Fernus is the first boss enemy you encounter. It is fought on a conveyor belt leading to a furnace, with chargeable blocks falling onto it.
Fernus will open its mouth, pulling in positively charged blocks, which it will eat, damaging its jaw. After eating three blocks, Fernus will breathe fire onto the conveyor belt, which speeds up. This is repeated three times. In the hall entered before battling Fernus, hieroglyphs indicating how to defeat it are drawn on the ceiling and floor.
- The name "Fenrus" comes from Fenrir, a giant, god-eating wolf from Norse mythology, and from "furnace".