Dr. Xa-Du was a Kryptonian researcher interested in improving technology revolving around suspended animation.
Xa-Du's unethical methods when experimenting led to the Kryptonian Science Council to suspend his research. Jor-El then proceeded to imprison Xa-Du in the Phantom Zone, where he became it's first inmate. Xa-Du swore revenge on the House of El. Spending his years of incarceration building an an "Ecto-Suit" to allow him to interact with the world beyond the Zone when a chance was provided. However, his one chance to capture the infant Kal-El failed when the dog Krypto attacked him and entered the Zone with him. Eventually, several Kryptonian criminals, including Non, Jax-Ur, Ak-Var, Ras-Krom and Vak-Ox joined Xa-Du.
His experiments and longevity in the Phantom Zone slowly changed and morphed Xa-Du's apperance. Eventually his body became wrapped up in a strange substance, giving him a mummy like appearance. Several years later, the Phantom Zone Projector was temporarly activated and Xa-Du managed to again return to the "normal dimension" by switching places with a grown Kal-El, who had taken the name Superman. However, Xa-Du was once again unsuccessful and was defeated by the Superman. Hence when Vyndktvx gave Xa-Du a chance to join the Anti-Superman Army, the sociopathic doctor quickly accepted. However, every attack on Superman by the army ended up failing, one way or another.
Xa-Du was then approached by Harrow, who wanted to aid the Tower in killing Superman. Deeming Doomsday to be the best instrument to do so, Xa-Du decided to release the beast from it's forbidden corner. Before he did so, however, Xa-Du conspired with Brainiac to infect Superman with the Doomsday Virus. Doomsday proceeded to escape and create unstable tears in the Phantom Zone. Xa-Du explained these events to a team led by Batman, he then attempted to capture them with Non, but failed. Escaping back to the material world, Xa-Du was captured by Steel.