Finn loses his arm in the Adventure Time episode "Escape from the Citadel". This episode is from Season 6 and is the second episode of the season. In this episode, Finn, Jake, and the Lich are transported to the Citadel, where Finn and Jake meet Martin, Finns father. The three are forced to run from the Lich, as he begins to destroy the Citadel. At this time, Martin flees, and Finn tries to give chase, but his grass sword takes over his entire arm, causing Finn to lose it.