Fistula

Fistula is a boss that can appear: The large Fistula has 60 HP, the medium forms have 15 HP, and the small forms have 8 HP.
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Behavior
All stages of Fistula deal a whole heart of contact damage.
 * Moves around the room slowly, bouncing off the walls.
 * When enough health is depleted, the one large Fistula breaks into four medium Fistulas.
 * Medium forms behave identically but move slightly faster and can bounce off other enemies.
 * Each of the four medium Fistulas breaks into two small Fistulas.
 * Small forms behave identically but move very fast.
 * Each of the smallest Fistulas spawns a Charger on defeat unless they are over a pit or other obstacle.

Champion Versions

 * Dark Grey: 15% larger with 25% more HP (75). Spawns Boom Flies instead of Chargers. Each piece travels much slower and splits into three medium Fistulas as opposed to the original's four, similar to Teratoma.

Trivia

 * Fistula appeared in the original The Binding of Isaac.
 * A fistula is an abnormal connection between organs, intestines, and other tissues.
 * Fistula’s splittings after taking enough damage is a reference to the game Asteroid, where asteroids would float diagonally and split after taking enough damage. This was confirmed by Edmund McMillen.
 * Fistula used to be a Caves/Catacombs boss in the original The Binding of Isaac, but as of Rebirth, it was a Cellar boss.
 * It has been moved back into the Caves as of Repentance.
 * Teratoma is the posthumous version of Fistula.
 * Unlike the original The Binding of Isaac, Fistula doesn't bounce off of rocks.
 * Fistula was inspired by a boss of the same name in another Edmund McMillen game, C-Word.
 * Fistuloid is an enemy variant of this boss combined with what looks to be a Charger or Spitty.
 * The Matriarch is a boss added in Booster Pack #5 that is simultaneously based of a Fistuloid, Chub, and this boss. The Matriarch will also spawn small Fistulas throughout the battle.
 * Mother will spawn a unique Fistula with a rotten appearance during one of her attacks.
 * A similar Looking Fistula can be encountered in the Corpse.
 * Fistula is one of four bosses that can be spawned by It Lives, the other three being Chub, Polycephalus and Teratoma.