The Lich (formerly named the Lich King in the original pitch and "His Hero," and referred to as Sweet Pig Trunks in his baby form, or just Sweet P for short) is a powerful undead being and a major antagonist in Adventure Time.
In "The Comet," it is revealed by the Purple Comet that the Lich, like Finn, started out as an incarnation of the Catalyst Comet. The Purple Comet explains that it has embodied both the ultimate good by once becoming what would later be Finn, and the ultimate evil by at some point becoming the Lich in their respective time periods.
His first episodic appearance is in "His Hero" where the hero Billy casts him down. He was imprisoned in the ancient tree in the Candy Kingdom, but escapes and plays a significant role in "Mortal Folly," "Mortal Recoil," "Jake the Dog" and "Finn the Human." The Lich goes on to kill Prismo and releases the criminals in the Crystal Citadel in "Wake Up" and "Escape from the Citadel." However, by the end of the latter episode, the Lich is turned into a harmless baby by the Citadel Guardian's healing blood, and is adopted by Tree Trunks and Mr. Pig and renamed "Sweet Pig Trunks," Sweet P for short.
In "Gold Stars," it is revealed that the Lich's evil is still contained within Sweet P, and can take over when Sweet P is threatened and emotionally charged. Sweet P does not realize the evil lurking inside him, attributing his blackouts and dreams of being the Lich to being 'just a dream.'
Sweet P also possesses the potential to use the Lich's powers as indicated when he shows his victims another dimension, although it seems to be more limited as the Lich's death breath does not kill its victims as it had done before with Prismo.
The Lich is portrayed by Ron Perlman who also played Gnarlack in Fantastic Beasts and Where To Find Them.