Underfist: Halloween Bash is an animated spinoff special of The Grim Adventures of Billy & Mandy. It aired on October 12, 2008.
Voices
- Mandy/Aunt Sis/Kid/Ike - Grey DeLisle
- Grim/Sperg/Giant Candy Bar - Greg Eagles
- Billy/Harold/Pumpkin/Trick-or-Treater Eater #2 - Richard Horvitz
- Irwin/Hoss's Mom - Vanessa Marshall
- Hoss Delgado - Diedrich Bader
- General Skarr/Candy Skeleton/Rat/Creepy Gnome - Armin Shimerman
- Fred Fredburger - C.H. Greenblatt
- Jeff the Spider - Maxwell Atoms
- Dracula/Grandmama/Bougersnatch - Phil LaMarr
- Mindy - Rachael MacFarlane
- Bun-Bun/Trick-or-Treater Eater #1/Jimbo - Dave Wittenberg
- Perkins/Peanutbutter Wheel/Tiny Skull - Greg Ellis
- Confic/Humicun - Dan Gilvezan
- Singer - Jess Harnell
- Nergal/Police Officer/Tarvis - Martin Jarvis