The Lich, also known as the Lich King, was a villain in Adventure Time and served as the main antagonist of the episodes "Mortal Folly" and "Mortal Recoil". After being defeated again in "Mortal Recoil," the Lich apparently continued to inhabit the body of the waving Snail, which appears in every episode. In the episode "The Lich ", he posed as Billy to force Finn to assist him to unlock the true secret of the Enchiridion.
Early on, the Lich (or Lich King) had a plan to suck all life from the Land of Ooo, the setting of Adventure Time. Using the stolen power, he planned to destoy all life, but he was attacked by the hero Billy. He was trapped in a prison of amber in the heart of the Candy Kingdom after Billy personally pounded him into it. In one of the recent episodes, it's implied that he was created during the Great Mushroom War, and didn't cause it.
One day, in the Candy Kingdom, Princess Bubblegum, the ruler of the Candy Kingdom, Finn the Human Boy, and Jake the Dog were thinking hard. While Finn and Jake were dreaming about regular day dreams, Princess Bubblegum saw a hellish scene where a threatning black shadow stands in front of of a backdrop of flames. Fearing that the amber prison was weakening, she takes Finn and Jake with herself to the prison. They wear protective gems first so they do not fall prey to the Lich`s mind-controlling voice, which was still a problem even though the Lich himself wasn't.
While they were up there, the Waving Snail (an Easter Egg which appears in every episode) comes out of Finn's backpack. Since the Snail was not wearing a gem, its eyes go big, hollow, and black with a strange glow coming from them. Slowly, Waving Snail moved to the sphere of amber. The Snail breaks the prison and an oily liquid comes out and forms the Lich, which turns into a black vapor and runs away, leaving behind a black trail of death.
Princess Bubblegum gives Finn the Gauntlet of Billy, which defeated the Lich the first time, and they start to chase the Lich. But Ice King comes in and pesters Finn and Jake for permission to marry Princess Bubblegum. Finn and Jake shut him out and run along where they come close to hit the Lich with a power beam from the Gauntlet several times but Ice King gets in the way.
Annoyed, Ice King decides to kidnap Princess Bubblegum instead. Just as the Lich is about to be blasted, Ice King comes in with the captive princess. This allows the Lich to escape to his Lair to regain his power via the Well of Power.
Following the Lich into the Lair, Finn and Jake are met by dead human skeltons which attack them, but are destroyed by the Gauntlet. Going down even farther, they look through a collapsed ceiling the Lich standing over the Well of Power (a radioactive pit of broiling green liquid). The Lich thrusts his hands into the Well and soaks up the power making pale green smoke come from the Lich`s eye sockets.
Finn then fires a beam down at the Lich and Jake uses his stretching powers to make wings that glide Finn towards him. Before Finn could deliver another blast, the Lich grabs Finn by the Gauntlet and brings him up to his huge skull face. The Lich brings up a finger to its mouth and says shhhhh. The Gauntlet then explodes in thousand of pieces leaving Finn`s hand singed and then the Lich throws Finn to the other side of the room.
Ice King and Princess Bubblegum come in, see the Lich and freak out. Jake then goes up to the Lich carrying Finn on his back and barks at him. The Lich then simply creates a ball of buring green fire that hits Jake in the face, sending him tettering backwards. Finn hits the floor and shatters his protctive gem and his world goes dark.
The Lich tries and make Finn walk into the Well of Power, but Finn fights the heavy voice of the Lich King. He goes back into his world and Finn brings out the sweater that Princess Bubblegum had made him and uses that as his weapon.
Finn pulls the sweater in one eye socket and then out the other and starts to pull. Surprised, he staggers but then regains balance and laughs at Finn. The hero breaks the Lich's skull and he turns to dust, sending him to the floor.
Jake then helps Finn up and the Ice King frees Princess Bubblegum. With that the ice wizard drops Princess Bubblegum into the broiling Well of Power. Later what remains of Princess Bubblegum is brought to a hospital where she is revived. But she seems strange. In her room she tells Finn to bring her some things. At that moment Jake comes in with Peppermint Butler but when Peppermint Butler sees Bubblegum talking to Finn, he goes on four legs and hisses at the Princess and scampers off.
When Finn leaves to get the 'things' for the Princess Jake tries to sing her a song but she is angered by this and twitches followed by yelling at Jake in an angry babble. Jake agains tries to play a song and she put her hand into the large amount of picked flowers brought to her by Finn and Jake and they all wilt and die. Again Jake tries to sing and Bubblegum and her bed fly into the air and all of flowers are consumed by a fire that has the faces of a thousand of tortured spirits.
