Hurlsvelgr flies from either the left or right of the screen and has two attack patterns. It either charges across the screen, or hangs back to spit rainbow projectiles at Specter Knight. The bottom half of his body is armored, so he can only be struck from above by taking advantage of the nearby lanterns. When hit, Hurlsvelgr gets knocked back and vomits a rainbow which create a solid platform to land on and strike it a few more times before he flies away. It deals one full damage point and has 12 hit points.
- The name Hurlsvelgr is a pun on "hurl" (as in the rainbow liquid he spews) and "Hræsvelgr", a giant eagle in Norse mythology sitting on northern edge of the heavens. The rainbow bridge-like platform he creates when hurt is a reference to Bifröst in the same mythology, a bridge that connects the Earth to Asgard, the realms of the gods.
- The Rainbow Bridge platforms in Shovel of Hope and Plague of Shadows are sculpted in the bird's likeness. In the games, Hurlsvelgr's fight is based off the Rainbow Bridge, but the bridge is based off the bird in-universe.
- In the game's files, his model is named "Rainbow Bird", composed of two separate sprite sheets: "Bird Boss" containing the creature's sprites and "Bird Boss Rainbow" containing his rainbow attack effects.