Marathon Boss / The Binding of Isaac

• If your damage output doesn't scale as you progress through the game, later bosses can take an obscene amount of effort to kill. This is especially true of Isaac and ???. However, those two pale in comparison to Mega Satan of Rebirth, who takes forever without the Chaos Card due to being a Boss Rush as well as a regular boss with two phases.
• Afterbirth adds in Hush and Ultra Greed, who not only have massive HP pools on par with Mega Satan, they are unique in that they have a Damage Reduction mechanic that scales with your damage output, so you won't wind up with a combo that can kill them in twenty seconds. No matter how well-prepared you are, expect to spend a lot of time killing them. But on the other hand, this mechanic does keep them from becoming Anti-Climax Bosses.
• Afterbirth+ adds Delirium, which doesn't make as effective use of the Damage Reduction mechanic as Hush and Ultra Greed due to a quirk of its own battle gimmick, but its health pool is even more stupidly massive.
• Repentance adds the Damage Reduction mechanic to Mega Satan and its own alternative endgame superbosses, Mother, Dogma, Ultra Famine, Ultra Pestilence, Ultra War, Ultra Death, and The Beast. Oh, and The Beast by itself has as much health as Delirium, doesn't sabotage its own Damage Reduction, and is the final enemy in a very long Boss Bonanza — Dogma, the Ultra Harbingers, and the Beast are all battled one after another. The Home battles are going to take you a long time.
• To provide hard HP numbers for all of the bosses we mentioned (and a couple that we didn't), as well as describe some gimmicks that drag these fights out even longer:^note Bear in mind that your baseline damage output in this game is 3.5 per tear, so even the smallest of these numbers are going to take a while for weaker builds to reduce to 0.
  • The "light path" Chapter 5 and 6 bosses, Isaac and ???/Blue Baby, have 2,000 HP each, and while they each have three phases, all three phases share the same life bar. The latter was the definitive True Final Boss of the Flash version of The Binding of Isaac with the Wrath of the Lamb expansion. Both start out immobile, and they remain that way for the entire fight in the Flash version, but in Rebirth, they start moving around in their third phases, as well as periodically teleporting around. Isaac tends to try to avoid your attacks, even periodically teleporting out of the room entirely to carpet-bomb most of the Boss Room with light beams. ??? fights more recklessly, electing to chase you down to shoot you point-blank or even attempt to Tele-Frag you, but makes up for his tendency to put himself in harm's way by deploying both a ring of Eternal Flies and a steady stream of flunkies to block your shots and distract you.
  • The "dark path" Chapter 5 and 6 bosses, Satan and The Lamb, are less straightforward. Satan himself (who was the True Final Boss of Flash Isaac's Halloween update) has only 1,200 HP, split across two phases^note yes, in this case, that means that each phase has its own life bar, so using a Chaos Card or Plan C on the first phase will just start phase 2 rather than automatically winning the fight; this is more than Mom's Heart/It Lives! (which has 1,000 HP), but still would be not quite enough to qualify as a Marathon Boss in this game if not for the fact that you need to go a round with The Fallen immediately before Satan fights you^note and, yes, using a Chaos Card or Plan C on The Fallen just one-shots The Fallen while leaving Satan himself unharmed. Satan's first phase drifts back and forth across the top of the room, while the second phase uses a variation of Mom's attack strategy, with the main body of the boss being unseen and un-attackable and the battle consisting of the player attacking the boss's limbs (specifically his feet) that rapidly burst into the arena to attack, then retreat after a moment. The Lamb has 2,600 HP (which used to be 2,000 before being buffed in a patch), divided in a single life bar across two phases^note yes, this means that you can one-shot The Lamb immediately and be done with the fight right away with a Chaos Card or Plan C, and unlike Isaac and ???, it actively pursues the player throughout the entire fight. "Division" is taken literally in the second phase, as The Lamb splits itself into two entities, its head and its body, both of which have half of the remaining life meter and must be defeated to win the battle. The body is immobile but briefly darkens the room when killed, while the head retains the mobility of the first phase and adds a Dash Attack. We didn't mention either of these earlier because Satan's HP still isn't as impressive as Isaac or Blue Baby's even with The Fallen as a cushion^note even though Satan can't be cheesed with a One-Hit Kill item like Isaac or ??? can, and The Lamb was never the True Final Boss of the game since it made its debut in Rebirth, so it was upstaged by Mega Satan from the very beginning.
  • Mega Satan has a total of 5,000 HP. Like Satan, this is split into two life bars for two separate phases (3,000 HP in phase 1 and 2,000 HP in phase 2). Also like Satan, Mega Satan's own HP by itself isn't the whole story; at certain HP intervals in the first phase, Mega Satan will retreat and summon some lower-grade bosses that you will need to kill before he returns to fighting and being susceptible to damage. While you can bypass the Boss Rush aspect of phase 1 by one-shotting it with a Chaos Card or Plan C, this will just advance to phase 2 rather than outright defeating Mega Satan. Oh, and just to add an extra dash of despair, Mega Satan's health bar isn't visible at all unless you have Spider Mod, though you can use the scripted sub-boss spawns in phase 1 and the visibly-degrading sprite in phase 2 to have a rough idea of how well you're doing. While Mega Satan didn't have Damage Reduction in Afterbirth or Afterbirth+, he gained it in Repentance (and it's present for both phases), bringing his longevity up to around Hush's level with the scripted interruptions factored in.