Jake runs out and tries to warn Finn that something is wrong and tries to prvent him from giving Bubblegum the 'things' she wanted. These things where bleach, gassoline, ect. but beofre Jake could finnish Bubblegum takes all of the things from Finn. They enter her room and find Princess drinking the mixture of the toxic things that Finn had gotten.
Mutated Princess Bubblegum grown into a huge black demon which puches Finn and Jake through a wall and hurtle them beside the Ice King who was listening through the wall with a cup. The Princess grows even bigger until she break through the ceiling sending out chunks that almost kill sevral candy citizens.
While in the mutated demon body the Lich runs to the gates of the Candy Kigdom and is fought by a giant Gumball Guardian who is easily smashed to bits. Finn and Jake go out to fight her but Finn is punched taking out some of his teeth he is sent back to the tower where he came from; Jake is then caught by the Lich and is strangled.
Finn and Ice King work together and Finn stalls the demon while Ice King frezzes it and the plan works. But then Bubblegums foot was frozen at a wrong angle and the ice covered Princess fell over and broke into shards.
A close up shot at the end of Mortal Recoil shows Waving Snail on a window ledge waving forcably. Its eyes were sunken and dark and filled with a green deadlight which showed that it was a host body for the Lich King who was angry and looking for revenge on Finn. It is likely that since he is a lich he would have a Phylactery, which would house his soul. The Snail is used by the Lich in "Mortal Folly" to free the Lich from his amber prison. The Lich does so by taking control of Snail's mind and making him/her/it break open the prison. After the events of "Mortal Recoil", the Lich returns to the Snail's body. As of season 3, the Snail has continued to appear in every episode, now in possessed form. In "Following In Your Footsteps" the bear gives the possessed snail the hero's Enchiridon. In "The Lich," the Lich ends up changing bodies moving to that of the deceased Billy. After this, the snail no longer shows any further sign of possession as revealed in "Up a Tree". The Lich possesses Billy in most of the episode "The Lich," and tricks Finn and jake into gathering the gems of power needed to activate the Enciridion. The events surrounding how the Lich assumed Billy's identity remained ambiguous until the Season 5 season premiere where it was shockingly revealed by Prismo that the Lich was in fact wearing Billy's dead body as a macabre disguise the entire time when he refers to the Lich as, "a ghost wearing a dead guy".
Possessed Farmworld Jake
Finn used his wish to wish that the Lich never existed. This however triggered a series of events that ended with Farmworld Jake coming in contact with the pool of green goo created by the Mushroom Bomb, mutating into a Lich/Dog hybrid and attacking Finn.
After seeing how horrible life was with the Lich never existing, Jake tries to save the day with his wish. Prismo tries to help him with the perfect wish and Jake ends up wishing to change the Lich's wish to sending him and Finn home. Jake and Finn go back in time and sees the Lich get ready to wish for the end of all life on Ooo. However, thanks to Jake's wish, the Lich ends up wishing for Finn and Jake to go home safely. Realizing what he said, a now suprised Lich tries to change his wish but it was too late and Finn and Jake go back to Ooo, with the Lich trapped in the Time Room, presumably for eternity. Finn is concerned with the Lich's wish but Jake assures him that everything is now fine.
Finn and Jake decide to go to Prismo for advice on how to find Finn's long-lost father. Finn and Jake talk to Prismo at a party he is hosting with The Cosmic Owl, Death, Peppermint Butler, and Party God. Peppermint Butler asks Prismo if it is safe to be taking "selfies" with an unconscious Lich, to which Prismo replies that ever since he wished for all life to be destroyed, The Lich had been in a paralyzed state. Jake begins to panic when Prismo runs out of cheese crackers, but Prismo creates more.
Jake begins to talk to Prismo about the Citadel, he says that since he and Finn had used their wishes he brought Shelby along to make the wish for him, but Shelby wishes for a pony for his girlfriend instead. Prismo explains that you cannot wish yourself into the Citadel. Prismo tells Finn that to gain access to the Citadel, one must commit a "cosmic crime," and to do so, all Finn needs to do was to bring a sleeping old man to Prismo's dimension, without waking him up. Prismo gives Finn and Jake pen-lights. Finn and Jake then set out to find the sleeping man which Prismo said was located on a rock that looked like an upside-down duck.
