In "The Delicious Last Course" DLC, there is a puzzle on the map for Isle IV involving a trio of contest winners towards the back of the Isle and a set of 9 gravestones towards the front of the Isle. Completing this puzzle will unlock Spoiler:a secret boss fight that takes place in a dream world held up by the skeletal remains of the Devil (seen in claymation), where you fight angel and demon variants of the Devil on both sides of the stage. However, they each switch places back-and-forth every time you turn left or right, presumably due to Cuphead's interactions with the Devil in the main game preventing him from seeing an Angel. This boss fight was originally designed as a second phase for the Devil's fight in the main game, but ultimately went unused until the release of The Delicious Last Course. A primitive version of the phase can be found in the game’s data. This boss cannot be refought after beating it. Beating it will unlock the Cursed Relic as a charm as well as the achievement "A Horrible Night to Have a Curse", a reference to the text box seen during the day-to-night transition from Castlevania II: Simon's Quest.
Contributed by MehDeletingLater