.

http://static.tvtropes.org/pmwiki/pub/images/gurdyjr.png
Debut: Wrath of the Lamb

Blighted Ovum

http://static.tvtropes.org/pmwiki/pub/images/blightedovum.png
Debut: Wrath of the Lamb
  • Back from the Dead: Gemini is now a zombie, and is accompanied by his conjoined brother's ghost.
  • Body Horror: The larger one has a gash on his head where his eye once was, lacks a lower jaw, and is either covered in blood or has no skin on his lower body.
  • Bullfight Boss: Like his living counterpart, except he never tires.
  • Frickin' Laser Beams: The ghostly twin fires Brimstone if you stand to the left or right of it for too long.
  • Invincible Minor Minion: The ghost can't be harmed; the only way to get rid of it is to get rid of his brother.
  • Nerf: In the original game, both parts of the Blighted Ovum would fire bullets in all directions upon death. This ability was removed in Rebirth.

Gurglings

http://static.tvtropes.org/pmwiki/pub/images/gurglings.png
Debut: Rebirth
  • Bullfight Boss: They'll charge you if you get in their line of sight.
  • Creepy Twins: When fought as a pair. Identical and dangerous.
  • Degraded Boss: They appear as regular enemies as early as the Caves, and don't even get a health bar. On the other hand, they constantly leave behind damaging blood trails (as opposed to only doing so when charging) and shoot six-way blood bursts rather than spawning flies, making them actually slightly more dangerous than the boss version.
  • Dual Boss: They appear as a pair.
  • Enfant Terrible: They seem to be even younger versions of Gurdy Jr.

Dingle

http://static.tvtropes.org/pmwiki/pub/images/dingle.png
Debut: Rebirth
  • Adorable Evil Minions: His Squirts.
  • Bullfight Boss: One of his attacks is to charge at you three times in succession, occasionally leaving a pile of poop on the first charge. He scrunches up just before the charge, and gets pooped (heh) after the third charge, allowing you to get damage in.
  • Cute Is Evil: He's probably the cutest boss in the game, and not in an Ugly Cute way. But just like every other boss, he still wants you dead.
  • Degraded Boss: Often appears in the Womb.
  • Dung Fu: Besides being made of poop, he will also frequently summon Squirts or leave more poop laying around.
  • Nobody Poops: Averted, obviously.
  • Not-So-Innocent Whistle/Enemy Summoner: He occasionally will whistle innocently (complete with a music note hovering above his head), before one to three Squirts appear beside him to fight. This is immediately followed by a bullet spread attack.

The Haunt

http://static.tvtropes.org/pmwiki/pub/images/thehaunt.png
Debut: Rebirth
  • Bedsheet Ghost: What's underneath its sheet is quite a bit scarier, however.
  • Flying Face: That rips itself open to reveal a skull when its minions are dealt with.
  • Flunky Boss: He is surrounded by three Lil Haunts which attack before he does. First one attacks, then the other two do. He's invulnerable until they're dead and he moves to the top of the room. His black Champion version spawns spiders instead of shooting Brimstone.
  • Frickin' Laser Beams: In his second phase, he'll fire Brimstone downwards as he strafes along the top of the screen.
  • Heel–Face Turn: The Lil' Haunt item gives you a friendly Lil' Haunt who chases your enemies.
  • Intangibility: Until his minions are dead.
  • King Mook: To the Lil' Haunts.
  • No Fair Cheating: He normally paces from side to side along the top edge of the screen, shooting downwards. If you try to move to the top where he can't shoot you, he'll immediately charge at you to cause damage. Which hurts like crazy.
  • Our Ghosts are Different: This one has a skeleton. Which he'll gladly show to you.
  • The Pawns Go First: Sends his Lil' Haunt minions to attack you while he himself remains intangible. When they go down, then he fights you.

Rag Man

http://static.tvtropes.org/pmwiki/pub/images/ragman.png
Debut: Afterbirth
  • Alien Blood: Unlike all other enemies in the game, his blood is purple.
  • Flunky Boss: He'll occasionally detach his head. Not only does the head move and attack independently from his main body, but if it isn't destroyed quickly, it turns into a Ragling.
  • Heal Thyself: Aside from the regrowing heads thing, Rag Man possesses the unique ability to revive the Raglings created from his heads. This also restores a small amount of his HP.
  • Losing Your Head: Detaching his head doesn't seem to bother him too much, especially since it turns into a Ragling and he can simply grow a new one.
  • Mummy
  • Psychic Powers: His primary attacks are homing psychic blasts.

Little Horn

http://static.tvtropes.org/pmwiki/pub/images/littlehorn.png
Debut: Afterbirth
  • "Get Back Here!" Boss: Spends a lot of the fight teleporting away from the player, often leaving a Troll Bomb behind.It also flees from the player if approached.
  • Homing Projectile: It fires large homing projectiles that will track you forever until you destroy them with your own attacks.
  • Portable Hole: Can make these to teleport itself around the room, teleport Troll Bombs around the room, or to send straight at you so that you will fall in and take damage.
  • Screw This, I'm Outta Here!: When his health gets depleted, instead of dying, he retreats into one of his portable holes.
  • Tactical Suicide Boss: Has a tendency of teleporting into its own Troll Bombs. The boss room also tends to have fires, into which Little Horn will run if you herd him in that direction.
  • Thinking Up Portals: Can open up two holes that are linked to each other, either to teleport itself around the room or as a lead up to the following trope.
  • Throw Down the Bomblet: Attacks with Troll Bombs by opening a pair of portals between his location and the player and throwing one in.

Turdlings

http://static.tvtropes.org/pmwiki/pub/images/turdlings.png
Debut: Afterbirth
  • Bullfight Boss: They'll charge you if you get in their line of sight.
  • Creepy Twins: When fought as a pair. Identical and dangerous.
  • Dual Boss: They appear as a pair.
  • Dung Fu: Leave behind brown creep when they charge.
  • Enfant Terrible: Like Gurglings, they're very young stages of Gurdy.
  • Flunky Boss: One of the major differences between them and the Gurglings is that they spawn several Corn Dips after their charge attack.
  • Mix-and-Match Critters: Essentially a combination of Gurglings and Dingle.
  • Palette Swap: A poop-themed version of the Gurglings, possessing the same attacks.

Dangle

http://static.tvtropes.org/pmwiki/pub/images/dangle.png
Debut: Afterbirth
  • Palette Swap: Of Dingle, possessing the same attacks but with slightly different properties.

    Caves, Catacombs, Flooded Caves 

Gurdy

http://static.tvtropes.org/pmwiki/pub/images/gurdy.png
Debut: The Binding of Isaac
  • Body Horror: She's what can only be described as a mountain of flesh and organs, with a smiling face at the top.
  • Damage-Sponge Boss: The bulkiest boss in the Caves, having more health than some of the later bosses.
  • Degraded Boss: Interestingly, the weaker version attacks very differently, charging and firing circular waves of bullets. Contrast with the other bosses made into minibosses, who are more or less identical to their original version, but weaker. Wrath of the Lamb separates the degraded form into a different boss: Gurdy Jr. But The Very Definitely Final Dungeon DOES feature Gurdy as a normal Degraded Boss.
  • Flunky Boss: Spawns Flies and Boils to assist her. Her Champion version only spawns enemies and never fires at you.
  • Stationary Boss: She always stays in the middle of the room. Even if you push her away with a fear effect, she'll move back into position when it wears off.
  • Wake-Up Call Boss: When it comes to the pre-Wrath of the Lamb bosses who gave new players trouble, Gurdy is the most infamous.

Peep

http://static.tvtropes.org/pmwiki/pub/images/peep.png
Debut: The Binding of Isaac

Chub

http://static.tvtropes.org/pmwiki/pub/images/chub.png
Debut: The Binding of Isaac

C.H.A.D.

http://static.tvtropes.org/pmwiki/pub/images/chad18.png
Debut: The Binding of Isaac
  • Bonus Boss: Occasionally replaces Chub once unlocked.
  • The Cameo/Mythology Gag: C.H.A.D. is a boss from Super Meat Boy. Dropping the item Little C.H.A.D. on defeat references his post-fight cutscene from the above.
  • Heel–Face Turn: The Little C.H.A.D item gives you a friendly C.H.A.D. who "gives kisses" (half red hearts) every few rooms.
  • Obliviously Evil: C.H.A.D. is charging at you not out of spite, but because he wants a hug.
  • Turns Red: As C.H.A.D. takes damage, he gets smaller and faster but no less dangerous.

The Husk

http://static.tvtropes.org/pmwiki/pub/images/thehusk.png
Debut: Wrath of the Lamb
  • Back from the Dead: The Duke of Flies' rotted corpse.
  • Flunky Boss: Spawns even MORE flies than the Duke does, with some spiders thrown in for good measure.
  • Rush Boss: Like his weaker counterpart, he has low health. Unlike his weaker counterpart, he can overwhelm you with flies if he has the chance.

The Hollow

http://static.tvtropes.org/pmwiki/pub/images/thehollow.png
Debut: Wrath of the Lamb
  • Back from the Dead: Larry Jr. devoid of blood with swollen eyes.
  • Money Spider: The gold champion variant, which drops coins when its segments are destroyed.

