| The Basement|
Boss Challenge Room
Normal Enemy (Later Floors)
Fistula's only attack is to move around the room, bumping into the walls. When the original Fistula is defeated, four smaller Fistulas will be spawned, and each of those pieces splits into two even smaller Fistulas when defeated. After these smallest Fistulas are defeated, they spawn Chargers unless they are over a pit or other obstacle.
- Black: Spawns Boom Flies instead of Chargers. Pieces travel much slower, and the original Fistula will only split into three smaller Fistulas.
- Fistula appeared in the original Binding of Isaac
- A fistula is an abnormal connection between organs, intestines, and other tissues.
- Fistula used to be a Caves/Catacombs boss in the original "Binding of Isaac", as of Rebirth, it is a Basement/Cellar boss.
- The posthumous form of Fistula is Teratoma.