|First phase: 350|
Second phase: 100
Hornfel rides around in a minecart for the majority of the fight going in and out boss doors. He will do one of the following actions as he enters the room:
- Throws one to two bombs at Isaac.
- Pulls out three black mines with red blinking parts and throws them around the arena, that will fire brimstone beams in the cardinal directions after a delay.
- Goes faster and faster on a single track while throwing regular bombs. Afterward, he gets sick and vomits a fireball that will release fire in the cardinal directions.
Some other occurrences can happen as well:
- A fake Hornfel can appear instead of Hornfel, and hitting this dummy will spawn several troll bombs.
- These bombs detonate faster than regular troll bombs.
- Several enemies can be seen riding on the Minecarts during the fight that can attack Isaac, such as Blasters, Quakeys, Gurgles and Bonies.
- Objects such as fires, barrels and nitro barrels will sometimes pass through in a minecart.
- Upon Hornfel's entry, several Fly Bombs have a chance to spawn.
When Hornfel's health is low enough (Around 50 or so), his minecart will be destroyed, and he will try to escape through one of the many doors. He can still be damaged and even killed while he runs, but even if he escapes, the fight ends and Isaac wins the battle.
- Killing Hornfel before he can escape has a chance of dropping bomb-related items that he used in the fight. A separate Boss drop will also spawn regardless of if Hornfel dropped an item on death.
- These include regular bomb pickups, Sad Bombs, Hot Bombs, Stone Bombs, and Boom!, but does not include the brimstone bombs.
- A hornfel is a type of metamorphic rock known for its animal horn-like texture and its usage as a whetstone by miners.
- Hornfel's Brimstone bombs look like the grenades from the Scotch Guardian, a weapon that was contributed to Team Fortress 2's Workshop by Antibirth's lead designer Kilburn, which would eventually become the Iron Bomber.
- Hornfel is one of only 2 bosses whose boss rooms can be either regular sized or double-sized, the other being The Visage.