The Wretched

http://static.tvtropes.org/pmwiki/pub/images/thewretched.png
Debut: Wrath of the Lamb

Dark One

http://static.tvtropes.org/pmwiki/pub/images/thedarkone.png
Debut: Rebirth
  • Blackout Basement: One of his attacks is to make the entire screen dark and hide himself in the shadows.
  • Bullfight Boss: He has a charge attack from one side of the screen to the other which angles at the player, similar to the charge attacks of Famine and War but harder to dodge.
  • Degraded Boss: In Sheol.
  • Frickin' Laser Beams: Will shoot Brimstone straight down if you stand below him.
  • Offscreen Teleportation: He can dash past the edges of the screen like the Harbingers.

Mega Fatty

http://static.tvtropes.org/pmwiki/pub/images/megafatty.png
Debut: Rebirth
  • Acrofatic: Normally slow and fat, but it has a leap attack which is executed in under a second.
  • Damage-Sponge Boss: Noticeably soaks up more punishment than most normal bosses.
  • King Mook: Of the regular Fatty enemy.
  • Vacuum Mouth: One of his attacks is to suck everything in the room toward him, mimicking the effect of the item Magneto.
  • Vomit Indiscretion Shot/Waterfall Puke: After the vacuum attack, he vomits up a barrage of projectiles that cover a portion of the screen.

Mega Maw

http://static.tvtropes.org/pmwiki/pub/images/megamaw.png
Debut: Rebirth

Polycephalus

http://static.tvtropes.org/pmwiki/pub/images/polycephalus.png
Debut: Rebirth

The Stain

http://static.tvtropes.org/pmwiki/pub/images/thestain.png
Debut: Afterbirth
  • Back from the Dead: Polycephalus reduced to bones and bits of connective tissue.
  • Bullet Hell: Fires a barrage of bullets in all directions around itself, often several times in succession.
  • Combat Tentacles: Has a pair of exposed blood vessels poking out from its sides, which it uses as such.
  • Flunky Boss: Occasionally spawns a pair of Chargers.
  • The Last of These Is Not Like the Others: Unlike every single other undead or upgraded version of a boss, who appear 1 floor later at least, The Stain is on the same floor as his living version.

The Frail

http://static.tvtropes.org/pmwiki/pub/images/thefrail.png
Debut: Afterbirth
  • Artistic License – Biology: Tapeworms don't normally have skeletons. Of course, this makes about as much sense as anything else in the game.
  • Back from the Dead: Pin has been reduced to a rotting undead corpse. And once you kill that, you have to deal with Dem Bones.
  • Glowing Eyes of Doom: While charging and firing its Brimstone beam in its second form.
  • Rush Boss: Even with multiple health bars, the Frail goes down quickly, but once he enters his second form, he can deal quite a bit of damage with his split shots and Brimstone lasers.
  • Sequential Boss: One of the few bosses in the game with multiple health bars, and the only one that isn't a Final Boss.
  • Took a Level in Badass: In life, Pin was one of the easiest bosses in the game. As the Frail, it's surprisingly tough.
  • Turns Red: Once its health is depleted the first time, it instantly undergoes an Adrenaline Makeover, regains full heath, and becomes faster, more aggressive, and gains several Brimstone attacks.

The Forsaken

http://static.tvtropes.org/pmwiki/pub/images/theforsaken.png
Debut: Afterbirth
  • Back from the Dead: Technically, the Haunt already was dead, being a ghost and all. So whatever state you put him in when you won his boss battle, he's back from it.
  • The Coats Are Off: The Haunt takes off his cloak/skin/ectoplasm/whatever, showing his full skeletal form instead of only his skull.
  • Dem Bones: Ever wanted to know what a Bedsheet Ghost's skeleton looked like?
  • Flunky Boss: Spawns Bonies during the fight, and just like his previous incarnation, he'll go intangible if he has any minions to fight in his place.
  • Frickin' Laser Beams: Can fire Brimstone beams in three directions, and he can rotate the beams. Before doing this, he will always turn intangible and run away from you, so make sure you herd him towards the middle of the room, or else this attack may be impossible to dodge.
  • Glowing Eyes of Doom: While firing his Brimstone attack.
  • Intangibility: Will use this to flee from you so that he can fire his Brimstone attack.

Rag Mega

http://static.tvtropes.org/pmwiki/pub/images/boss_rag_mega.png
Debut: Afterbirth+
  • Composite Character: He's a combination of Mega Maw and Rag Man, with a bit of The Haunt thrown in for good measure.
  • Detachment Combat: His orbitals will break off and attempt to chase you when his health gets low.
  • Frickin' Laser Beams: Fires 3 purple beams from his orbitals.
  • Glass Cannon: He has relatively low health, but his orbitals block shots and can hurt you, he can shield himself inside his rags, and he has a projectile version of Rag Man's resurrection move. You'll spend most of the fight just trying to land a hit.
  • Mook Maker: Spawns Raglings throughout the fight.

Big Horn

http://static.tvtropes.org/pmwiki/pub/images/boss_big_horn.png
Debut: Afterbirth+
  • Damage-Sponge Boss: Not as tanky as Mega Fatty, but still on the high end. His constant teleporting doesn't help.
  • Degraded Boss: Can summon a significantly weaker version of Little Horn during the fight.
  • Feed It a Bomb: When his mouth opens, you can kick a bomb into it. This will stun him for a bit and cause him to take 20% more damage.
  • Flunky Boss: Occasionally summons Little Horn, though thankfully he doesn't have nearly as much health as he did as a stand alone boss.
  • "Get Back Here!" Boss: Ducks underground constantly, then reappears somewhere else in the room.
  • Portable Hole: He creates two or three holes around the room as his main attack, which either expand to become pitfalls, or his hands will stick out and either throw bombs or chase the player. His whole body is also sticking out of one.
  • Throw Down the Bomblet: One of his attacks has him throw several bombs at the player. These can be a few troll bombs, a hot bomb, or even a mega troll bomb.

    Depths, Necropolis, Dank Depths 

Loki

http://static.tvtropes.org/pmwiki/pub/images/loki.png
Debut: The Binding of Isaac
  • Hoist by His Own Petard: If you can kill his Boom Flies as soon as they spawn, they will blow up on him.
  • Flunky Boss: Summons Boom Flies periodically.
  • Heel–Face Turn: The Little Loki item gives you a friendly Loki who uses its four-way-bullet attack. It isn't dropped by the Loki boss, though.
  • Tactical Suicide Boss: The Boom Flies it spawns will often explode on him, given how powerful you should be by the time you fight him.
  • Teleport Spam: Frequently teleports away during battle.

Monstro II

http://static.tvtropes.org/pmwiki/pub/images/monstroii.png
Debut: The Binding of Isaac
  • Back from the Dead: He is the zombified husk of Monstro.
  • Body Horror: He's three-fourths skeleton, and the one-fourth of him left isn't looking much better.
  • Damage-Sponge Boss: Much bulkier than the other Depths bosses.
  • Degraded Boss: In Sheol and the Cathedral.
  • Flunky Boss: Summons Suckers on his second jump attack.
  • Frickin' Laser Beams: Replaces his shower of bloody tears with a Brimstone attack. He'll only fire it if you're standing directly to his left or right, though.
  • Weaksauce Weakness: In the original game, Monstro II could be locked into his Brimstone attack by standing beside him to trigger the attack, then moving to his other side to make him shoot Brimstone in the other direction, and repeating the process as necessary. It's no longer possible with his Rebirth incarnation.

Gish

http://static.tvtropes.org/pmwiki/pub/images/gish.png
Debut: The Binding of Isaac

Mask of Infamy

http://static.tvtropes.org/pmwiki/pub/images/maskofinfamy.png
Debut: Wrath of the Lamb
  • Attack Its Weak Point: The mask is invincible until you destroy its heart. Even after that, you need to hit it from behind.
  • Cat and Mouse Boss: The first phase of the fight: you chase the heart, while the mask chases you.
  • Chasing Your Tail: The second phase of the fight has you trying to attack the mask's vulnerable rear, while it continues to try and run you down.
  • Interface Screw: The only boss without a health bar in the original version, though fixed in Rebirth. The fact that it's not easy to land a shot in the last stage makes the fight look depressingly long.
  • King Mook: A beefed-up version of a standard enemy also added in Wrath of the Lamb.
  • Shout-Out: Inspired by Carmillia's mask in Castlevania.

The Bloat

http://static.tvtropes.org/pmwiki/pub/images/thebloat.png
Debut: Wrath of the Lamb
  • Ascended Meme: Its infamous reputation as a hated boss by the community is reflected in Afterbirth +'s challenge April Fools, where he's the boss on every floor.
  • Back from the Dead: What remains of Peep's bones are broken, and his eyes are swollen.
  • Combat Pragmatist: Since he fires his Brimestone lasers the moment you're to his side or in front, chances are he may well attack you the instant the battle starts.
  • Detachment Combat: Like Peep, but from the beginning.
  • Eye Scream: His eyes are covered in red growths, and aren't attached to him.
  • Frickin' Laser Beams: He'll fire dual Brimstone lasers from his eye sockets if you so much as put a toe in front of or beside him. This is one boss you'll always want to attack from the back, even more so than Mask of Infamy.