Once they reach the rock, Finn and Jake take the old man and his bed back to Prismo's dimension, but on the way the old man begins emitting strange black beings that can be vanquished by the light of the pen-lights. Upon arriving, Prismo comments that he wanted to see himself before they wake the man up, puzzling Finn and Jake.
Prismo explains that he is nothing more than the manifestation of a dream being had by the old man. He then says that if the man had woken up, Prismo would cease to exist, until the man fell back asleep in 1000 years. And also that killing him would be a "cosmic crime" capable of sending them to the Citadel.
After reassuring Finn and Jake, Prismo tells them to wake the man, but before they can, The Lich springs to life and grabs the old man, waking him. Prismo disappears, and then the Lich blows toxic gas into the old man's nose, turning him into dust. This causes Jake to becomes enraged at his friend's death. Before Jake can act, a Citadel Guardian imprisons the Lich in a crystal cell and takes him away, but not before Finn chases after the Lich.
Finn and Jake hitch a ride on a Citadel Guardian after he imprisons and transports The Lich to the Citadel. Upon arrival, the Guardian attaches The Lich's crystal prison onto a much larger crystal "cell block." Finn and Jake then look around for Finn's long-lost father, whom Finn initially suggests might be a Guardian. Jake points out a strange looking "old kid" who resembles Finn, and it turns out to be Finn's father incarcerated inside a crystal jail cell. They then notice that the Lich's cell is excreting a gray slime that is beginning to infect the other prisoners and Guardians. The Lich's influence over the citadel enables a massive jailbreak. One of the prisoners is a voidmaster, who has the power to open portals, so he uses his power to attack a guardian by opening a void in front of a train that Starchie is driving in the Candy Kingdom, which crashes into a guardian. As gray slime fire forms on Finn's father's cell, Finn uses Jake as a morning star to free his father, who introduces himself as Martin. Martin is reluctant to answer either Finn nor Jake's questions about his motives for abandoning Finn. Jake begins to encourage Finn to leave his father behind, calling him a loser. Martin notes that the fighting will cease once the last Guardian dies. He then points out that the last Guardian is currently sinking into a gray swamp. The Guardian shoots a beam that hits Martin in the lower right shin, which obliterates all the flesh from a segment of his leg. Martin tells Finn that the blood of a Guardian contains healing properties. Finn and Jake retrieve the regenerative guardian blood and put it on Martin's leg, which is quickly restored to a healthy state. The Lich then entrances Finn, telling him that only death will be imminent for all life and that he is all alone in the world now. Jake then appears unconscious next to Finn. Finn wakes up and calls for Martin, who had fled with the other prisoners, abandoning Finn once again. Finn feebly attempts to strike the Lich as he encroaches. His attack was ineffective, but there was some guardian blood left on his hand from healing his father's wounded leg, which causes the Lich to scream as flesh begins to grows on his bones. Finn then throws more guardian blood onto The Lich, restoring more flesh around his chest. Finn then tells Jake to finish him off while he chases after his father. Martin and the other prisoners cut a stem connecting a floating bit of the citadel to the ground, and at Martin's request, the fugitive voidmaster creates a void to escape into, which begins to suck the floating chunk of land upward. Finn grabs the two halves of the severed stem, preventing his father from leaving, but he struggles to hold on. Jake tells Finn to let it go, but Finn refuses and the Grass Sword takes over Finn's right arm, allowing him one last tremendous pull. Finn's arm gives out and is severed and Martin says "Wait, what about air" really fast right before escaping through the portal. Finn falls into the water below where a small piece of Guardian blood touches his arm, making the stump of an arm sprout a flower. Jake pulls Finn to shore where Shelby is revealed to have been hiding out the entire time. Finn is reminded of the Lich and asks Jake if he finished him off. Jake says there's no need, as the Guardian blood transformed the Lich into a baby-like creature, rendering him seemingly harmless. Finn, Jake, Shelby, and the infant Lich return to the Candy Kingdom by way of the portal that Starchie drove through during the battle. Later, Tree Trunks and Mr. Pig are sitting in her house when outside Finn and Jake's chatter can be heard. The doorbell rings, interrupting a somber Tree Trunks saying, "I think we should get a d-". The couple sees outside their door an infant Lich in a stereotypical abandoned-baby basket. Tree Trunks looks excited and implies that this sudden addition to their family has changed her feeling about whatever she was about to say.