History ThatOneBoss / TheBindingOfIsaac

* The Haunt. Initially, he's completely invincible and relies on his three [[DemonicSpiders smaller ghosts]] to assist him, but once they're killed, he starts his attacks. The Haunt is actually very fast once he starts attacking, making it difficult to lead your shots to him. Not only does he have an ''untelegraphed'' bullet spread attack, but he can also fire a Brimstone laser vertically across the screen with barely a second of warning. And if you think you can just run off to the side of the Haunt to dodge this, think again. Doing this causes him to charge you for a '''full heart''' of damage. And the worst part? He's a ''Cellar/Burning Basement'' boss, meaning he can potentially be the first boss encountered in a run. Some fans have come to the conclusion that he's even harder than the Widow, the previous infamous Cellar boss. And woe be to you if you encounter the pink champion version which fires slanted, ''bouncing'' Brimstone, forcing you to stay up close and potentially get hit by bullets. While other bosses like the Bloat and the Matriarch were nerfed in some updates and [[DownloadableContent DLCs]], the Haunt was not nerfed in any way. As a matter of fact, not many of the ''Rebirth'' bosses, if ''any'', were nerfed at all, including some of the ones listed below.

* If you thought the TrueFinalBoss fight against Mega Satan in ''Rebirth'' was brutal, you're far from the worst. Enter '''[[EldritchAbomination Hush]]'''. Reach Mom's Heart or It Lives and beat them in under 30 minutes to get access to its lair, a place called ???. Before the fight, the room gives you two item rooms (with two item options to choose from on each room), four golden chests, and a shop. Needless to say, ''you will need them''. You'll first face Hush as a glorified ???, but defeating it is not the end of it, as it reveals [[OneWingedAngel its true form]]. With [[SequentialBoss five phases to the true form's battle]], loads and loads of BulletHell attacks that make bosses from ''Franchise/TouhouProject'' blush (and one of which [[ThatOneAttack wraps around the room's boundaries]]), the ability to [[FlunkyBoss spawn loads and loads of enemies]] to block your shots unless you have a piercing attack, the ability to merge with the floor to completely avoid your attacks, and the [[DamageSpongeBoss second-highest HP pool]] at [[NumberOfTheBeast 6666 HP]] on top of damage resistance that scales with your current damage, you're in for [[MarathonBoss the actual longest fight in the game]], no matter your item build. It's easy to get hit by its attacks as it is, so you also better hope you came into its battle packing tons of health... unless you're playing as [[OneHitPointWonder The Lost]] or [[TierInducedScrappy The Keeper]], in which case... [[ThisIsGonnaSuck good luck]], '''[[NintendoHard you'll need it]]'''. Granted, you can skip this boss fight in ''Repentance'' by using IV - The Emperor?, you fight [[WarmUpBoss Monstro]] instead of Hush, and beating [[BreatherBoss him]] will count as beating Hush.

* Mother, the renamed Witness, is far more of a threat than in ''Antibirth''. For one thing, you don't have much room to work with compared to ''Antibirth'', making dodging her attacks far more difficult. She also doesn't have a health bar and has a crapload of health too. And the attacks themselves include summoning a crapload of gapers that take up tons of space in the already small room walking up to her and her hitting towards you. The worst one, though, has to be when she pukes out a Fistula and smacks it around with bullets (and if you live, then the pieces of it) going everywhere. And that's just phase 1. Phase 2 begins after shooting a laser out of nowhere and then coming out to fight you the same way she did in ''Antibirth''. She doesn't move around much, but she's also blocking off a good part of the room. Her attacks include jumping up and charging before crashing down in the center sending shockwaves and her sucking bullets up to them spit out at you. [[SarcasmMode Have fun with the Tainted characters]], especially [[OneHitPointWonder The Lost]] and [[TierInducedScrappy Jacob]]!

* The Scourge is the KingMook to [[DemonicSpiders the Whipper and Cultist families]]. Sounds painful enough? '''''Because it is!''''' While its green multi-projectile attack is a fairer one to dodge (even if it spawns [[TheGoomba Small Maggots]]), it also makes tons of use of its tendrils to [[WhipItGood lash at you]] and the walls. It has three variations to this attack: lashing one at you and dragging it at the wall to fire projectiles and maggots, firing one at you and another at the wall then rotate them while firing rings of projectiles, or embedding both at the wall while indulging in BulletHell. And no, you can't leave the area you are in if it corners you; the tendrils are solid, so you ''will'' have to survive the projectile storm. And lastly, it hovers to the center of the room while sucking you and streams of shots in, often changing the directions of the streams, meaning that you'll have to backtrack to not get cornered. Also? The Scourge has [[DamageSpongeBoss a colossal amount of HP]][[note]]950 HP, which is that of ''Mom's Heart''[=/=]''It Lives''[[/note]], so you will stay a very long while either [[MarathonBoss trying to outlast this hellspawn]], or [[HaveANiceDeath die trying]].