Carrion Queen

http://static.tvtropes.org/pmwiki/pub/images/carrionqueen.png
Debut: Wrath of the Lamb
  • Attack Its Weak Point: Can only be hit from behind, except when exploiting its other weak point (see Feed It a Bomb below).
  • Back from the Dead: A mostly-skeletal Chub.
  • Dung Fu: Randomly produces Red Poop that damages you if you touch it and always regenerates when it's destroyed. Also can bounce off the poop as if it's a wall.
  • Feed It a Bomb: Same as Chub, though it becomes impossible once she starts moving diagonally as she no longer charges with her mouth open.
  • Turns Red: Will bounce aimlessly around the room once her health is down to half.

The Adversary

http://static.tvtropes.org/pmwiki/pub/images/theadversary.png
Debut: Rebirth

The Cage

http://static.tvtropes.org/pmwiki/pub/images/thecage.png
Debut: Rebirth
  • Back from the Dead: He is a post-mortem version of Mega Fatty, with his head being absent (assuming it got chopped off) and his body rotting, along with a new stitch across his stomach, making him resemble a giant Vis.
  • Damage-Sponge Boss: Notably, he has more health than Mom.
  • Dishing Out Dirt: His jumping attack now creates sharp rocks like the Adversary's, but he creates lines of them that travel outwards in several directions until they hit a wall.
  • Dual Boss: The pink champion variant pits the player against two Cages.
  • Giant Mook: He's basically been turned into a Vis, making him this.
  • Mook Maker: He can spawn discoloured Vis that fire sickly gray Brimstone lasers.
  • Rolling Attack: He can curl up in a ball and roll around the floor in an attempt to hit Isaac.

The Gate

http://static.tvtropes.org/pmwiki/pub/images/thegate.png
Debut: Rebirth
  • Back from the Dead: He is a post-mortem version of Mega Maw, as the flesh from his face leaks to the lower half of his head, his skull remains animated for the time it's alive.
  • Flunky Boss: Spawns a series of Leapers as a lead-up to the next trope.
  • Frickin' Laser Beams: One of his attacks is to spawn three Leapers and then fire a Brimstone laser that connects between the three and deals a whole heart of damage.
  • Meat Puppet: When defeated, something that looks like a tiny Isaac peeks outside a crack right before expiring.

Brownie

http://static.tvtropes.org/pmwiki/pub/images/brownie.png
Debut: Afterbirth
  • Abnormal Ammo: Corn.
  • Bullet Hell: Tends to fire much larger volleys of projectiles than most bosses. Also drops piles of dung that explode into even more projectiles.
  • Flunky Boss: Spawns Dips in large numbers throughout the fight.
  • Rush Boss: Although its attacks and minions can be difficult to dodge, Brownie doesn't have a particularly large health pool. The fact that one of its attacks is essentially Cast from Hit Points doesn't help its case.
  • Tactical Suicide Boss: The exploding turds Brownie places down can not only damage it, they can also poison it. Zig-zagged in that Brownie also trails poisonous creep while poisoned.
  • Toilet Humor: How else does one describe a giant ball of poo that shoots corn?
  • Turns Red: Inverted. When its health drops low, it turns into Dangle.

Sister Vis

http://static.tvtropes.org/pmwiki/pub/images/boss_sisters_vis.png
Debut: Afterbirth+
  • Distaff Counterpart: To The Cage. They're female and look almost identical to him, except they have heads. They were also supposed to debut in Afterbirth, but were cut and replaced with Pink Champion Cage (which is, of course, two Cages).
  • Dual Boss: Usually. Can come in as many as eight at once in Greedier Mode.
  • Frickin' Laser Beams: Much like the Vis enemies, their main attack is opening up their stomach and firing blood lasers. Except they use Mega Satan's giant version and can sweep the beam left to right.
  • Giant Mook: A pair of giant Vis, although they don't really look like them.
  • Rolling Attack: Just like The Cage's.
  • The Cameo / Mythology Gag: Originally appeared as bosses in Edmund's earlier game, Gish.

    Womb, Utero, Scarred Womb 

Blastocyst

http://static.tvtropes.org/pmwiki/pub/images/blastocyst.png
Debut: The Binding of Isaac
  • Asteroids Monster: As it gets damaged, it splits into smaller blobs. The smallest blob variety leaves behind the worm-like creature inside when destroyed.
  • Blob Monster: Judging by the name, it's a gigantic, blob-like cell.
  • Bullet Hell: Can happen if too many of them are left onscreen. Since they only fire in a set direction, however, it's pretty tame compared to most other examples in the game.
  • King Mook: To the Embryos found in early stages.
  • Mega-Microbes: They're cells/developing embryos that are bigger than Isaac.
  • Weaksauce Weakness: Poison. Normally a boss would only take a bit of damage from poison as their health bars reach in the hundreds but if Blastocyst is poisoned when it divides it's instantly killed, ending a long boss fight with in a few minutes.

Scolex

http://static.tvtropes.org/pmwiki/pub/images/scolex.png
Debut: The Binding of Isaac
  • Attack Its Weak Point: You can only damage it by hitting it in the very back segment.
  • Fragile Speedster: It has the lowest health of any boss in the game, but it makes up for this with its speed and hard to hit weak point.
  • Tactical Suicide Boss: One of its attacks has it poking its vulnerable tail out of the ground for a fairly lengthy period of time before firing three Ipecac shots, giving the player a perfect opportunity to take off a lot of its health.
  • Underground Monkey: It's a much tougher version of Pin, but unlike the Back from the Dead versions of other bosses, Scolex is noticeably different in appearance from its easier version.

Daddy Long Legs

http://static.tvtropes.org/pmwiki/pub/images/daddylonglegs.png
Debut: Wrath of the Lamb
  • Easter Egg: There is an item that summons the Daddy Long Legs to stomp enemies in an area the player is standing in without hurting them. Using this against the Daddy Long Legs reskins it as the Triachnid's claw. Using it against Triachnid, however, flat-out doesn't work.
  • Giant Foot of Stomping: Yes, this game has a spider that stomps you. No, it (most likely) does not take place in Soviet Russia.
  • Giant Spider: Bigger than any other spider in the game, taking up more than the screen can show.
  • Heel–Face Turn: The Daddy Longlegs item makes him periodically stomp on your enemies for you.
  • King Mook: A giant spider, same as Widow.

Triachnid

http://static.tvtropes.org/pmwiki/pub/images/triachnid.png
Debut: Wrath of the Lamb
  • Bonus Boss: Occasionally replaces Daddy Long Legs once unlocked.
  • The Cameo/Mythology Gag: Triachnid is a reference to a previous flash game by Edmund McMillen. Furthermore, Triachnid is an alternate of Daddy Long Legs; in his own game, Triachnid was finding a home for his young.
  • Dummied Out: As hinted here, Triachnid was originally cut before the game was released. It wasn't removed entirely, though. Due to a glitch, it originally would replace the Cathedral's True Final Boss. This was eventually corrected, and Triachnid was added in as a normal boss.

Lokii

http://static.tvtropes.org/pmwiki/pub/images/lokii.png
Debut: Wrath of the Lamb
  • The Artifact: With the "Double Trouble" mechanic in Rebirth, Lokii is kind of redundant; the fact that the halves summon Red Boom Flies instead of regular Boom Flies is the only way they're different from a Double Trouble Loki fight, aside from appearing in standard 1x1 boss rooms.
  • Back from the Dead: Loki has been split in two, but both halves live on.
  • Dual Boss: He's the equivalent of fighting two Lokies at once, only with Red Boom Flies.
  • Flunky Boss: Unlike standard Loki, they summon Red Boom Flies. This gives them an advantage, because they won't be blasted in the face if their minions die near them.
  • Half the Man He Used to Be: They're Loki after being split in half, and somehow they're completely fine.
  • Teleport Spam: Just like normal Loki.
  • Visual Pun: The last two "i"s of his name have a line crossed through them on the bottom, making them resemble part of the Roman numeral II.

Teratoma

http://static.tvtropes.org/pmwiki/pub/images/teratoma.png
Debut: Wrath of the Lamb
  • Asteroids Monster: Except now filled with spiders instead of Chargers.
  • Back from the Dead: The Fistula has shriveled up and now shelters spiders.
  • Body Horror: Like Fistula, it's named after a real life Body Horror phenomenon in which the body forms a growth containing all three germ layers, one of which can potentially manifest as bone shards and teeth!

Mama Gurdy

http://static.tvtropes.org/pmwiki/pub/images/mamagurdy.png
Debut: Rebirth
  • Ascended Meme: Apparently a reference to a fan-made animation.
  • Expy: The fourth form of Gurdy to appear.
  • Spikes of Doom: She is able to create spikes around the room.
  • Wall Master: She can hide in the walls of the boss room, immune from damage.

Mr. Fred

http://static.tvtropes.org/pmwiki/pub/images/mrfred20.png
Debut: Rebirth
  • Flunky Boss: Starts the fight accompanied by four Homunculi whose bonds he can break at will, and spawns Freds throughout the fight.
  • King Mook: A giant boss version of the enemy Fred that appears in Chapter Four.
  • Stationary Boss: Never moves from the centre of the room.

