Hornfel

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

Behavior

Phase 1

Hornfel appears with his own specially designed boss room: each wall has multiple boss doors with minecart tracks connecting each pair of doors. After a short delay, he appears out of a random door riding a minecart and performs one of the following attacks:

Occasionally, when Hornfel enters the room through a door, he is accompanied by 2 Fly Bombs.
Throws 1-2 Bombs at Isaac. Each bomb has a small chance of having the Hot Bombs or Sad Bombs effect.
Throws 3 Brimstone Bombs that resemble mines. Each bomb has a delay and detonates after a few moments.
Accelerates to a very high speed along one set of tracks, traveling in one direction, while frequently throwing out bombs. After multiple cycles, he slows down, gets sick, and vomits a fireball that releases fire trails in the cardinal directions upon landing.
Sends out 1-2 decoy dummies with 4 HP on other tracks that burst into 6 Troll Bombs if killed or walked into.
Other minecarts occasionally appear on other tracks independent of Hornfel, carrying a Fire Place, TNT, or a variety of other enemies:

Hornfel and his decoys deal a whole heart of contact damage.

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. In this phase, his contact damage is reduced to half a heart.

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, Boom!, Brimstone Bombs, Hot Bombs, Remote Detonator, or Sad Bombs, and unlocks Brimstone Bombs if not already unlocked.

Gallery

Hornfel in-game.
Hornfel after losing his cart.
Hornfel fleeing.
Hornfel's decoy dummy.
Concept art of Hornfel and his behaviors.

Unlockable Achievements

Brimstone Bombs - After breaking Hornfel's minecart, kill him before he can escape.

Trivia

Hornfel will sometimes taunt Isaac by sticking out his tongue while sending out his decoy dummies.
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.
Hornfel and Loki are siblings.^[1]