The Joker of Earth-43 was, like in many realities, the greatest enemy of Batman, who had become a vampire.
The full details of the Joker's earlier history in this universe are unknown, but he and Batman have clearly fought before, their history at this point most likely resembling the pre-Robin encounters they had in Earth-1.
Taking ControlHaving somehow learnt of the presence of vampires in Gotham, the Joker tracks down the remaining nest of vampires, armed with holy water and a crucifix, and confronts them with their inability to think beyond their next meal in their current condition with Dracula's death, offering to take control to organise their expansion efforts and help them deal with their mutual enemy of Batman. Impressed by his nerve and acknowledging his point about their short-term actions, the vampires (with the vampire Creach as the Joker's 'right-fang-man') agree to let the Joker lead them as a human so long as his leadership produces results.
Although the Joker successfully coordinates their efforts to turn and take control of all of Gotham's major crime families (intending to expand from there to turn the rest of Gotham, and then the rest of the country), he soon learns while dining at a club owned by now-vampire Manny the Shark that his old enemy, Batman, has also become 'involved' with vampires, the difference being that Batman is now a vampire himself where the Joker merely works with them. Reasoning that he needs a new and unexpected trap to catch Batman off-guard, the Joker kills the priest of a local church to use as the setting for his 'bat-trap'. However, while the Joker prepares his trap, Batman, now aided by were-cat Selina Kyle, is able to destroy the Joker's minions, tracking down their new nests with Selina's aid and then contacting his daylight allies Commissioner Gordon and Alfred Pennyworth to eliminate the turned mobsters while they slept. With most of the vampires having settled into the mobsters' homes and thus making themselves easy targets, the only vampires left in the Joker's army were the fifteen who had been hiding in the church basement.
The Final Confrontation
Leading these last few vampires into a warehouse, the Joker attempted to capture Batman and Selina in an ambush, but the plan resulted in the deaths of all his minions, the vampires' attempts to shoot the two with crossbows completely missing their targets. Disgusted at how the vampires have gone 'soft' from 'living large' Joker steps in himself after Creach's death, only for Selina to intercept the crossbow bolt aimed at Batman as he killed the last vampire. Enraged at Selina's death, Batman pursued Joker into the church, Joker having filled the interior with crosses as part of his last trap. However, the Joker was shocked upon discovering that Batman is immune to traditional vampire weaknesses (such as crosses) as he has never drunk human blood.
Even as Batman grimly informed the Joker of his still-pure heart, his anger and grief at Selina's death swiftly overtook his senses, prompting Batman to surrender to his vampire instincts after striking the mocking Joker hard enough to break his neck. Driven by pain and grief at the knowledge that Selina's love had been destroyed by the Joker, Batman drank the Joker's blood, reveling in the taste for the moment before he realized what he had done and became vulnerable to the crosses. Despite the pain of the crosses as his body is burned, Batman remembers to stake the Joker in the heart before the bloodloss can kill him, slaying the undead before he dies and preventing the Joker coming back as a vampire.
Stricken with guilt and self-loathing at his murder of the Joker, Batman fled the church where the Joker had died, convinced that, at the last, the Joker has won by turning him into a monster as bad as Dracula and even the Clown Prince of Crime himself, Batman having drunk the Joker's blood in a predatory act of rage rather than any sense of 'justice'. Although he arranged for Gordon and Alfred to stake him to stop him harming anyone else, the subsequent months Batman spent rotting and paralysed by the stake- staking only immobilizing the vampire unless decapitation occurred as well- drove him even more insane, fully aware of the decay his body was experiencing and unable to do anything other than think about the blood he had taken from the Joker.
Eventually, Alfred convinced himself that his master couldn't be evil despite his transformation given that the Joker had been his sole victim, prompting him to remove the stake, thus freeing Batman from the paralysis. However, Batman's new thirst for blood drove him to stalk the streets of Gotham and hunt down his other enemies, draining their blood and decapitating them to prevent them returning as more vampires. He was often shown reflecting that he has been cursed "by the Joker's foul blood" as much as he was cursed by Dracula's bite when he originally became a vampire, seemingly convinced that the Joker's blood had contributed to his current insanity.
He is revealed to have survived and attacked Pre-Crisis Gotham with Catwoman and Poison Ivy. He vampirizes Steve Trevor and Etta Candy before being killed by Wonder Woman.