    Shared 

The Fallen

http://static.tvtropes.org/pmwiki/pub/images/thefallen.png
Debut: The Binding of Isaac
  • Asteroids Monster: After about 75% of his health his gone, he splits into two versions of himself.
  • Bullfight Boss: Does this when he isn't shooting. The charge is stopped by shooting him enough times, after which he'll shoot blood in every direction.
  • Classic Video Game Screw You: It's uncommon, but this guy can be the boss of any dungeon save ones with predetermined bosses. He's notorious for showing up in the Basement/Cellar, often before you've had a chance to buff your speed and survive his charge.
  • Degraded Boss: In Sheol. He's later fought at full strength when the Satan fight begins.
  • Early-Bird Boss: He can replace any boss that's not pre-set by the level. If you're unlucky, he can appear on the first or second floor.
  • The Dragon: To Satan. He even acts as the first stage of his master's boss fight!
  • High-Pressure Blood: He shoots blood beams in all four cardinal directions at once for one of his attacks.

Krampus

http://static.tvtropes.org/pmwiki/pub/images/krampus23.png
Debut: Wrath of the Lamb
  • Bad Santa: Drops a Lump of Coal.
  • Chest Monster: Randomly appears in Devil Rooms. He's more likely to appear if you have already entered a Devil room, even more so if you've made a deal.
  • Classic Video Game Screw You: Krampus can appear in the first Devil Room, potentially invalidating any chance to see further Devil Rooms merely by being there.
  • High-Pressure Blood: Just like the Fallen, but he'll do it if you pass by him in any direction rather than after a charge. Afterbirth adds in a chance of him rotating his beams, and Afterbirth+ adds in a chance of him firing them off diagonally.
  • The Krampus: It's in the name, after all.
  • Palette Swap: Of The Fallen, except he doesn't split into two parts and is mildly weaker. He also has no charge attack, instead firing off his blood beams if you stick to any cardinal direction of him for too long. Killing him may drop the Head of Krampus, which lets you use this attack.

Uriel and Gabriel

http://static.tvtropes.org/pmwiki/pub/images/urielgabriel.png
Debut: Rebirth
Secret bosses in Rebirth, a fight with one of twonote  Angels is initiated by bombing the statue in an angel room at any point throughout the game. Beating one unlocks a piece of a key each time, and beating both and combining the keys allows the player to unlock the true final boss of the game.

  • The Blank: Even when not in statue form, they lack faces.
  • Bolt of Divine Retribution: They shoot coherent beams of holy light.
  • Color-Coded for Your Convenience: Uriel wears silver or blue, Gabriel wears red.
  • Degraded Boss: Two Fallen Angels attack you as Mega Satan's flunkies, and to get to that point you're almost required to have already killed the normal ones in this run. One interpretation of this is that they kept the pieces of the Golden Key safe, failed when Isaac took the pieces, and were subsequently punished for it.
  • Dismantled MacGuffin: They each hold a piece of the key required to open the Gold Gate.
  • Guide Dang It: Bomb the angel statue in an Angel Room to provoke it into a fight. Less aggravating than other examples since there are more in-game clues and it is often next to blue fires, which can be bombed for soul hearts, so the player might figure this out by mistake if nothing else.
  • Hidden in Plain Sight: A fight can be initiated at any point in the game, provided you have beaten either ??? or The Lamb.
  • The Last of These Is Not Like the Others: You can fight a different-colored Uriel after beating him and Gabriel, but it doesn't drop a key piece.
  • Palette Swap: Each one has different colored rags.
  • Personal Space Invader: Unlike Uriel, Gabriel likes to fly directly at Isaac to deal contact damage.
  • Winged Humanoid: Albeit without a face.

    The Harbingers 

In General

The four Horsemen of the Apocalypse, unlocked after beating Mom for the first time. Famine, Pestilence, War, and Death each have the ability of replacing one of the bosses of the Basement, Caves, Depths, and Womb respectively, while the Headless Horseman has the capability to replace any of these bosses, and Conquest was introduced as an alternate boss to Death.
  • Degraded Boss: In later stages.
  • Horsemen of the Apocalypse: Each boss is based on a separate horseman.
  • Offscreen Teleportation: Most of them charge against you through the screen.
  • Quirky Miniboss Squad: Despite being full bosses in their own right and fought one at a time, they still fit the bill. Their item drops, the Cube of Meat and the Ball of Bandages, become more powerful by collecting more Cubes or Balls from multiple Horsemen, they can be summoned by the Book of Revelations, multiple Horsemen will appear at the same time as normal enemies in later levels, and they're easily recognizable.
  • Sequential Boss: With two stages. Except Conquest and the Headless Horseman, the latter being a Dual Boss.
  • Uncatty Resemblance: Each of the Horsemen's horses look similar to their rider.

Famine

http://static.tvtropes.org/pmwiki/pub/images/famine.png
Debut: The Binding of Isaac

Pestilence

http://static.tvtropes.org/pmwiki/pub/images/boi_pestilence_5.png
Debut: The Binding of Isaac
  • Body Horror: No eyes, rotting body with swollen parts, filled with explosive, possibly-toxic fluids, and surrounded by maggots and flies.
  • Green Around the Gills: His entire body is sickly green, swollen, and leaking some sort of disease-ridden fluid. So is his horse.
  • Incurable Cough of Death: His hoarse coughs summon flies.
  • Losing Your Head: Only the body (still riding the horse) remains after the first part.
  • Mook Maker: Maggots at first, Flies later.
  • Walking Wasteland: As the personification of disease and toxin, he leaves a trail of creep everywhere he moves.

War

http://static.tvtropes.org/pmwiki/pub/images/war.png
Debut: The Binding of Isaac
  • Bullfight Boss: After losing his horse.
  • Bull Seeing Red: He's red, and his horse has bull horns and charges.
  • Mad Bomber: One of his attacks drops bombs randomly.
  • Oh, Crap!: After losing his horse, hit him enough times and he'll fall to the ground stunned, with a terrified look on his face.

Death

http://static.tvtropes.org/pmwiki/pub/images/death.png
Debut: The Binding of Isaac
  • Death's Hourglass: A literal example, in that one of his first stage's attacks is to use the Hourglass item on Isaac.
  • Dual Boss: He hops off his ride on his second stage, which goes on ramming aimlessly while he hovers around spawning Knights.
  • The Grim Reaper: The skeletal kind, complete with black cloak and scythes.
  • Mook Maker: Creates Knights.
  • Shout-Out: One of his attacks is very similar to one that a certain other Death you may know of uses.
  • Sinister Scythe: He can summon flying, spinning scythes to attack.
  • Visual Pun: His champion form is black. Reinforced by the enemies summoned by the champion version: the Red Maws summoned by the first form reference the swelling caused by the disease, while the Kamikaze Leeches refer to the use of leeches as a treatment for the blood build-up.

The Headless Horseman

http://static.tvtropes.org/pmwiki/pub/images/headlesshorseman.png
Debut: The Binding of Isaac
  • All Your Powers Combined: The head has Famine's triple shot and War's triple charge, while the body is Pestilence minus spawning enemies. Plus, you fight two bosses at the same time, which is what happens when Death loses half his health.
  • Classic Video Game Screw You: He can replace any horseman. Are you on the Womb/Utero 1, with Book of Revelations and Level 3 Meat Boy or Bandage Girl, hoping to finally get the corresponding achievement for Level 4? Too bad, have a pony instead.
  • Degraded Boss: The head alone can appear in normal rooms once you reach the Depths. The body can as well starting from Sheol/Cathedral.
  • Dual Boss: The boss is split into: his head, that charges and shoots small bullets, and his body, which shoots large bombs and leaves trails of blood.
  • Headless Horseman: Well, yeah.

Conquest

http://static.tvtropes.org/pmwiki/pub/images/conquest.png
Debut: Wrath of the Lamb
  • Be Careful What You Wish For: Some players complained that War was "too easy" for a boss in the Depths.
  • Bolt of Divine Retribution: One of only two bosses in the game to summon beams of light from on high.
  • Classic Video Game Screw You: Like the Headless Horseman, he can replace other Horsemen (although he will only replace War or Death in vanilla Isaac, or just Death in Rebirth), dicking players out of getting the Meat Boy/Bandage Girl achievements countless times.
  • Continuity Nod/Genius Bonus: invoked The secret calls him "A Forgotten Horseman", reflecting the fact that he's been replaced by Pestilence both in the game and in nearly every other version of the Horsemen of the Apocalypse. Also, it was replaced because it was too similar to War. Conquest has improved versions of War's attacks.
  • Doppelgänger Attack: He can charge across the screen alongside five or so clones.
  • Eyepatch of Power: A full blindfold in his case.
  • Holy Hand Grenade: His beam attack.
  • Light Is Not Good: As a Horseman of the Apocalypse, this is a given.
  • Missing Secret/Dummied Out: Weird variation. Conquest does not need to be specifically unlocked, but there is an unused picture for unlocking him. Subverted in Rebirth, where taking ten angel room items unlocks him.
  • Sixth Ranger: An additional horseman added later. Technically, the original passage from the Book of Revelations likely referred to Conquest, not Pestilence, but most modern usage of the horsemen ignore Conquest.

