Catacombs

The Catacombs is one of the environments of Chapter 2.

The Catacombs are unlocked once each boss in has been defeated. It generally posseses more spiders and decayed enemies/bosses than caves, like gurgles or guts.

After is unlocked, a boarded door similar to the one leading to a Bedroom will appear in this floor's Boss Room upon defeating the boss, leading to the  /. This door, like a Bedroom, requires two bombs to be opened. Unlike Angel Rooms and Devil Rooms, this door never disappears.

Monsters

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Trivia
In the original Binding of Isaac, Blighted Ovum could appear as a boss.

Music
The music for the Catacombs is titled Capiticus Calvaria. Calvaria is Latin for "Skull", and while Capiticus does not have a true definition, it most likely comes from the word Caput, meaning "Head", or Capitulum, meaning "Chapter".

Binding of Isaac: Rebirth OST