Fear Feaster is a reccuring villain in Adventure Time. Serving as the main villain in Ocean of Fear and Billy's Bucket List, and as a minor villain in King Worm. He is voiced by Mark Hamill.
The Fear Feaster makes his appearance in the episode "Ocean of Fear." He first introduces himself as the manifestation of Finn's fear, but Finn and the Three Wise Men later call him the Fear Feaster. He appears to live inside Finn's stomach and emerges from his bellybutton when Finn realizes that he is afraid of the ocean. He mentions repeatedly that he hates this arrangement, as Finn's guts aren't very pleasant. The Fear Feaster is defeated when the Three Wise Men—who appear to be similar kinds of entities—emerge from Finn's belly and convince him that he is still a hero. After his defeat, the Fear Feaster is banished back into the depths of Finn's guts. Finn said to the Fear Feaster, "Everybody has a flaw, and it looks like yours is smelling like my nasty guts."
Fear Feaster makes a brief appearance in "King Worm" unlocking a chest that contains Finn's deepest gut fears in order for him to defeat King Worm.
The Fear Feaster makes his final appearance in "Billy's Bucket List," where when Finn tries to complete Billy's bucket list, he is turned away by a wave (similar to "Ocean of Fear"). After Finn knocks himself out in order to prevent the Feaster from increasing his Fear. He has a dream, he wakes up, and, to his dismay, he's in the ocean. The Feaster, using this to his advantage, tries to mess with Finn. However, as the Fear Feaster does so by saying "Wake Up... WAKE UP!..." and laughing evilly, Finn's sword takes control of his body once again and slashes through the Fear Feaster, cutting him from Finn and ultimately destroying him. Very soon after, Finn realizes that he is not afraid of the ocean anymore.