    The Seven Deadly Sins 
Debut: The Binding of Isaac
Debut (Super variants and Ultra Pride): Wrath of the Lamb

In General

  • Ambiguously Human: It's not clear if they are humans under the effect of various itemsnote , humanoid monsters from the start, or the actual Sins themselves given physical embodiment. The implication that every character is an alternate Isaac as he only imagined the events of the gameplay just complicates things further.
  • Anthropomorphic Personification: Of their respective sins.
  • Degraded Boss: Like most of the bosses, they can be found in regular rooms towards the end chapters of a run. However, they will still have specific "miniboss rooms" complete with the music and "Vs" text appearing at the top of the screen in chapter 5.
  • King Mook: The "Super" variants introduced in Wrath of the Lamb.
  • Inescapable Ambush: Unless you hold certain items, they won't show up on the map at all until you run into them, and you cannot leave unless you carry a teleportation item. Their rooms also have metal bars around the doors so that you cannot even bomb your way out as you could with normal enemy rooms.
  • Quirky Miniboss Squad: They fight you individually, but in any of the levels following the fourth chapter, you may encounter up to three different Sins to fight in the same room.

Envy/Super Envy

http://static.tvtropes.org/pmwiki/pub/images/envy.png
  • Asteroids Monster: When defeated, he splits into two heads, which each split into two smaller heads, which finally each split into two tiny heads. Super Envy splits into threes instead of twos, except for the smallest heads.
  • Damage-Sponge Boss: To make up for the fact that he is easily the least dangerous Sin in either form, he has more aggregate health than the other Sins.

Gluttony/Super Gluttony

http://static.tvtropes.org/pmwiki/pub/images/gluttony21.png

Greed/Super Greed

http://static.tvtropes.org/pmwiki/pub/images/greed9.png
  • Bandit Mook: His shots (and mere contact with him!) take away a couple of pennies from you, while making you drop a variable amount on the floor that you can try to retrieve. Just be careful while doing so, if you don't want to be killed in the process. Oh! And guess what? His mook-look-alikes have the same ability as him. Yay!
  • Breakout Character: He's the most memorable and ubiquitous of the The Seven Deadly Sins mini-bosses, and Wrath of the Lamb cemented his status by adding a quite a few new versions of him. And Afterbirth makes Ultra Greed the final boss of Greed Mode.
  • Character Blog: The Afterbirth Alternate Reality Game gave him a Twitter. On the other hand, most of the entries were made by the players, who completed the ARG by logging into the account and posting tweets.
  • Chest Monster: Replaces shops and secret rooms. He's more likely to appear as you go further in the game.
  • Expy: He shares some traits with Spelunky's Shopkeeper, including his multi-shot ability, potential to turn hostile on the player without previous warning, and overall dickishness.
  • Flunky Boss: Normal Greed summons Hoppers, Super Greed summons Keepers (which are Greed heads).
  • King Mook: Wrath of the Lamb added Super Greed, who is bigger, meaner, scarier, and can spawn Greed-heads. Said Greed-heads also appear in the Depths, the Catacombs, and the Necropolis. Also, the final boss of Afterbirth's Greed Mode is Ultra Greed.
  • Mook Promotion: Inverted. The game's standard version features him as a mini-boss, but the expansion added mook-versions of him that share some of his most aggravating attributes.note 
  • The Scrappy: Invoked. Given how he can replace shops and secret rooms, can knock out your money when he hits you, and can be fought multiple times in one run, players usually don't enjoy finding him. You'd be hard-pressed to find anything in the game that will make you madder than fighting Greed in the Depths/Necropolis 2's shop or a Secret Room later and having him drop the Steam Sale, when you have no use for it by that point in the game. He becomes an Ensemble Darkhorse if you have the D20.
  • Shopkeeper: When he's not out for your blood, anyway.
  • Undead Child: If the Alternate Reality Game is canon, he may be Isaac's corpse.
  • Wingding Eyes: Super Greed either has cent signs for eyes or actual coins jammed in his eye holes.

Lust/Super Lust

http://static.tvtropes.org/pmwiki/pub/images/lust.png
  • Badass Normal: Pride can spawn bombs from nowhere and either has electrical powers or can stretch out tentacles. Greed can sap money away from Isaac through collision or even bullet contact and can spawn monsters. Lust, however, just seems to be a fast girl with a harmful virus. Super Lust averts this by having a trail of harmful blood that doesn't damage her, which is not "normal" by any standard.
  • Body Horror: Super Lust's horribly deformed face.
  • Bullfight Boss: She does nothing but charge at you. Thankfully, she can't turn on a dime and there are plenty of rocks around to kill her momentum.
  • The Smurfette Principle: The only one of the Sins/Super Sins that has been identified as female.

Pride/Super Pride

http://static.tvtropes.org/pmwiki/pub/images/pride.png
  • Combat Tentacles: It's either this or Shock and Awe.
  • Mad Bomber: Occasionally, Pride randomly spawns troll bombs in an attempt to catch you in the blast.
  • Palette Swap: In Wrath of the Lamb, Super Pride's appearance was just that of Pride, but larger and pink instead of gray. In Rebirth, the face is slightly different, but he still resembles a giant, pink Pride.
  • Shock and Awe: Both forms of Pride can do this.

Sloth/Super Sloth

http://static.tvtropes.org/pmwiki/pub/images/sloth11.png

Wrath/Super Wrath

http://static.tvtropes.org/pmwiki/pub/images/wrath.png
  • Cartoon Bomb: His bombs are identical to yours, skull marking and all.
  • Expy: Of Bomberman.
  • Hoist by His Own Petard: Wrath's bombs can be pushed around like player-dropped bombs, allowing you to push them back towards him for a nice chunk of damage. This doesn't work on Super Wrath's troll bombs, though. Even more amusingly, any items that affect bombs will affect the bombs dropped by Wrath/Super Wrath. This can result in hilarious situations where, if you have the Bobby-Bomb collectible, every bomb the boss drops proceeds to walk back toward him.
  • Mad Bomber: Throws bomb after bomb at you. Since he's Wrath, this doubles as a Stealth Pun.
  • Tactical Suicide Boss: Probably the finest example in the game, as he has exactly one attack which can easily blow up in his face. Less so with Super Wrath as he throws Mega Troll Bombs, which follow the player around and cannot be pushed. However, these can potentially be lured into him.

Ultra Pride

http://static.tvtropes.org/pmwiki/pub/images/ultrapride.png
  • Creator Cameo: He's a stylized representation of artist Edmund McMillen, while his familiar is one for programmer Florian Himsl. Hence why he's named Ultra Pride.
  • Death by Cameo: You do fight them, after all.
  • Dual Boss: He comes with his familiar, and both have to be defeated.
  • Meaningful Name: The creators are ultra proud of their product. Additionally, putting yourself as a cameo in your own game is a flagrant show of pride.
  • Palette Swap: He's basically Sloth with different moves, while the familiar is a graphically altered Baby with a different shot type.

    Final Bosses 

Yes, bosses. At first, you end the playthrough in Depths II and kill Mom, but as you beat the game repeatedly, you unlock more chapters, each with a Final Boss bigger and tougher than before.

All spoilers are unmarked from here on out. Proceed at your own caution!

Mom

http://static.tvtropes.org/pmwiki/pub/images/momleg.png
Debut: The Binding of Isaac
At the end of the Depths, Isaac finally confronts his mother. As her foot stomps down on Isaac from above, she looks and sends her hands through the doors of the boss room while spawning enemies. But even if Isaac defeats Mom, it's not the end.
  • Attack of the 50-Foot Whatever: Her boss fight.
  • Ax-Crazy: Her goal is to kill her own son, and she doesn't sound too upset about it during the fight, what with the laughing and all.
  • Body Horror: The way Mom's body parts move throughout the fight (such as both eyes appearing on opposite sides of the room while she tries to stomp on you) suggest an ability to contort herself in rather...impossible ways.
  • Boss Banter: ISAAAAAC!
  • Death by Irony: Crushed by her own bible!
  • Disc-One Final Boss: Demoted to this on your second playthrough and beyond.
  • Evil Is Visceral: Likely the reason her body parts are detailed and somewhat deformed.
  • Evil Laugh: She lets out one every time she successfully damages Isaac.
  • The Faceless: Her entire face is never shown at any point in the main game (the intro and epilogue don't count, as they're Isaac's cartoon drawings). This is subverted after you beat the Chest in WOTL or Rebirth, but only briefly. In photographs, to be precise.
  • Fan Disservice: When Isaac uses the Mom's Bra item, a picture of his mother's shirtless torso flashes on the screen. A very unflattering picture of his mother's less-than-fit shirtless torso.
  • Fat Bastard: As shown by her giant leg and the picture that shows up when using Mom's Bra.
  • Final Boss: In your first playthrough. She's nowhere near the True Final Boss, though.
  • Flunky Boss: She'll summon groups of random enemies to battle Isaac
  • Gameplay and Story Integration: Using the Bible instantly defeats her in her boss fight.
  • Giant Foot of Stomping: Her main method of direct attack.
  • Gonk: See that leg up above? That's all the proof you need.
  • Gross-Up Close-Up: One of her leg in the boss portrait, and one of her body in the picture that appears when using Mom's Bra.
  • Humanoid Abomination: Most of what she does in-game and in a few cutscenes blatantly disregards normal body movements if not physics. She can step on her own eye, generally alter size, and her arm can fit in a chest inside her heart inside her womb
  • Leitmotif: Thine Wrath... in the original, Matricide in Rebirth.
  • Mook Maker
  • Mother of a Thousand Young: If you go by the theory that the majority of the enemies are her former children.
  • Say My Name: ISAAC!
  • Skippable Boss: Provided you have the Bible on you by the time you reach her, and it's fully charged.
  • Tactical Suicide Boss: It's not actually mandatory, but making her stomp herself is possible, and deals lots of damage.
  • Womb Level/Bigger on the Inside: After defeating her, you will unlock her womb as a playable level.
  • Your Size May Vary: Just how big is she? In the cutscenes, she's a normal-sized human being, but her underwear is the right size to be worn by a child visibly smaller than her, in the boss fight her leg is the length of the screen, and then her womb is big enough to have two entire levels in it, with her heart being larger than Isaac.