* Damage resistance, although [[ScrappyMechanic a controversial mechanic as a whole due to how it works]], is generally accepted because the only bosses that have it in ''Afterbirth'' are "superbosses". ''Afterbirth†'' breaks this rule with Big Horn, a Catacombs DamageSpongeBoss who will usually take the same amount of time to kill no matter where you fight him. The only way to deal decent damage is via FeedItABomb, but it's hard to pull off. His other tricks include tossing many varieties of bombs (all of which deal a full heart of damage), trying to corner you with slapping hands, and summoning a weakened [[DegradedBoss Little Horn]]. An update quickly reduced his health [[note]]from 600 HP to 480 HP[[/note]] and gave his attacks a little more delay, whereas one released a month later (most importantly) removed his damage resistance. His Little Horn summon also got reworked in ''Repentance'' so that you no longer have to fight it alongside Big Horn. Nevertheless, despite these changes, [[GoddamnedBoss he can still be quite painful to fight]].

* Thought that bosses like The Bloat, The Adversary, The Cage, or Sisters Vis were painful? The Forgotten update adds The Matriarch, an unholy mix between Fistula, Chub, and the Chubber and Fistuloid enemies that's [[DamageSpongeBoss even sturdier]][[note]]at 700 HP[[/note]], and who can appear on ''any'' environment of the Womb (but as of ''Repentance'', appears most commonly in the Scarred Womb). The Matriarch shoots Chargers in the eight cardinal directions, spawns Fistuloids that surround her, and shoots a jumbo tear that leaves a puddle of toxic blood creep. As she takes damage, she sheds small Fistulas and Fistuloids which shoot Chargers like Chubber does. At medium health, she starts summoning [[ZergRush an enormous wave of Fistuloids that immediately shoot their Chargers at you]]. When "killed", the Chub sheds her Fistula armor completely, spawning even more Chargers, Fistulas and Fistuloids. Lastly, unlike the normal version of Chub, this one [[DefiedTrope defies]] the FeedItABomb strategy by [[HoistByTheirOwnPetard spitting it back at you]]; only The Forgotten can avoid this attack by [[TennisBoss swinging the bomb back at her]] with his bone. It's virtually impossible to avoid getting hit simply because the Matriarch is big enough, her spawns are numerous enough, and her attacks are quick enough, that there is just nowhere in the room you can hide. Therefore, to many, this boss became the new Bloat for the final days of ''Afterbirth†'', being considered ''even'' harder than the non-tankiest final bosses in the game, and unlike bosses like Sisters Vis and Big Horn, was never nerfed - it took until ''Repentance'' where this hellspawn was '''''finally''''' {{nerf}}ed, causing her to [[CastFromHitPoints take 30 HP of damage when she summons a Fistuloid]], making the Fistuloids that orbit her attack less aggressively, and shedding less pieces of Fistula armor whenever her health is reduced a certain amount - nevertheless, she's remains a potentially very dangerous boss.
* '''Delirium'''. In addition to having [[DamageSpongeBoss an absolutely titanic amount of HP (10,000, the highest HP of any enemy tied with The Beast)]], his attacks are hard to dodge as they are; they have very convoluted and tight patterns of bullets fired with barely any downtime, some of which can be ipecac shots. No matter what Delirium transforms into, though, expect to be annoyed by his propensity to teleport around the room at random (often [[TeleFrag right beside you]]) and the fact [[TurnsRed he gets faster as his health gets lower]]. Even so, the amount and type of bosses that you encounter, combined with the [[GameBreaker build you amassed before the fight]], are what will make Delirium either a boss whose forms are [[CurbStompBattle only a nuisance while dealing with its bullet patterns]], or a [[NintendoHard true nightmare of a boss]] that [[MarathonBoss you'll have to outlast]]. It doesn't help that the level you find Delirium in, [[MarathonLevel The Void]], has [[GoddamnedBats enemies]] of [[DemonicSpiders all types]] that will cause you to hope you can hold onto your life, and bosses that Delirium can transform into during the fight... yes, that includes all and each of the other bosses on this page (up to ''Afterbirth†'')[[note]]sans Hush, unless you encounter ''very'' few bosses throughout your run[[/note]]. It gets to the point where most consider it even harder than '''The Beast''', ''Repentance'''s TrueFinalBoss.

