Death

"Those are the rules of the Land of the Dead, lose a music battle, lose your life" -Death

Death is the ruler of the Land of the Dead in the episode "Death in Bloom". He plays the double-bass drums in a competition against Finn. He almost kills Finn, but then Finn brings up Peppermint Butler and Death gives back Jake's memory and the life of Princess Bubblegum's plant. He is shown to have various powers, such as the "Kiss of Death", encasing people in forcefields, and sending people back to the Land of Ooo.

Peppermint Butler
Apparently, Death has had what seems to be a friendly relationship with Peppermint Butler prior to the episode "Death in Bloom," and possibly the show. 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 with him.

Abilities

 * Death can teleport two (possibly more) people or objects with some red beam that is cast from his eyes.
 * He is a skilled musician (apparently especially at death metal).
 * Death can create an energy prison to execute anyone who fails to best Death in a music battle.
 * In association with the prison, he can create flames from his skull to execute the loser as seen in "Death in Bloom." However, what it does is currently unknown.
 * Death can restore memory to those who drank from the "River of Forgetfulness" by the "Kiss of Death".

Trivia

 * Death was originally going to be some sort of Japanese cosmic alien, according to Jesse Moynihan's blog. This is why Death's Castle resembles a large Japanese rock garden. Moynihan later changed it to a design inspired by Alejandro Jodorowsky's The Holy Mountain, and later 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 this universe, there are some characters who can "cheat Death" and thus not be able to be controlled by him, such as The Lich and Marceline's dad. Marceline referred to this as being "deathless".

Quotes
See Death/Quotes