Mom's Heart/It Lives!

http://static.tvtropes.org/pmwiki/pub/images/momsheart.png
http://static.tvtropes.org/pmwiki/pub/images/itlives.png
Debut: The Binding of Isaac
Defeating Mom once unlocks the Womb in subsequent playthroughs. At the end of the Womb lies Mom's Heart, which, summons enemies like Mom does. Defeating Mom's Heart reveals a Chest with something special inside. But once it's beaten enough times, it will become "It Lives!", which is Isaac as a fetus for some odd reason.
  • Boss Banter/Say My Name: Has the same voice lines as Mom.
  • Boss Rush: "It Lives!" has the ability to spawn Degraded Bosses such as Monstro, Larry Jr., and Chub to fight you alongside the enemies.
  • Bullet Hell: Rebirth's version shoots red tears nearly constantly throughout the whole fight, even when retreating to the back of the room.
  • The Bus Came Back: The original Mom's Heart form, to an extent, in Afterbirth +. While unlocking It Lives used to make it so that you can no longer fight Mom's Heart, Afterbirth + lets you fight Mom's Heart again in the Void, likely so that the Bestiary is completable for those who already unlocked It Lives.
  • Damage-Sponge Boss: Its lack of mobility is made up for by a big health bar. Its Eternal version in the Eternal Edition has even more.
  • Enemy Without/Fetus Terrible: "It Lives!" is not just any embryo, it has been confirmed by Word of God to be Isaac's embryo.
  • Flunky Boss: Will summon enemies intermittently. "It Lives!" ups the ante by spawning Degraded Bosses.
  • "Get Back Here!" Boss: Despite not moving, it accomplishes this by occasionally retreating above the screen and leaving you to deal with its flunkies for a while.
  • Leitmotif: ...Be Done in the original, Ventricide in Rebirth.
  • Mook Maker: Hides at the back of the room while it spawns normal enemies to fight you, then jumps back to the centre to spawn more.
  • Not Quite Dead: It seems that Mom survived their fight, or at least her heart did.
  • Skippable Boss: Using the Bible instantly kills it like it did with Mom.
  • Stationary Boss:
    • In the main game, ll it can do movement-wise is pull itself upwards to hide from attacks, making bombs very useful against it.
    • The Eternal form in the Eternal Edition averts this greatly; it will pursue you relentlessly around the room until it dies, still spawning enemies.
  • Tactical Suicide Boss: In Rebirth, it moves to the top of the screen instead of disappearing entirely. Anything that wanders up there, enemies included, takes damage.
  • True Final Boss: Was initially this to Mom's Final Boss. However, this is subverted by the Halloween update, and further subverted by Wrath of the Lamb and Rebirth, to the point that it's no longer anywhere near the real final boss.

Satan

http://static.tvtropes.org/pmwiki/pub/images/satan.png
Debut: The Binding of Isaac
The original True Final Boss (although this isn't true as of Wrath of the Lamb and Rebirth), Satan himself is the boss of what was originally the game's secret floor. Despite offering Deals with the Devil throughout the game, he turns on Isaac and tries to kill him. His boss fight is comprised of three phases, detailed below under Sequential Boss.
  • Attack of the 50-Foot Whatever: His final form is him flying far above the screen and stomping on Isaac with giant feet a la the Mom boss fight.
  • Black Speech: During the fight with him, he sounds somewhat like this. They are actually Mom's quotes, played backwards and in a much lower tone.
  • Bigger Bad: He gives Isaac all of these cool items, then deceives him in Sheol. Not to mention he is (or was) the True Final Boss via Mega Satan.
  • Deal with the Devil: He offers them throughout the game before you reach Sheol. Possibly even as you reach Sheol or after, if you have a Joker card on hand.
  • Evil Laugh: Since he uses Mom's voice clips, he can do this.
  • Flunky Boss: His first form is spawning the Fallen to fight you instead. His third phase continuously spawns Kamikaze Leeches.
  • Frickin' Laser Beams: His second phase can fire dual Brimstone lasers from his hands (that can curve), or a single one from his mouth.
  • Giant Foot of Stomping: His third phase has him attempting to crush Isaac with his giant hooves. They work like Mom's, but he does it much faster.
  • Leitmotif: Enmity of the Dark Lord in the original, Hericide in Rebirth.
  • Living Statue: His second form is a unique boss where the statue of Satan comes to life, grows much larger, and attacks Isaac with tears and lasers.
  • The Man Behind the Man: Implied; after all, he did at the very least string Isaac along through the Deals he makes in the devil rooms earlier on, before turning on Isaac at the last minute.
  • No-Sell/Press X to Die: Using the Bible on him, which killed Mom and Mom's Heart instantly, kills you instantly instead. The same applies to later bosses, too.
  • One-Winged Angel/Sequential Boss: Has three forms (see Flunky Boss, Living Statue, and Attack of the 50-Foot Whatever respectively).
  • Satan: It's in his name, and he looks the part as well.
  • Say My Name: Satan's battle cry when you defeat his first phase is Mom's Boss Banter, only lower pitched and in reverse.
  • Sequential Boss: Three phases: first a fight with The Fallen, then a fight with Satan himself on the ground with you, and finally a fight against his giant stomping legs.
  • Tactical Suicide Boss: Those leeches he spawns don't just hurt Isaac.
  • Theme Music Power-Up: When you fight the Fallen, his first phase, the regular boss theme plays. When Satan actually starts fighting, however, Hericide kicks in.
  • True Final Boss: He was the final boss of the game in the Halloween update, but this has been since trumped by Wrath of the Lamb and Rebirth.

Isaac

http://static.tvtropes.org/pmwiki/pub/images/isaacboss.png
Debut: Wrath of the Lamb
After beating It Lives, the Cathedral is unlocked. At the end of the Cathedral resides Isaac himself... and one of the toughest challenges in the game.
  • Ascend to a Higher Plane of Existence: Appears to do this upon his defeat.
  • Bald of Evil: Justified, being a young child and all.
  • Beam Spam: One of his attacks in Rebirth has him covering most of the room with beams of light, except for a couple safe spots in the corners. His third phase in old versions of Wrath of the Lamb had no attacks other than rapidly dropping the beams on random spots in the room.
  • Bolt of Divine Retribution: Like Conquest, he can summon several pillars of light. The safe spot is in either corner of the room perpendicular to the direction the pillars are spawning.
  • Bullet Hell: Fires bullets in all directions, and fires at you if you hit him. Having a high-fire rate can backfire horribly.
  • Children Forced to Kill: A subtle example. While it's easy to miss, Isaac doesn't attack at all until he takes damage.
  • Counter Attack: In addition to the projectiles he fires periodically, he also fires at you whenever you damage him. In Rebirth, he constantly fires in your general direction when not using another attack, instead of only doing so when hit.
  • Damage-Sponge Boss: Later updates in Wrath of the Lamb made him this. Rebirth instead adds more Bullet Hell, and makes him mobile in his third form.
  • Enemy Without: Maybe.
  • Evil Counterpart/Good Counterpart:
    • He seems to be the latter to Satan. He appears in a Cathedral instead of Sheol, summons angels instead of demons, and has an angelic appearance.
    • He could qualify as either to the Player Character. The fight against him is most likely more symbolic than literal anyway.
  • Flunky Boss: Spawns Angelic Babies, which is fitting.
  • Go Out with a Smile: Well, he is smiling in his third phase.
  • Holy Hand Grenade: He can cover the screen in rays of deadly light.
  • Leitmotif: My Innermost Apocalypse in the original, Infanticide in Rebirth.
  • Messianic Archetype: His One-Winged Angel.
  • Mirror Match: He is Isaac. Note that he's not a Mirror Boss, as he can do things you'll never be able to pull off.
  • Power Gives You Wings: In his final form. An item called Fate can be unlocked, allowing the player to have those same wings.
  • Purposely Overpowered: You know how almost all the items added to Wrath of the Lamb are much more powerful than the ones in the base game? There's a reason for that.
  • Sequential Boss: Three phases: one lying in the fetal position, one standing up, and one with wings and teleportation.
  • Stationary Boss: For the original game and the first two phases in Rebirth.
  • Troubled Fetal Position: The first part of the fight.

???

http://static.tvtropes.org/pmwiki/pub/images/bluebabyboss.png
Debut: Wrath of the Lamb
Completing the Cathedral with the Polaroid item allows Isaac to enter The Chest. At the end of this super-secret stage lies...the Blue Baby?
  • Back from the Dead: Is a post-mortem version of Isaac, naturally.
  • The Cameo/Mythology Gag: From Edmund McMillen's flash series Dead Babies.
  • Flunky Boss: One way he varies from the previous boss is that he summons flies in addition to his other attacks.
  • Leitmotif: In Rebirth, in order to further differentiate ??? from the preceding boss, he's given his own track, Ascension.
  • Invincible Minor Minion: In Rebirth, he gets a halo of flies, which block your tears.
  • Palette Swap: He is very similar to the previous boss, Isaac, so most of the tropes above apply to him too. This resemblance is downplayed in Rebirth by giving him different attack patterns from Isaac.
  • Teleport Spam: Utilized in his third phase in Rebirth, with a tendency to reappear right on top of the player.
  • True Final Boss: Of Wrath of the Lamb, being the boss of the only sixth chapter in the game, and since beating him unlocked the game's real ending. Subverted in Rebirth.
  • Weaponized Teleportation: Tries to teleport into the player during his third phase in Rebirth.

