Bosses are very strong monsters found in boss rooms at the end of a floor. The boss at the end of the floor must be defeated in order to advance to the next floor. In later chapters, bosses are able to spawn as normal enemies. This starts in the caves, where the Gurglings are often encountered. The boss for a particular floor is chosen from the pool of bosses for the environment of the floor. Bosses may also spawn as champions, which can change various attributes such as health, damage, and attack pattern. Upon defeat, most bosses will drop an item from the boss item pool.
Bosses boast additional resistances to status effects. For example, fear occurs less often and the Night Light is unable to slow them down.
Additionally, some floors may spawn a room with a Mini-Boss.
The Seven Deadly Sins are mini-bosses that can be encountered on any floor, and will appear on post-Womb/Utero floors as regular enemies. Unlike with regular bosses, there is no special door leading to a mini-boss room, meaning that there is no way to tell whether a room is inhabited by a mini-boss unless the player is in possession of an item that reveals map icons such as Compass or a Map item. Once in a mini-boss room, the player cannot escape through means of bombing the door, as it will be boarded off.
Each of the Sins has specific drops, that are consumables or a pedestal item (less frequently).
In a room containing multiple Sins, only the last one will drop something, allowing you to choose what drop you want by killing a certain mini-boss last.
There is also an Ultra variant of both Pride and Greed. Ultra Pride resembles Edmund and Florian, while Ultra Greed serves as the final boss for Greed Mode.
"Originally every boss/enemy in Isaac was going to resemble Isaac in some way. this is the case for most."[1]
Double Trouble boss pools can have bosses from any of the previous chapters' pools, and are the same for both versions of that chapter's floor. This means that, for example, Double Trouble boss fights in the Caves can feature bosses from the Cellar or the Basement, and every Double Trouble fight in the Caves can also be encountered in the Catacombs.
Bosses often come in three versions: normal (For example, Monstro), posthumous (Monstro II), and alternate (Gish). Usually, the posthumous boss looks like the original with some horrible mutilation; for example, Monstro and Chub had most of their skin ripped off.
Bugs
Bug!
Defeating a boss quickly enough might cause the boss music to play, even after the boss dies.
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Bug!