- This article is about the character. For the story arc he originated in, see Emperor Joker (Story Arc)
Emperor Joker was a form The Joker took after tricking Mister Mxyzptlk into donating 99.99% of his reality powers to him. As a direct result, he became near-omnipotent and created a hellish world in his own image.
At some point, the imp from the fifth dimension, Mister Mxyzptlk, attempts to see what a member of the regular realm would do with his power and selected The Joker. He intended to donate only 1% of his power to the Joker, but the mad clown tricked him into revealing his true name and thus ended up gaining 99.99% of Mxyzptlk's powers, and proceeded to transform existence within 69 seconds into a realm made in his own image, which was a twisted caricature, and with everyone stuck in a loop.
Emperor Joker in the meantime proceeded to entertain himself with various forms of murder, such as killing Lex Luthor over and over and devouring the entire population of China (even with such vast power Emperor Joker did many of these killings with a twisted sense of play). In the latter instance, he also mused jokingly that someone should have shouted "Peking, DUCK!!!"
Emperor Joker continually tortured and killed his adversary, Batman, every day, only to bring him back to life and do it over and over again - as could be expected Emperor Joker took extra pride in these particular killings due to the unique relationship the two characters continued to share.
Eventually, Joker managed to have Superman brought to him by Jimmy Olsen, who offered to kill him, but Joker proceeded to have Olsen killed instead via bludgeoning from robots wielding rubber chickens, as he felt that killing a dumb animal was unfunny, even for him (Superman had been transformed into a dog by that point). After Superman is returned to his regular form, they then fight, but then Joker uses his powers to crack the ground below Superman, also stating that the "next time I say 'Freeze! My Finger is loaded', maybe you'll believe me."
However Superman's powerful will allowed him to fight off Emperor Joker's influence enough to make contact with the weakened Mxyzptlk, who along with a less-powerful Spectre, encouraged Superman to work out the weakness of Emperor Joker before the mad clown destroyed reality due to his misuse of Mxyzptlk's power, as well as viewing his own existence as meaning the world is irredeemable.
As time runs out, Superman realizes that Emperor Joker's weakness is Batman himself - who the omnipotent villain can not erase from existence, since he still shares the iconic relationship his former self had with the Dark Knight (defining himself by his antagonism with Batman).
As a result of this bond Emperor Joker would be unable to truly destroy reality as it would result in the destruction of Batman - this causes Emperor Joker to lose control of his power and revert to his usual form - Mxyzptlk and the Spectre manage to reconstruct reality from the moment the Joker disrupted everything, but Batman is left broken from experiencing multiple deaths. Superman is forced to steal Batman's memories so that the hero can go on.
Powers and Abilities
As a direct result of his absorbing virtually all of Mxyzptlk's powers, Joker possessed not only his madness (which was amplified as a result), but also near-total omnipotence and omniscience, able to warp reality and time to his will. He was even capable of pushing reality further than Mxyzptlk had ever been capable of in the past, although this was because his insanity meant that he simply didn't care about the damage he was causing to reality where Mxyzptlk didn't want to risk leaving himself with nothing to do. However, as he lacked 0.1% of Mxyzptlk's power, the Joker was still limited by his human desires and obsessions, allowing Superman to undermine his control when he realized that the Joker literally couldn't erase Batman from existence.
In other media
Batman: The Brave and the Bold
An alternate (and tamer) incarnation Emperor Joker is formed in the cartoon series Batman: The Brave And The Bold - this time as a result of the manipulations of the troublesome Fifth Dimensional Imp known as Bat-Mite - who was seeking the "ultimate" Batman battle as part of his obsession with Batman. Bat-Mite began by freeing the Joker from Arkham Asylum using his power, prompting Batman to make the imp swear not to use his powers again. Bat-Mite tries to bypass this however by transferring his power to Batman during a battle with Joker - only to accidentally transfer it to the Joker instead: creating Emperor Joker in the process. Emperor Joker proceeded to cause mayhem and even created his own "Joker-Mite" to attack a powerless Bat-Mite. The rest of the episode details a lighter (but still sinister) reshaping of reality by the omnipotent Emperor Joker - it is also surprisingly brutal despite the obviously lighter tone (especially when Emperor Joker begins to kill Batman in many twisted ways, only to bring him back to life - much as he had done in the original comic book storyarc). After repeated deaths Batman informs Emperor Joker that he has lost everything save his sanity - prompting Emperor Joker to enter Batman's mind in an attempt to drive him completely insane. In Emperor Joker's absence Bat-Mite and a now mute Harley Quinn team up to defeat "Joker-Mite" but are unable to help Batman. However Batman's incredibly powerful willpower allows him to turn the tables on Emperor Joker and shows him a world without Batman, a world where crime runs rampant which The Joker likes until he realizes without Batman, there could be no Joker and in that world he is perfectly sane - which frightens Emperor Joker into surrendering his powers (much like in the comic book storyarc) - unwilling to live in a world without his arch-nemesis.