It's an average day in Metropolis: Superman breaks out of ArkhamAsylum, overseen by Solomon Grundy, Lois Lane is the richest, vilest, andbaldest woman in (and owner of) the city, and Bizarro #1 is the most beloved (and destructive) hero in the city....sorry, what?Well, it turns out that this is what happens when Mr. Mxyzptlk accidentally gives most of his power to The Joker, who creates a mind-bendingly odd world for giggles so that he will be able to kill Batman over and over and over again. Of course, in doing so he's risking tearing the fabric of reality apart at the seams, but when has he ever worried about that?What started out as a mind-boggling inversion of everything Superman knows to be true turns into a race against time to stop the Joker's reality warping and get the associated powers back to their rightful owner.
But for Me, It Was Tuesday: Satanus is completely unfazed with The Joker's take on him, chiefly because of the I Have Many Names nature of him. He also proves that he can step away from the Joker's incarnation of him very easily. It's implied he actually enjoys Joker's take of him as a piano playing crooner.
Cardboard Prison: The Joker is ultimately sent to a real prison called Slabside Penitentiary, which takes pride in the fact the no one has ever escaped from it. This didn't last as during Joker: Last Laugh, after being lied to and told he has terminal cancer, the Joker engineers and pulls off a jailbreak, and subsequent stories would see Joker back in Arkham.
Determinator: Superman offers to take Batman's memories of being horribly killed over and over and over in order to mend his (Batman's) utterly shattered mind. Superman was also the only being who never gave up under Joker's reign.
Did You Just Scam Cthulhu?: Mxy decides to give the Joker just 1% of his power to see what he will do with it. The Joker scams him into handing over 99%.
Enemy Mine: Darkseid of all people tries to do this with the council known as the Quintessence, which contains his mortal enemy Highfather, because The Joker could potentially gain the Anti-Life Equation with his powers, but The Joker warped them, too.
Even Evil Has Standards: The Joker will kill Batman and everyone associated with Batman, repeatedly, with creative and horrible ways. He will eat everyone in China. He plans to destroy the universe. He will not kill Superman after he's been turned into a dog, simply because killing a dumb animal isn't funny.
Fate Worse than Death: Satanus invokes the trope as he kicks Superboy and Supergirl out of Hell after they've been killed - after offering a standard Deal with the Devil, he gets annoyed with Supergirl's incessant prayer and sends them back to the Joker's world, commenting there weren't many Hells worse than that.
Gonk: Gorgeous Gilly is ugly as sin, but uses her powers to make men fall in love with her.
Gut Punch: Superman first assumes (when he finally realizes that this isn't right) that Mxyzptlk is responsible for all the changes. Standing on the Moon, he received a truly epic gut punch when he looks back at the Earth and sees its a cube with the Joker's face as the landmass on each side.
Late-Arrival Spoiler: If you bought the original monthly issues, then who exactly was responsible for this was kept secret until The Reveal halfway through, with the title of the arc being given as "Superman: Arkham!" The Emperor Joker oneshot at the halfway point was solicited as Emperor ?????. If you buy the trade, of course, it says Emperor Joker on the cover.
Logic Bomb: Leads to the Joker's defeat. How can he be all powerful if he can't just kill Batman once and for all?
Magic Kiss: The Joker has one. When Harley Quinn thinks she's about to die after all she's done for him, he says, "You kooky kid — c'mere," and as they both kiss, his is a powerful one that turns her into a constellation. So heartwarming.
Not-So-Omniscient Council of Bickering: Darkseid is aware of the Joker's ascension and the peril it places reality in (and how the lunatic has access to the Anti-Life Equation), so he contacts the Quintessence, a group of cosmic entities devoted to the good of the universe - to convince them not to bother him while he deals with the situation. Unfortunately, by the time he's presented his case, the Quintessence has already been reduced to imbeciles by the Joker.
Omnicidal Maniac: The Joker is prepared to unmake all reality simply because it created him.
Only Sane Man: Aside from the heroes, Joker left a single person untouched and drew him into his demented world - Lex Luthor. He was not pleased. Not pleased at all.
Reality Bleed: Small hints of standard reality leak through the Joker's rewoven fabric of the universe, offering the heroes hints that something—okay, everything isn't right.
Reality Warper: As annoying as Mr. Mxyzptlk is, the Joker with his power is worse.