Underfist is a spin-off special of The Grim Adventures of Billy and Mandy. It aired on October 12th, 2008. It tied as the number 1 program for the week among all broadcast and basic cable networks with boys 6-11, and earned between 19% and 76% growth across all kid demos. thumb|300px|right
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On Halloween night, Irwin, Billy, Mandy and Grim are trick-or-treating. Billy, Mandy and Grim decide to go home after having no success with Dracula. Irwin realizes that his life is passing him by. Mindy (secretly turned into a witch by an evil marshmallow bunny named Bun Bun) fools Irwin into opening an Underworld portal releasing an army of evil candy warriors and Bun Bun. Irwin is tricked into following her into the Underworld alonside Hoss Delgado, and the ensuing conflicts lead to the formation of Underfist. The Underfist team is Irwin, Hoss Delgado, Mr. Reginald Skarr, Jeff the Spider and Fred Fredburger. Supporting characters are Billy, Mandy and Grim.
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Maxwell Atoms started production on Underfist a little bit after he finished with The Grim Adventures. The special was finished several months before it aired, but as it was a Halloween special it was delayed until October. Cartoon Network did not pay for a permanent crew for a single film, so much of the work was done freelance or by Atoms himself. The film also used a different overseas animation crew.
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Underfist tied for first in ratings for the week it was aired and has been nominated for Best Animated Television Production Produced for Children in the 36th Annie Awards. Jacob Paschal of Toon Zone gave the film a negative review, saying "Overall, Underfist is your typical annual Halloween film fluff designed to fill the airwaves and get younger folk hyped on something other than candy. Maybe you'll find that enough".