Underfist is a unmade spin-off show of The Grim Adventures of Billy & Mandy (and Grim & Evil), however due to Maxwell Atoms's contract with Cartoon Network expiring the seires was never made. But one special movie was made and was classed as the final episode of The Grim Adventures of Billy & Mandy, the episode was called Underfist: Halloween Bash.
The show is about a unusual team of friends who go about saving the world (or the town Endsville) from monsters. The Underfist is made up of Hoss Delgado, Irwin, Jeff the Spider, Fred Fredburger and Skarr.