The Lamb

http://static.tvtropes.org/pmwiki/pub/images/lamb.png
Debut: Rebirth
Defeating Satan and completing Sheol with the Negative item allows Isaac to enter The Dark Room, a dark reflection of The Chest. The skeletal demon known as the Lamb is the boss of this stage, and it fights with powerful attacks.
  • All Your Powers Combined / Confusion Fu: The Lamb's volleys of shots mirror those of previous bosses. This, combined with how it switches the type of bullet with each volley, and that all of its bullets are the same color, make it a very difficult opponent to predict.
    • Shadow Archetype: The Lamb's attacks also resemble combinations that can be used by Isaac — for example, Monstro's Lung plus Homing Tears. Its Detachment Combat is a lot like the Pinking Shears. It even begins the battle in Isaac's crying position!
  • The Antichrist: In the Bible, the Lamb was a symbolic reference to Jesus. In this game, however, the Lamb is an undead demonic creature fought after Satan. The implication is that this Lamb is a demonic parody of Jesus.
  • Blackout Basement: When the body on his second phase is defeated, the room will darken for a brief time.
  • Creepy Cool Crosses: Specifically, an upside-down one on his forehead.
  • Dem Bones: The Lamb is essentially a demonic skeletal child with curved horns.
  • Detachment Combat: His second form involves his head floating around the room attacking you while his body lies limply on the ground spawning flies and sometimes tossing out Monstro-style shots.
  • Dual Boss: His second form.
  • Flash Step: One of his attacks qualifies as this.
  • Frickin' Laser Beams: His second form gains the ability to shoot four-way Brimstone in the cardinal directions. He can also start rotating the beams like Mom's Heart/It Lives.
  • Horned Humanoid Skull for a Head: He looks like an undead demon.
  • Leitmotif: The Fallen Angel
  • Losing Your Head: During the second phase of the battle.
  • Mythology Gag: His name is a reference to the expansion for the original Isaac, named Wrath of the Lamb. He is also visually similar to Wrath of the Lamb's horned skull logo. Given that Edmund had more planned for the original Isaac, it's possible he was always intended to be the last boss of the expansion, but Flash's limitations meant he couldn't add him in.
  • Nightmare Face: Its skeletal face isn't pleasing to look at, especially in the vs. screen.
  • Off Model: The Lamb's portrait in the versus screen is dark in color, but in-game and in the Afterbirth+ trailer, The Lamb is colored white.
  • Sequential Boss: Has two forms.
  • True Final Boss: Averted. He was available from the launch of Rebirth, but so was Mega Satan, meaning he never qualified as the True Final Boss.

Mega Satan

http://static.tvtropes.org/pmwiki/pub/images/megasatan.png
Debut: Rebirth
In Rebirth, a giant golden door appears in the Chest and Dark Room stages. This door can be unlocked with a special item, which leads to Rebirth's ultimate boss battle.
  • Attack of the 50-Foot Whatever: Although the name probably gave it away.
  • Bragging Rights Reward: Besides unlocking the final and definitive ending, there is no reason to fight him other than to boast you did it. The only thing you get out of winning is a new picture for your save file. There are no associated item or character unlocks, not even a mark on the character file to indicate that character beat him. Averted in Afterbirth. He gets both character unlocks and a mark on the sticky note upon beating him, and beating him on hard mode is required for getting the final item (or co-op baby, in the case for everyone except The Lost) for every character.
  • Bullet Hell: What some of his attacks delve into.
  • Cognizant Limbs: His hands.
  • Damage-Sponge Boss: He's just shy of Hush in terms of overall health, and his individual forms have as much or more health than any other boss save Ultra Greed. He could actually be considered to have more health than any boss if you include the health of the mandatory boss fights. However, Mega Satan doesn't have the same damage-scaling property of those two, so he's functionally a whole lot weaker than they are.
  • Final Boss Preview: Regular Satan's third phase can be considered this. In Sheol, you only fight his lower half. In the Chest/Dark Room's secret room, you face the much more difficult upper half.
  • Flunky Boss: Summons The Harbingers, the Super Sins, and fallen versions of the Angels. If you take too long on one of his phases, he starts summoning regular Sins and lesser bosses such as The Hollow and Fistula.
  • Giant Hands of Doom: Accompanied by a giant head. The hands serve mainly to block shots aimed at the head, which is treated as the main boss.
  • Guide Dang It: The only in-game hint you have to his existence is "The Angels await!" You're supposed to bomb the Angel statues in the Angel Rooms, prompting them to engage you; both will drop a piece of a Golden Key on defeat, and getting both pieces opens a Golden Door in the Chest/Dark Room that leads to Mega Satan.note  Fortunately, completing the key also unlocks "Dad's Key" as a regular item, which can also be used on the Golden Door for future playthroughs.
  • Leitmotif: Hericide
  • Marathon Boss: He has two forms, each of which has as much if not more health than every other boss save Hush and Ultra Greed. This is on top of all the other bosses he summons. Winning takes several minutes at least, and that's if you have a good enough run.
  • Red Eyes, Take Warning: Considering he's the (current) True Final Boss of the game before the title being uplifted by Delirium, you will want to take warning.
  • Sealed Evil in a Can: Heavily implied to be sealed by the beings in the Angel Rooms.
  • Sequential Boss: Including his summons, he has twelve phases.
  • Soundtrack Dissonance: Due to a bug, doing too much damage to any form will cause the calm, somber "boss defeated" music to start playing. This was eventually patched.
  • True Final Boss: The second to last True Final Boss of the game (which is saying something, considering there have been seven other candidates before it), hidden at the final stage and having an extra unlock process required each run. Delirium took over the role in Afterbirth+, but even then it is the final boss of an alternate route.
  • Turns Red: Has two full health bars, and when you take down one of them, he really cranks up the Bullet Hell.

Hush

http://static.tvtropes.org/pmwiki/pub/images/hush.png
Debut: Afterbirth
Defeating Mom's Heart or It Lives in under thirty minutes of total play time unlocks a short, hidden areanote . This area only has one enemy: Hush, which appears to be another Palette Swap of the Isaac and ??? bosses. Then he enters his second form, which is soundly described below.
  • Body Horror: It represents Isaac suffocating inside the chest; unsurprisingly, its "face" looks like it's being asphyxiated inside a bag.
  • Bonus Boss: He is the "Isaac" counterpart to Mega Satan. While beating it does grant you a new ending, it doesn't require you to end the game. Instead, you can opt to go to Sheol/Cathedral once the fight is over. This was later patched out; defeating Hush now plays the ending and ends the run, but only the first time you beat him. And as of Afterbirth +, Hush is no longer a Bonus Boss. Beating Hush is the only guaranteed way to reach the Void and fight Delirium.
  • Bullet Hell: Its attack patterns during its second form. They even resemble the dense ring-patterns found in danmaku shooters like the Touhou series.
  • Damage-Sponge Boss: Has the second highest health of all bosses in the game. Furthermore, his defenses scale with your damage output, so no matter what items you have, he'll take a long time to wear down. Combine this with his numerous Bullet Hell attacks and absurd enemy spawns, you'll have a brutal boss fight.
  • Flunky Boss: Begins spawning minions as the battle goes on.
  • Leitmotif: Morituros, although its first phase shares Ascension with ???.
  • Marathon Boss: Hush has 6666 health and has a damage resistance mechanic which hardcaps how much damage you can do at once. Even an overpowered build will still take ten minutes to kill this thing.
  • Meaningful Name: To hush is "to put at rest", and seeing as this boss represents Isaac slowly suffocating in the Chest.
  • Number of the Beast: Its base HP is 6666, and unlike other bosses you can't do absurd amounts of damage instantly because he resists damage about a certain threshold.
  • Ocular Gushers: When low on health, Hush will fire out geyser-like streams of tears from its eyes that track you like a pair of Kill Sat lasers.
  • Sequential Boss: Fights like Isaac and ??? at first, but then it teleports away and its huge face comes out of the floor, firing loads of bullets.
  • Stationary Boss: Mostly. It begins laying on the ground, as a variant of Isaac and ???, then turns into a large bullet-spewing head. The head can move around, but generally it stays still when attacking.
  • Suspicious Video Game Generosity: Before the fight, the game gives you two item rooms, four golden chests, and a shop. Needless to say, you will need them.
  • Theme Music Power-Up: The first phase of the fight has Ascension playing in the background. Once the second phase begins, the music changes to Hush's own Leitmotif, Morituros.
  • Trick Boss: Shares a boss theme with ??? and fights like him, with a health bar that goes all the way down to zero, so you'd think that's all there is to him. Nope.
  • Wrap Around: It fires shots that go out of the room and then come back from the other side. Notably, these shots actually get faster after the wrap-around.

