What if the Lich is a woman... and Marceline is her daughter? Think about it. If Jake turned into the Lich in the alternate world, then who turned into the Lich in the normal world? It would have to be someone durring the time of the Mushroom War.
The term "Lich" was actually founded by Wizards of the Coast, aka Dungeons and Dragons. It is a character template a wizard takes when he trades his mortal soul for ultimate magical power ... but becomes a monsterous undead creature.
Sa bout to go dauouuunnnn!