The Joker (Earth-43)

The Joker of Earth-43 was, like in many realities, the greatest enemy of Batman, who had become a vampire.

Pre-Bloodstorm
The full details of the Joker's earlier history are unknown, but he and Batman have cleary fought before.

Bloodstorm
Having 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 coordinate their expansion efforts. 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-, the now-vampire Batman- aided by were-cat Selina Kyle- is able to destroy the Joker's minions, tracking them down with Selina's aid and then contacting his daylight allies Commissioner Gordon and Alfred Pennyworth (Earth-43)|Alfred Pennyworth]] to eliminate them.

Leading the last of his 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, Joker only managing to kill Selina when she intercepted a crossbow bolt aimed at Batman. Enraged at her death, Batman pursued Joker into a church that Joker had filled with crosses as part of his last trap, only for the Joker to be shocked to learn that Batman is immune to traditional vampire weaknesses as he has never drunk human blood. However, due to his anger and grief at Selina's death, Batman surrenders to his vampire instincts after striking the Joker hard enough to break his neck, drinking the Joker's blood and becoming vulnerable to the crosses. Even as his body is burned, however, 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.

Legacy
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 drove him even more insane when the stake was finally removed, leaving him driven to stalk the streets of Gotham and hunt down his other enemies, often 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.