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"Mortal Folly" is the twenty-fourth episode of the second season of the American animated television series Adventure Time. The episode was written and storyboarded by Adam Muto and Rebecca Sugar, from a story by Mark Banker, Kent Osborne, Patrick McHale, and series creator Pendleton Ward. It originally aired on Cartoon Network on May 2, 2011. The episode guest stars Ron Perlman (pictured) as the Lich. Perlman's character would become the series' chief antagonist, and would reappear in several fourth and fifth season episodes. "Mortal Folly" was the first part of a two-part episode; this marked the first time an Adventure Time episode carried into another episode. The plot was concluded with "Mortal Recoil" which aired on the same day. The episode introduced the Lich, who had first been proposed in the series' pitch bible. Sugar and Muto decided to make the Lich a scary and dangerous character, and purposely played up the darker elements of the episode. The episode was watched by 1.92 million people and received largely positive critical attention; many critics enjoyed the way the episode began to set up the series' mythology.

Cartoon Network's first on-air logo used from 1992 to 2004.