Hornfel

Hornfel is a boss added in The Binding of Isaac: Repentance that can appear in.

Phase 1
He appears with his own specially designed boss room, where the walls are covered in boss doors, with minecart tracks connecting them. Hornfel rides a minecart along these tracks and performs the following attacks upon entering the room: Hornfel and his decoys deal a whole heart of contact damage.
 * Occasionally, when Hornfel enters the room through a door, he is accompanied by 2 Fly Bombs.
 * Throws 1-2 Bombs at Isaac. Additionally, each bomb has a small chance of having the or  effect.
 * Throws 3 that resemble mines. Each has a delay and detonates after a few moments.
 * Accelerates along one set of tracks, while frequently throwing out bombs for the duration. Slows down after multiple cycles, before getting sick and vomiting a fireball that release fire trails in the cardinal directions upon landing.
 * Sends out 1-2 decoy dummies with 4 HP on other tracks that burst into 3 Troll Bombs if killed.
 * Other minecarts occasionally appear on other tracks independent of Hornfel, carrying fire, TNT, or a variety of other enemies:
 * Bony
 * Faceless
 * Danny
 * Gurgle
 * Quakey

Phase 2
Once Hornfel has taken enough damage, he falls out of his minecart and attempts to escape from the room via one of the doors surrounding the arena. He makes no attempt to attack Isaac but he can still deal contact damage.
 * Other minecarts carrying other monsters and objects can still appear as usual during this phase.
 * If Hornfel escapes successfully, the fight ends as normal.
 * If Hornfel is defeated, he drops either 3 Bomb pickups,, or  and unlocks , if not already unlocked.

Trivia

 * Hornfels is the group name for a set of contact metamorphic rocks. They are known to be very hard and durable.
 * The Horny Boys are a stitched amalgamation of Hornfel and Loki.