* Bumbino's attacks are aplenty and fairly unpredictable, not helped by the arena you will be pitted in. The room will always be full of rocks, which will do nothing to protect you, as he will always break them out of his way. His dash attack covers an excellent range and is also very fast, and makes rocks rain all over the battlefield, so even if he leaves himself open afterwards, you will not have much time to get some counterhits. His swipe attacks also [[BanditMook cause you to lose coins, which he will take for himself]] and will not be replenished when he's defeated - he prioritizes getting the coins before attacking you, too. He can also throw a bomb at you, much like he can do so when he's a familiar. Finally, all of his attacks deal one full heart of damage, so if you're not careful, you're going to lose lots of health to this boss. Additionally, this is only in Chapter 2, and he can appear in all of its environments.

** Ultra War makes use of very special explosive attacks, launching bombs that explode in a constant, cardinal cross pattern and shoot projectiles in all directions, making dodging the next bomb harder. Once he does this three times, he surfs the lava below and raises giant lava balls (like the ones that raise from Ultra Pestilence's ipecac bombs). Of the two, the first attack is the most dangerous. Once you've whittled his HP to half, he TurnsRed and chases you for a killer bear hug, followed by long and numerous streams of fire if he misses (some of which will not be put out by your tears), and after doing this twice, he hugs a bomb that will make him bounce across the entire screen and then into the lava to raise many more lava balls. If you're The Lost and you let War perform this final attack, pray it doesn't hit you because it's very easy to get tagged by it. It's ironic how the original War was a BreatherBoss while Ultra War is much more dangerous.

* Colostomia. Not only winning [[NauseaFuel the most disgusting boss]] in ''Repentance'' yet she's also one of the hardest. Along with being [[DamageSpongeBoss a fairly tanky boss]][[note]]380 HP[[/note]] (even tankier than the Basement's Dangle), all of her attacks are dangerous. She slides around really fast, leaving explosive gas clouds which react to bomb explosions. She also fires out butt bombs that detonate these same gas clouds, and the explosions have [[HitboxDissonance varying range to their size]]. Unless you pay attention to the gas the charge leaves behind, you won't have a tell at all. When low on health, Colostomia will shed her "armor" and change her scheme, starting to spew out the bombs while jumping around and leaving gas clouds and brown creep that, unlike other types of the same creep, hurts you. And here's the worst part: this is only in chapter 1! Thank goodness she only appears in, fittingly enough, [[SewerLevel Dross]].
* Hornfel, from the Mines. This bugger tends to use hit and run methods to kill you. And they are nearly all explosive (with one exception though that comes ''after'' he pelts bombs like a maniac) so unless you have great reflexes and speed, or defensive items like the Wafer, or something that makes you immune to explosives like Host Hat, you're going to lose a ton of hearts (or runs) from him. Also, remember his attack where he sends dummies of himself in ''Antibirth''? It was ''buffed'' to include three now. All with bombs if you hit one by mistake. Prepare to rage, especially if you are playing any version of the Lost!

* The alternate path's Mom is much more deadly than her Depths counterpart. Her monster summons include Psychic Horfs, [[DemonicSpiders Raglings]] and Cursed Globins which are much more deadly than the Depths could ever be. If you face her in the alternate floor, [[ThatOneLevel Gehenna]], you'll also have to contend with that floor's DemonicSpiders too. Her "safe" eye will now fire twin Brimstone lasers if you are in sight. And finally, her stomp has a chance to instead be three quick stomps, with the third one [[ThatOneAttack leaving rows of explosions in four directions that can't be blocked by Host Hat or Pyromaniac]], because why not? The good thing is that she will never use her hand attack, and that she can't stomp the ground thrice if she's firing her Brimstone, and vice versa. However, if there was a time to use the Bible, then now would be the time.

* Mother, the renamed Witness, is far more of a threat than in ''Antibirth''. For one thing, you don't have much room to work with compared to ''Antibirth'' making dodging her attacks far more difficult. She also doesn't have a health bar and has a crapload of health too. And the attacks themselves include summoning a crapload of gapers that take up tons of space in the already small room walking up to her and her hitting towards you. The worst one though has to be when she pukes out a Fistula and smacks it around with bullets (and if you live then the pieces of it.) going everywhere. And that's just phase 1. Phase 2 begins after shooting a laser out of nowhere and then coming out to fight you the same way she did in ''Antibirth''. She doesn't move around much but she's also blocking off a good part of the room. Her attacks include jumping up and charging before crashing down in the center sending shockwaves and her sucking bullets up to them spit out at you. [[SarcasmMode Have fun with the Tainted characters]], especially [[OneHitPointWonder The Lost]] and [[TierInducedScrappy Jacob]]!