  • Ultra Greed has 3,500 HP and Damage Reduction. While his raw longevity pales in comparison to Mega Satan or Hush, he does have a Defend Command to drastically reduce the amount of damage he takes (which he tends to use after summoning minions), as well as the ability to regain small amounts of HP with one of his random slot-machine coin minions (though the player can prevent this by killing said minions) or by picking up coins that his attacks knock out of the player (which the player can prevent by picking up the dropped coins before Ultra Greed can or just not getting hit in the first place). In Greedier Mode, he gains a second phase with 2,500 HP; this is a separate life bar (so you'll still have to deal with it the hard way if you use a Chaos Card in Phase 1, and you'll just kill yourself by using Plan C on phase 1) and loses the Defend Command and minion-spawning in exchange for harder-to-dodge attack patterns.
  • Hush's fight starts with a warm-up round against a reskinned ??? with only 1,000 HP. Once that's swept aside, Hush emerges in its true form, which has a colossal 6,666 HP and Damage Reduction, not to mention enough Bullet Hell to have you constantly scurrying all over the Boss Room. It also periodically spawns flunkies to get in your way. At least Hush doesn't move around very much (much like Mega Satan), though it does have a couple of attack patterns where it retreats into the floor and becomes temporarily invulnerable. Oh, and as of Repentance, it gains Gradual Regeneration, though this only kicks in if you don't attack it at all for an unreasonably long time, and even then, only when it's almost dead. Chaos Card and Plan C are effective at skipping the full Hush fight, but using them on the ??? warm-up round will just be wasting them, and since Hush isn't actually the end-point of the run (at least after being defeated the first time), using Plan C on it without any extra lives will end up counting as a loss on your win/loss streak.
  • Delirium has a whopping 10,000 HP, but while it does technically have Damage Reduction, it doesn’t amount to much, since the scaling reduction resets every time Delirium changes forms (which it does very often). It makes up for this with its potential to change into bosses that can outright dodge or deflect your attacks, such as Scolex, Polycephalus, or Mom, as well as its own form-independent ability to outright teleport wherever it feels like being in its Boss Room at a moment's notice with absolutely no tells whatsoever. And just like its shapeshifting, Delirium uses its teleportation frequently, especially when it's heavily damaged. As such, you'll be spending at least as much time trying to figure out where Delirium went and chasing it down to get back into position to attack it as you will be dodging its continuous Bullet Hell and whaling on it with your own tears/knives/missiles/whatever you're equipped with. (At least, when it doesn't decide to teleport right on top of you (good luck dodging that one, especially if you're playing as The Lost, The Keeper, or their Tainted versions and really need to avoid all damage at all costs).) Oh, and just for one more kick in the teeth, Delirium is completely immune to the Chaos Card, and its death animation is long enough that trying to cheese the fight with Plan C will result in the player dying before Delirium is officially defeated (meaning that you'll need to be Lazarus, have Lazarus' Rags, a Soul Stone of Lazarus, or Inner Child to actually be credited for the kill — and the former two only work for Lazarus' completion record). In other words, there is absolutely no way to cheese this fight.
  • Mother has 4,222 HP, spread across two separate life bars (2,222 HP for phase 1 and 2,000 HP for phase 2), and yes, she has Damage Reduction like the rest of the superbosses (and Chaos Card and Plan C only deplete one life bar). The first phase is immobile, but frequently spawns smaller enemies to get in your way (with one such attack pattern having Mother retreat into the wall and become invulnerable until she re-emerges for good measure, forcing you to focus on the minions to avoid being overwhelmed), while the second phase stops spawning minions but instead slowly chases Isaac around while firing Bullet Hell attacks, with one attack pattern in particular having Mother suddenly leap out of view before wildly dashing all over the room (obviously making it hard to aim at her until she settles down). Oh, and like Delirium, Mother's death animation takes longer than 3 seconds, meaning that Plan C will kill the player before Mother's death animation completes, denying the player the completion mark (or the opportunity to dash for the chest and finish the run) if they don't have an extra life. Unlike Delirium, even extra life items that kick the player out of the room won't undo a Plan C kill, so as long as the player wasn't dumb enough to use Plan C on phase 1, they can still get credit without having to redo the fight. Also, just like Mega Satan, Mother's health meter isn't visible without Spider Mod, and unlike Mega Satan, Mother has absolutely no other indicators of how close she is to defeat. Furthermore, she can't be one-shotted with The Bible.
  • Dogma has 2,400 HP split across two life bars. In the first phase, Dogma's main body is invulnerable and the TV it's connected to is the actual target to destroy, which means that while the target is stationary, it has a meatshield to block your attacks with if you don't position yourself well. The second phase is more straightforward, with Dogma itself being the target, but it's mobile and has one attack pattern where it briefly leaves the Boss Room. And yes, both phases have Damage Reduction.
  • Each of the Ultra Harbingers has 2,000 HP (except for Ultra Pestilence, which has 1,800), and each of them has Damage Reduction. None of them have any defensive measures in and of themselves (though the switch in perspective from a top-down Dungeon Crawler to something akin to a side-scrolling Shoot 'Em Up does make certain items, including bombs, considerably less effective, as gravity is suddenly a factor and they are liable to drop to the bottom of the screen). All in all, Dogma and the Ultra Harbingers are just a Boss Bonanza designed to wear the player down before they face the real True Final Boss.
  • The Beast has 10,000 HP, spread across three phases in a single life bar (but, like Delirium, she takes absolutely no damage from the Chaos Card; Plan C, on the other hand, is very effective). Aside from the same side-scrolling perspective as the Ultra Harbingers that screws with certain items, the Beast doesn't really have any defensive gimmicks aside from occasionally moving from one side of the screen to the other, either by charging or by ducking into the lava and popping up elsewhere. The Beast doesn't have a visible health meter, but she Shows Damage, especially at very low HP.