Ultra Greed

http://static.tvtropes.org/pmwiki/pub/images/ultragreed.png
Debut: Afterbirth
The final boss of Greed Mode, fought in a stage named after him. A giant version of Greed that uses coins to attack, summon minions, or heal. And he still makes you drop money when hit.
  • Animation Bump: Ultra Greed moves around a lot, and the animations are extremely smooth and detailed.
  • Attack of the 50-Foot Whatever: A massive variant of Greed.
  • Bandit Mook: Like his smaller versions, his attacks will cause you to drop money. Unlike the others, however, he will actively pick up the money you drop, which heals him slightly.
  • Boss Remix: His battle theme incorporates elements of the shop and arcade themes, the latter of which is itself a remix of the title screen.
  • Damage-Sponge Boss: His health is slightly higher than that of Mega Satan's first form, making him third overall, and the damage scaling means he'll last a whole lot longer.
  • Defend Command: One of his abilities is to shield himself with his arms, reducing all damage dealt to him, which he usually uses when he has flunkies or bomb coins active.
  • Fan Disservice: Has a visible large ass. He's also a massive corpse-like monster that ultimately has a little boy as the base of his design, and might even be an alternate version of a little boy.
  • Final Boss: Of Greed Mode.
  • Flunky Boss: Can spawn Keepers close to himself, call more minions from the arena's doors, and drop a bunch of large coins that will cast buffs on him if they're not destroyed.
  • Frickin' Laser Beams: In his second form, the doors shoot gold Brimstone beams whenever he pounds the ground.
  • The Gambler: His primary attack depends on the outcome of a slot machine. Although it's not that much of a gamble, since every outcome is pretty terrible for you. You're just hoping that he gets one of the less terrible ones.
  • King Mook: To Greed and all of his mook counterparts.
  • Leitmotif: Chorus Mortis.
  • Mad Bomber: One of his slot machine attacks is to drop giant bomb coins all over the arena. His statue form in Greedier Mode gains an explosive effect on every coin he fires.
  • Marathon Boss: His health is about half that of Hush, but he shares the same damage reduction mechanic to prevent you from winning too fast. Furthermore, he has the ability to heal during the fight, though not by much. Greedier Mode is even worse, as there's a second half to his fight.
  • Money to Throw Away: Uses large coins as projectiles. At times, huge ones fall from above. They can explode, heal Ultra Greed, or allow him to summon minions if you don't get rid of them.
  • No Cure for Evil: A notable aversion. He's unique in his ability to heal through heart coins, though it isn't very much. He'll also heal a little if he picks up the coins you drop.
  • Sequential Boss: In Afterbirth +, he becomes this in Greedier Mode. After beating his first form, his statue will start attacking.
  • Spin Attack: Does spinning lariats when you're close.
  • Stone Wall: His attacks, unlike most other final bosses, don't employ Bullet Hell and are comparatively easy to avoid. He makes up for this by being able to tank attacks that only Hush could withstand, with his damage reduction ensuring that he will always last for a few minutes. The challenge comes not from dodging him, but from outlasting him.
  • Tactical Suicide Boss: Downplayed compared to most examples, but his bomb coins can damage him as well. He uses his Defend Command to block most of the damage. His second form is even worse in this regard, since everything is bombs. Half of the time you won't land the final hit that kills him.
  • Taken for Granite: Ultra Greed turns into a solid gold statue when killed. Midas' Touch (touched enemies temporarily turn into gold) destroys his damage scaling property, letting you deal greatly increased damage. In Greedier Mode, he keeps attacking as a statue, and explodes into a fine gold dust on death.
  • Turns Red: As he loses health, his attacks become faster and he spawns more giant coins, which also take effect more quickly. He also visually becomes redder as an indication of this. He also does this whenever heals via his slot machine ability or by picking up coins; the more he heals, the redder he gets. This is only temporary, however.

Delirium

http://static.tvtropes.org/pmwiki/pub/images/portrait_delirium.png
Debut: Afterbirth+

The final boss of an area added in Afterbirth+, the Void. Beating Hush will unlock a floor made up of various different environments from the game, with this as the boss of one of its several boss rooms. It first appears to be a giant, melting skull, but will shift forms to white variations of the previous bosses.
  • All Your Powers Combined: This is the boss' primary attack method. It will randomly transform into almost any other boss found throughout the game (including some of the other final bosses!) and take on the transformed boss' attack patterns, all while still firing off its own bullet rings. Once it Turns Red, it will shift through bosses much faster, and each form attacks far more quickly.
  • Berserk Button: Attempting to use the Chaos Card on him will not only fail, but also make him turn red and attack much faster.
  • Bullet Hell: Delirium's bullet patterns manage to be worse than Hush's, because he'll shift into random bosses to change the pattern around and never sits still. This is on top of his own unique bullet patterns being fired from the bosses along with their modified patterns. By the time you've got him down to 1/4 health, he'll be firing off a new pattern before the last one even finishes.
  • Contractual Boss Immunity: The single enemy who isn't harmed at all by the Chaos Card. In fact, all it does is piss him off. Plan C (kill everything then yourself three seconds later) does technically kill him, but his death animation is so long that Isaac will die before Delirium does. Lazarus is the only one who can get around this thanks to his ability to resurrect in the current room (Lazarus' Rags on another character will work, but Lazarus still gets the credit).
  • Damage-Sponge Boss: Clocks in at a whopping 10,000 health, though it mercifully doesn't have the same damage scaling that Hush and Ultra Greed do. Even so, it teleports all over the place, so expect to spend a long time trying to kill this thing.
  • Dying Dream: This creature, its environment and the ending you get for defeating it drive home the point that most of what is seen during the game was Isaac's dying hallucinations.
  • Eldritch Abomination: A physical manifestation of a complete mental breakdown? Yeah, it qualifies.
  • Eyes Do Not Belong There: When transformed into The Husk or Conquest, Delirium will have a single eye or a pair of eyes respectively inside its mouth. Its Teratoma form also has an eye inside.
  • Final-Exam Boss: It's only able to transform into bosses you've previously encountered during your run, thus learning how each boss attacks is vital for surviving the fight with Delirium.
  • Flunky Boss: Can spawn minions depending on what form it is in. Keep in mind that the Lil' Haunt you see sometimes during the battle is no mook, but Delirium itself.
  • "Get Back Here!" Boss: With a 2x2 arena to fight in, you'll spend a fair portion of the fight trying to figure out where it teleported to. This isn't helped by it transforming into wall-hugging bosses like Mama Gurdy or extremely small ones like Isaac.
  • Glowing Eyelights of Undeath: Has yellow pinprick eyes.
  • Interface Screw: Will occasionally cause the arena to glitch and shift from one tileset to another, causing static to briefly fill the screen in the transition. This can happen anywhere in the Void, but is far more likely to happen when fighting Delirium.
  • Leitmotif: The self-titled Delirium.
  • Marathon Boss: With his high health, constant teleporting, and shifting into whatever boss he feels like, you'll spend a large amount of time chipping this guy down to nothing.
  • Meaningful Name: "Delirium" is a disturbed state of mind. The battle plays out in a deranged and bizarre way, almost like some sort of hallucination.
  • Mind Screw: It's a giant, melting skull that turns into the other bosses wildly and at random (many of which are considerably smaller than it), and it's encountered in an area made of all of the past floors and fails to keep a consistent theme. Considering the name, nature, and location of this boss, it's implied that the fight is Isaac's mind breaking down.
  • Morphic Resonance: Delirium retains its coloring and often its facial features regardless of what form it takes, allowing you to tell it apart if it happens to spawn another boss monster.
  • Nightmare Face: A melting skull certainly isn't pleasant by any stretch of the imagination. And since its features are retained when it transforms, this can result in similar faces appearing on any boss in the game.
  • Shapeshifter: It can shapeshift into any other boss, taking on their attack patterns.
  • Shape Shifter Swan Song:
    • The less health it has, the more often it changes forms. At critical health it can't stay in a form for more than a few seconds!
    • Interestingly, the arena suffers this too. During Delirium's death animation, the arena glitches and shifts between several different appearances, finally stopping as he dies.
  • Teleport Spam: Can teleport around in any of its forms. As it loses health, this becomes more frequent, to the point that he'll barely spend more than five seconds without doing it.
  • Timed Mission: Played with. If you want to guarantee access to the Void and thus to Delirium, you have to beat Hush, which means you need to beat It Lives before the 30 minute mark (or have Mama Mega! on that floor). That said, the portal to the Void has a chance to spawn after beating Mom or any subsequent boss, increasing as you go further. Mega Satan has a 50% chance at best. He's also the only boss for whom completion is tracked even on victory laps, however, so one valid strategy is to take your time gathering items on the first run, defeat The Lamb, and then speedrun through the victory lap with your items to reach Hush.
  • True Final Boss: Of Afterbirth+, and as that is the last major update to the game, the game itself. Defeating it earns you the final ending to the game. Oddly enough, however, you can encounter it way before Mega Satan thanks to how its requirements and the level progression are set up.
  • Turns Red: The lower his health, the faster he shifts between forms and teleports.
  • Weaponized Teleportation: Even moreso than the ??? boss; Delirium loves to teleport directly onto you.

http://tvtropes.org/pmwiki/pmwiki.php/Characters/TheBindingOfIsaac