Emperor Joker refers to The Joker during his role as an omnipotent being after successfully usurping the powers of Mister Mxyzptlk - recreating the world in his twisted image Emperor Joker became the central antagonist of a major crisis and crossover event between Batman and Superman.
Superman Story arc
Emperor Joker was the result of the regular Joker stealing the unlimited reality-warping abilities of the Fifth-Dimensional Imp known as Mister Mxyzptlk.
Armed with the power of a god the newly named "Emperor Joker" proceeded to remake the entire world into a twisted caricature, with everyone in it stuck in a loop.
Emperor Joker 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).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.
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 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 (in which he defined himself with 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 back 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.
Batman: The Brave And The BoldAn 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 story arc).
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 were crime runs rampant which The Joker likes until he realizes without Batman then 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 story arc) - unwilling to live in a world without his arch-nemesis.
Emperor Joker was insane, even by Joker standards, perhaps due to the fact that with his new-found omnipotence he was capable of living out his violent fantasies in a way he never could before - he casually committed unthinkable acts of mass murder and mayhem only to correct them so he could do it all over again, leaving the world in a state that can only be described as Hell.
Emperor Joker still retained a twisted sense of comedy and sadism, revealing in death and misery just as his normal self has done for generations - the main difference is that Emperor Joker is arguably slightly more cruel and psychotic than most depictions of the mainstream Joker (though both are without a doubt vicious).
Perhaps the one portion of Emperor Joker's personality that was unique was the establishment of an emotional dependency on Batman - although almost all versions of the Joker have showed obsessive behavior towards Batman, Emperor Joker took the concept a step further: Emperor Joker could not exist without Batman - defining his very self by his continual struggle with the Dark Knight.. yet in his psychosis he still felt the need to torture and kill his foe, only to bring him back to life again and again - unable to actually destroy him (despite having the power to do so effortlessly).
He also at one point planned to destroy the entire universe, himself included, due to feeling his own existence automatically made the universe irredeemable, hinting he hated himself.
Emperor Joker was nigh-omnipotent, and displayed reality-alteration on a scale unimaginable by mortal minds -but apparently very common in the Fifth Dimension - however none of these powers were his to begin with, having been stolen from Mister Mxyzptlk via trickery.
Although very creative with the use of this power Emperor Joker was still the same murderous madman he has always been and thus he openly abused this power and actually began to destabilize reality as a result.
As with many "omnipotent" beings in fiction Emperor Joker did display a weakness, namely his emotional dependency on Batman, without which he would have arguably been unstoppable.