Lich (Adventure Time)

The Lich is an undead monster of nearly infinite power. His origins are still vague, but it is known that he was created after the detonation of a nuclear bomb. The nuke was of unknown explosive yield. The leftover waste gathered in the crater, and it is assumed The Lich was a human that mutated near the impact point. How The Lich gained his magical powers are unknown, but even if they are magic is also ambiguous.

Personality
The Lich is cold and calculating. He does not waste any time on monologues or complicated plans.

The Lich has one goal born from his mutated, corrupted mind. He will do anything to end all life in the multiverse, even at the cost of his own life.

As The Lich
In his purest form, the Lich is a creature with unnatural powers that could bring the end of the world if he wishes. He is immortal, unable to be killed, only leaving the option to trap him.

He only wishes to destroy all life, his mind corrupted by the nuclear bomb. He has no other wish than to turn the universe into a lifeless void, and has the power to do so.

Common abilities the Lich demonstrates is mind control and possession. He can stop even the strongest-willed heroes in their tracks just by muttering the word "Stop". He also has possesed Princess Bubblegum and turned her into an extremely deformed version of herself, in an attempt to destroy the Candy Kingdom.

Other powers he uses are shooting green flame bolts, pyrokenesis, kill weak-minded beings just by touching them, extremely fast reflexes, and immense physical strength.

The only two ways to truly kill the Lich is use The Gauntlet of The Hero against him, or to wear protective gear that will deter his power against you. However, his soul will still exist, looking for a new vessel to possess.

Eventually he was defeated, and his skeleton was trapped in crystal. His prison was broken during a nearby battle, against Citadel Guardians. His skeleton was then splashed by a Citadel Guardian's blood, regenerating his flesh and reverting his age. The Lich was now a giant infant, named Sweet P, and adopted by Tree Trunks and Mr. Pig.

As Sweet P
All former motivations of The Lich are gone from his mind. He is now as innocent and intelligent as the average five-year-old. However, this does not mean the Lich is gone forever. It has been shown Sweet P can use the Lich's powers when he is threatened or has strong negative emotions.

Princess Bubblegum
The Lich possessed Princess Bubblegum in an attempt to destroy the Candy Kingdom, and ate toxic waste to grow into a giant monster. They were defeated, but most of her matter was corrupted and poisonous. Doctors put her back together, but she was now 13 years old physically.

Lich Snail
During the Lich's first appearance, in order to escape, he possessed a snail. With it's newfound strength it broke the amber encasing the Lich. His consiousness then transferred back to his normal body, but after he was defeated, he went back to the snail's body.

For the entirety of seasons three and four, the Lich Snail can be seen hidden in every episode. Eventually he finds a suitable host in the legendary hero Billy.

Billy
Technically, Billy wasn't possessed. Instead, because Billy was invulnerable to the Lich's mind control, he instead was killed and worn as a costume. He pretended to be Billy to open a rift in dimensions.

The Lich then went to the interdimensional entity Prismo to wish for the extinction of all life. Finn and Jake, the main characters of Adventure Time, survived this because they were outside of a dimension.

Finn then wished for the Lich to never had existed in the first place, creating a new universe where the nuclear bomb never dropped. Instead it was frozen in a thick layer of ice, stopping it on the ground. This world is called "Farmworld".

Farmworld Jake
In this world, Jake the Dog never became sentient or gained his magic powers. But eventually, Farmworld Finn accidentally set off the bomb anyway. Farmworld Jake was near the blast, and became that world's Lich. This is all we see of him, as soon after real Jake goes back in time to make sure the Lich never reached Prismo.