UnderFell is an UnderTale AU. AU stands for Alternate Universe. UnderFell is fan-made so please don't scream at me for this, It does have it's own fan-game that's gonna be released soon.
UnderFell is a universe where all good characters become evil, and all evil characters become good. Fell Pap is the leader of the Royal Guard. He hates his brother Sans and makes sure he does his work. He is cruel down to the bone and is only nice to Frisk when they complete all his puzzles. His laugh is weird. I just had to say that. "NYEH HEH HEH!"