The story of The Lich from Adventure Time.
Plotting Multiversal Destruction[]
According to the Lich himself, he was one of the monsters created before time existed, which means that he is older than existence itself. He at some point became fixated and began to worship the embodiment of chaos GOLB and vowed to bring about the extinction of all life in the universe to appease him and become his vessel. He came to Earth in the form of a Catalyst Comet during the Cretaceous Period, which caused the extinction of the dinosaurs. His essence was awoken once the Mushroom Bomb exploded, most likely because the material from the comet was used to produce the bomb. It is implied that at one point a human fell into his toxic waste, thus causing said person to mutate into the Lich's full form. Later on, the Lich was attacked by the hero Billy and trapped in a prison of amber in the heart of the Candy Kingdom after Billy personally pounded him into 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 threatening black shadow stands in front of a backdrop of flames. Fearing that the amber prison was weakening, she takes Finn and Jake with her 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.
The Lich after escaping from the Amber Prison.
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 turn black with green pinprick pupils similar to the Lich's eyes. Slowly, Waving Snail moved to the sphere of amber, headbutting the side until it cracks. The Lich quickly escapes through the tiny crack by turning into an oily black vapor and flying out the door, leaving behind a trail of death in his wake.
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 skeletons 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 burning green fire that hits Jake in the face, sending him teetering backwards. Finn hits the floor and shatters his protective gem and his world goes dark. The Lich tries to 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.
Possessing His Victims[]
The Lich possessing PB.
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 again tries to play a song and she put her hand into a 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 the 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 prevent him from giving Bubblegum the 'things' she wanted. These things where bleach, gasoline, etc. but before Jake could finish 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 punches 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 several candy citizens.
While in the mutated demon body, the Lich runs to the gates of the Candy Kingdom 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 freezes it and the plan works. But then Bubblegum's 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 forcibly. 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.
"Yes! One step closer!"-Snail possessed by The Lich.
The Snail is used by the Lich in "Mortal Folly" to free him 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" a bear gets the Enchiridon from Finn and gives it to the possessed snail. 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."
Time Room Incident[]
The Lich in Billy's dead body.
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 two-parter premiere "Finn the Human/Jake the Dog" 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". This implies that the Lich killed Billy as vengeance for imprisoning him. Finn and Jake pursue The Lich as he finds his way into the Time Room, where he wishes to Prismo the Wishmaster for the extinction of all life, laughing at the realization of his goal before vanishing, inadvertently creating a reality where he achieved his goals.
Aiming at undoing the Lich's wish and prevent him from doing it again, Finn uses his wish to wish that the Lich never existed. As a result, Finn is transported to a different timeline known as the Farmworld Timeline, where he is a human farm boy living with his normal, non-talking dog, Jake, his parents and his little brother. In said universe, Simon Petrikov sacrificed himself by getting in the way of the Mushroom Bomb, thus preventing it from exploding and awakening the essence of the Lich. However, once Farmworld Finn gets a hold of his Ice Crown to save his family from the Destiny Gang, he is driven insane by its power and is granted the same powers as the Ice King, which triggers a bunch of chaotic events that cause the bomb to explode and awaken the Lich's essence. Farmworld Jake then falls into the pool of said essence, thus mutating into a Lich/Dog hybrid and attacking Finn.
After seeing how horrible life was with the Lich never existing and that the Lich will always come to exist no matter what, 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 surprised Lich tries to change his wish but it was too late and Finn and Jake go back to Ooo, with the Lich now stuck in the Time Room, his wish wasted, with no means to leave. Lich is now extremely furious at Jake for undoing his wish. Finn is concerned with the Lich's wish but Jake assures him that everything is now fine.
At the Citadel[]
Some time after the events of Billy and the Time Room, Finn learns of a place called The Citadel, where his father has been imprisoned. The Lich is currently in a standstill, with no one to kill and lying lifeless in the Time Room. Finn tries to get the Citadel by taking Prismo's human body, in which killing him will commit a cosmic crime and take Finn and Jake to the Citadel. However, with a new presence in the room, the Lich springs to life and knocks Finn and Jake aside, killing Prismo and destroying his human body. A portal opens and the Lich laughs, before being crystallized and drawn into the portal.
The Lich as a skeleton.
Finn and Jake grab on to the Lich's crystallized form, and it is drawn to a sphere in the middle of the Citadel alongside countless other villainous creatures. As Finn and Jake search for Finn's dad, the Lich's breath corrupts the crystal and releases a noxious gas throughout the citadel, lighting a black fire and melting the crystals imprisoning the creatures within. After Finn and Jake reunite with his father (who does not recognize Finn) the Lich appears in the water and the crystal containing him explodes, reducing him to a skeleton.
The Lich, as a skeleton, knocks out Jake and Finn, slowly walking towards them. He proclaims that he will use the Citadel's denizens as an army and extinguish all light and life. He reaches out his hand to Finn to kill him, but Finn sprays the Lich's hand with a Guardian's blood, causing flesh to grow on the Lich. In pain, the Lich starts uttering nonsense, and Finn, realizing this weakness, sprays Lich with the blood, causing him to grotesquely mutate and collapse. When Finn returns to Jake after failing to reach his father, Jake reveals the Lich has been reborn as a human child, and it is though that his mind is rebooted. At the end of the episode, "Baby Lich" says a proud "Hello!"
An Evil Lurking Within[]
However even turning into a baby wasn't the last we have seen of the Lich In the episode "Gold Stars" his human child form, Sweet Pig-Trunks (Sweet P) has a nightmare about his past life as the Lich as he appeared in "Mortal Folly" laughing and destroying everything. It can be implied that he has frequent nightmares. When the King of Ooo and Toronto threatens to burn down the orchard, the Lich regains control over Sweet P and almost kills them. With this said, it is revealed the Lich is still alive, but just trapped inside Sweet P's subconscious and awakens when he sees fires or is threatened.
Status After the Finale[]
An older Sweet P, now a heroic adventurer like Billy and Finn, a far cry of the threat the Lich used to be.
Although the Lich himself did not appear during GOLB's arrival in the series finale "Come Along With Me", it is certain that his spirit still exists within Sweet P's consciousness, who, as the epilogue montage shows, kept living with Sweet Trunks and Mr. Pig and even managed to graduate from school or college, though what career did he specifically choose or what degree he graduated with are both unknown.
One thousand years after the events of the show, Sweet P is shown as an adult hero wandering the Land of Ooo, so it can be assumed that the Lich's spirit is still dormant. Nevertheless, as Sweet P is depicted as having become a hero like Finn and Billy, even shown to be friends with Ooo's new adventurers Shermy and Beth, it's highly likely that the Lich's control over him is possibly gone or is at least controlled.