Xa-Du, also referred to as The Phantom King, was a Kryptonian researcher, interested in improving research into suspended animation. However, the unethical nature of his studies led to the Kryptonian Science Council suspending his research and choosing him as the first inmate of the Phantom Zone. A sociopath, Xa-Du swore revenge on the family of the Phantom Zone's discoverer, Jor-El.
Over the course of his incarceration in the Phantom Zone, Xa-Du built an "ecto-suit" to allow him to walk free. However, Krypton exploded before it was complete, and his one attempt at revenge was thwarted by Krypto, the El's dog.
Years later, Xa-Du was able to use the temporarily reactivated Phantom Zone Projector to swap places with Kal-El, son of Jor-El, the hero Superman. Unfortunately his scheme was unsuccessful, and he was banished to the Phantom Zone.
At some point in the future, Xa-Du will become a member of the Anti-Superman Army, a grouping of super-criminals who oppose Superman. They will return through time to fight their enemy; however, they will be stopped.
In the New-52 Batman/Superman series, Xa-Du was the initially hidden main antagonist of the "Superman's Joker" story arc. In this story, after several of Superman's allies and admirers were killed my mysterious sentient bullets, it was revealed that the bullets were actually tiny kryptonians who had been brainwashed by Xa-Du into believing that Kal El and his father Jor El were the ones who had shrunk and imprisoned their city in a vase, instead of Brainiac. Batman compared Xa-Du's chaotically motivated horrific actions to those of the Joker, observing that the two had almost identical personalities. Even after Batman and Superman were able to imprison Xa-Du back into the Phantom Zone, the psychological damage that he had caused to many of Kandor's residents (including Superman's grandmother and aunt) remained, and they were stuck in an eternal unending thought loop.
DC Universe Online
Xa-Du is revealed as the main antagonist of DC Universe Online's "Death of Superman" arc, where he unleashes a modified Doomsday upon Metropolis so as to infect Superman with Doomsday Spores - making everyone think Superman was dead, he then captures Superman's "body" from the Fortress of Solitude and has him put on display in the Phantom Zone, revealing that Superman was alive but trapped in a painful state of paralysis.
Xa-Du's goal is to keep Superman a prisoner for all of eternity in the Phantom Zone, trapped in perpetual suffering, yet the player heroes and Supergirl team up to raid the Phantom Zone and defeat Xa-Du, bringing Superman back to Earth and stopping Xa-Du's sadistic plan.