Underfist: Halloween Bash was the attempt to spin a new series out of the remains of the Grim Adventures of Billy & Mandy, but it wasn’t to be. Unifying various secondary characters from the former series, Underfist is more conventional adventure to Billy & Mandy’s more slapstick oriented cartoon insanity. Nonetheless it was a pretty enjoyable short TV-movie with a lot of verve and energy.
A former classmate of Mandy gets turned into a witch who wants to take over the world with the help of sentient candy. To the rescue comes a notorious action hero, Mandy’s reverse-unrequited love interest Irwin, Billy’s son (a gigantic spider), a retired super-villain (now a perfectionistic gardener) and a character from the original show who managed to be even more annoying than Billy himself. Since Unferfist is slightly modeled after superhero team narratives, you have the typical internal strife that nearly breaks the team, but later on they come all together to win the day.
Overall, fun diversion and could have been a fun series too, though whether it could have had the longevity of the main series remains questionable due to the more conventional plot structure.