Death

Death is the ruler of the Land of the Dead in the episode, "Death in Bloom." He also rules over the 37th Dead World, as seen in "Sons of Mars." He plays the double-bass drums in a competition against Finn. He almost kills Finn but gives back Jake's memory and the life of Princess Bubblegum's plant after Finn mentions Peppermint Butler. He is shown to have various powers, such as the "Kiss of Death," encasing people in force fields, and sending people back to the Land of Ooo.

Peppermint Butler
Death has what seems to be a friendly relationship with Peppermint Butler in the episode "Death in Bloom," though it is unknown how long they have been acquainted with one another. It was at the revelation that Finn and Jake know Peppermint Butler that Death allowed them to return to the surface, even though Finn had lost a music battle to him.

Abraham Lincoln
Death seems to have encountered Abraham Lincoln, the King of Mars before. In "Sons of Mars," he rejects Lincoln's initial offer of a penny by replying, "Not this time." He seems to have respect for his king status, referring to him as "your majesty."

Abilities

 * Death can teleport two (possibly more) people or objects with a red beam that he casts from his eyes.
 * He is a skilled drummer. His genre seems to be death metal, a pun on himself.
 * Death can create an energy prison to execute anyone who fails to beat him in a music battle.
 * In association with the prison, he can create flames from his skull to execute the loser.
 * Death can restore memory to those who drank from the "River of Forgetfulness" by the "Kiss of Death," in which he kisses the victim to restore their memory.
 * He is able to become intangible.
 * Death can apparently bring things back from the dead, such as the Princess Plant and Jake.
 * He can also take away a person's immortality. He is shown to do this by wobbling his head.

Episodes featured

 * "Death in Bloom"
 * "Sons of Mars"

Quotes
See Death/Quotes﻿

Trivia

 * Death was originally going to be some sort of Japanese cosmic alien, according to Jesse Moynihan's blog. This is why Death Castle resembles a large Japanese rock garden. Moynihan later changed it to a design inspired by Alejandro Jodorowsky's The Holy Mountain. Afterward, he changed the character's head into a horse skull because he got self-conscious about the similarity.
 * Although Death is one of the most powerful beings in the universe, there are some characters who can "cheat Death" and thus not be able to be controlled by him, such as the Lich and Hunson Abadeer. Marceline referred to this as being "deathless."
 * Death looks like White Spy from Spy vs. Spy, only their noses are different. Death's nose is heart-shaped while White Spy's nose is triangle-shaped.
 * Jesse Moynihan confirm what the Death of Adventure Time will the Death of Regular Show.