* Gurdy Jr. doesn't look tough, right? ''Think again.'' After summoning Pooters and/or firing eight bullets around her, she [[ThatOneAttack charges at you at a ridiculous speed with ridiculous accuracy]]. And if she misses, she might just do a little homing slide into you. After losing half health, this will be her only attack she'll do. Only bombs or slowing tears will fend off her charge attack. One of her Champion variations has two of them battle you at the same time, and they will only charge at you, only making her that much harder to defeat. ''Rebirth''[='=]s Double Trouble mechanic can actually saddle you with ''four of these gals simultaneously''! Thankfully, unlike "her evolved form" Gurdy, Gurdy Jr. only has decent health.
* Carrion Queen, the posthumous version of Chub, can be somewhat difficult without not because the boss herself is strong, but because she leaves behind damaging red poop that quickly regenerates if destroyed. Also, much like Scolex, [[AttackItsWeakPoint only her end segment is vulnerable to attack]] (though getting her to [[FeedItABomb swallow bombs]] works, too). If you don't damage her fast enough to trigger her second phase, the arena will be a minefield... not that her second phase is much better, since she starts erratically bouncing around the room at high speed and can no longer be [[FeedItABomb fed bombs]], making it an utter pain to take off her last sliver of health, as her rapid diagonal movement makes her [[AttackItsWeakpoint weak spot]] incredibly hard to hit. The champion version is actually better, because she instead poops out tiny heart enemies which are easy to kill and deal damage to her on death, and she doesn't [[TurnsRed enrage at low health]]. She's both a blessing and a curse during the BossRush, in which her poop can blend into the all the bloodstains and become hard to see, but will also cause major damage to the other bosses, potentially killing some of them ''in a few seconds''.
* The Mask of Infamy, a [[KingMook giant copy of the mask-heart enemies]]. Her AI is [[ArtificialBrilliance much better than the mooks she takes from]] (not that they weren't smart before), and ''both'' her mask and heart need to be killed. The mask can only be hit from behind, and has near-perfect turning reflexes, making the already annoying CatAndMouseBoss harder. If you don't have a piercing attack, bomb tears or enough bombs, expect to spend a long time fighting her.
* While Peep himself is more of a GoddamnedBoss than this {{trope}}, the same isn't true for '''[[MemeticTroll The Bloat]]'''. There are very good reasons for why he used to provide the page image. And why he has [[https://www.reddit.com/r/FUCKBLOAT/ his own Subreddit]] where the community dedicates themselves to LoveToHate him. As the [[BackFromTheDead posthumous version]] of [[KevLard Peep]], Bloat has the same amount of health as him, and shares most of his same strategy. However, unlike all versions of Peep[[note]]sans his dark cyan champion variant[[/note]], the eyes detach at the start, not [[TurnsRed after taking enough damage]]. He also shares the same attack pattern, with one notable and ''extremely hated'' exception; if the Bloat is idle and you so much as graze the area directly to the sides or in front of him, he'll [[ThatOneAttack fire Brimstone beams from his eyes]] (either two forward or one to each side). This attack is not telegraphed, and will hit you unless you've got enough speed to run past him in the split second between the trigger and firing. It's possible to get hit ''[[TheComputerIsACheatingBastard the same second you enter the room]]'' with this boss! The only safe place you can attack from is behind him, but this can't always be done as: his eyes can be behind him, he can leak out red creep behind him (which hurts you unless you have flight), and he sometimes changes position. His green champion version will also send out a spray of Ipecac bombs towards you whenever he jumps, and [[FlunkyBoss he]] can spawn [[GoddamnedBats Chargers]] when he creates damaging creep. It is also possible to fight him in both Necropolis and chapter 4 on the same run and, in Greed, the Boss switch might pit you with '''two of them'''. He's even worse in an ''Afterbirth†'' challenge called "[[https://bindingofisaacrebirth.gamepedia.com/Aprils_Fool April's Fool]]", on which '''''[[RecurringBoss every boss on every floor is The Bloat]]''''', even the final boss in the Womb! You can thank [[https://www.reddit.com/r/bindingofisaac/comments/5mbnqs/i_got_banned_from_rfuckbloat_for_coming_up_with/ this user on Reddit]] for coming up with this challenge. Granted, due to the wacky mechanics of the trial, it's functionally impossible to die, but that doesn't make it any less obnoxious. Also fortunate is that this expansion introduced the Analog Stick item, which allows you to fire diagonally and mitigates some of the difficulty if you happen to get it.