Reaper

Reaper makes an appearance in "Wizard." In the episode, he offers Finn and Jake the chance to earn magical powers for free, but the evil catch is that they must stop a comet from hitting the academy. Finn haggled with the Reaper, saying he wanted more than just free Wizard Powers, so Reaper snapped off his arm and made it into a keychain. At the end of the episode, he offers them a Demon Heart. He may have been one of BUFO's minions. He is a Lich, which is an undead creature, usually associated with magic. He also appears in "Dungeon," where he challenges Jake to a duel.

He also appears in the FusionFall game in Eternal Vistas, guarding a portal that leads to a dungeon where you can find Finn and Stanley.

Quotes
"Hey kid. You want some magic powers? Magic powers for absolutely free?"

"Yes. Then the deal is struck!"

"Hey naked kid. You want some free... Demon Heart?"

Triva

 * In "Dungeon", one of his arms seems to pass through his robe.