The Lich (formerly known as The Lich King) is a necrotic villain in Adventure Time. His first appearance is in "His Hero" during a flashback about the adventures of the great hero Billy. He plays the main villain in "Mortal Folly" and "Mortal Recoil" where he nearly kills Princess Bubblegum. The episode "His Hero" showed that Billy cast him down, and "Mortal Folly" shows that he was ultimately imprisoned in a large glob of amber in the ancient tree that serves as the castle of the Candy Kingdom. After being defeated again in "Mortal Recoil," the Lich apparently continued to inhabit the body of the waving Snail, which appears in almost every episode.
Since Lord Death created the DWMA to combat those that become Kishin he then saw him as a serious threat.