Ice King's crown

Ice King's Crown is the source of his Ice Powers. The crown is golden, topped with three points and adorned with three red gems. One of the gems within the crown protects Ice King from the evil brainwashing spells of the Lich.

In "Holly Jolly Secrets Part Two" it is revealed that long before the setting of the story, possibly before the Mushroom War, Ice King purchased the crown in Northern Scandinavia from "an old dockworker" when he was still a human named Simon Petrikov, as a part of his studies to be an antiquarian of ancient artifacts. He rushed home to his fiancée Betty and put it on to try and make her laugh. Upon wearing it, Simon experiences visions, which he frantically fought with and shouted at. He also underwent a personality change that he apparently couldn't remember afterward. He realized the crown was doing these things to him and took it off quickly, only to find his fiancée terrified and looking at him with "such contempt." She soon left him. After repeated exposure, the proto-Ice King developed his Wizard Eyes and his body underwent radical changes with subsequent use of the crown. His skin turned blue, his nose grew long and pointed, his body temperature dropped, his hair and beard grew long and white. After so much exposure to the crown he gained power of snow and ice, causing his home town to freeze solid around him unintentionally (which may have been how he created the Ice Kingdom). Simon realized that the crown's effects were becoming permanent, but knew he was "too far gone" to do anything. After the first time he put on the crown he started a series of videos chronicling what had become of him, in hopes that one day someone would help him to free himself from the things the crown was doing to his mind. The crown causes Ice King's emotional confusion and madness, as compared to Simon's calmness and intelligence. Simon's desire to make Betty (his "Princess") love him again surfaces in Ice King's desire to marry a princess.

In "What Have You Done?," Ice King mentions that he did not steal the crown, although he did make it with stolen magic. It is proved that this is untrue, as stated in the Christmas special. Adam Muto commented Ice King is an unreliable narrator and disassociated from the past of Simon Petrikov. So he may not remember where he got the crown and simply made up the fact that he made it himself. In the episode " The Lich" Ice King has one of the gems used to unlock the Enchiridion's true power. It's not known if this gem is also the source of the crown's other abilities or it's ability to keep the Ice King alive from I Remember You.

Trivia

 * The gender-swapped version of the Ice King's Crown is a Tiara, as tiaras are worn by female Ooo royalty.
 * In the gender-flipped episode, Fionna smacks the tiara off of Cake's head saying "You'll catch her crazy!" Since Ice King wrote this fanfic, it is logical to assume that somewhere deep down in his psyche, the Ice King knows that the crown can drive someone mad.
 * In "What Have You Done?" the crown is shown to speak and feel regret in the scene where it cries on the shelf of penitence. It has a feminine voice. Whether this proves the crown is sentient or is only the work of the shelf is unknown.
 * In "What Have You Done?" the Ice King states that he "made the crown with magic that he stole," but in "Holly Jolly Secrets Part Two" it is revealed that the Simon Petrikov purchased the crown in Northern Scandinavia from an old dockworker, contradicting the first story.
 * Jake has been seen wearing the Ice King's crown in the episode "Thank You," though it had no effect. This is because Jake is already magic according to Adam Muto. Since Cake was able to use the Ice Queen's Tiara, it is likely the Ice King is unaware of this fact or he made the fan fiction before "What Have You Done?."
 * Shelf-of-Penitence.png the episode Paper Pete, when Finn knocks all the books off the shelf one of the books pages has a picture of the Ice King's crown.
 * In the short "Let's Work Together" Ice King's Crown has five points for a brief moment after he sacrifices himself to stop the Magic Freakin' Wand. It is also shown with five points when Ice King wakes an unconscious Finn in "Mortal Recoil."
 * In "The Lich," The gems of the crown were snatched by Finn, Jake and "Billy" (the Lich).