Jax-Ur is possibly the most amoral villain ever to be imprisoned in the Phantom Zone. He is a scientist with absolutely no regard for human, kryptonian or any other form of life that would stand in the way of his research. the fact that he has all the power of Superman makes him all the more dangerous.
Long before the destruction of Krypton, Jax-Ur actually destroyed one of Krypton's moons in a failed experiment for interstellar travel. He showed no remorse for this action, and for his transgression was imprisoned by the Kryptonian high council in the Phantom Zone. The destruction would prove to have far greater consequences later on, as it would eventually draw the attention of Brainiac, resulting in the disappearance of the city of Kandor.
When General Zod was released from his own imprisonment in the Phantom zone, he freed Jax-Ur to fight Superman, but is returned to the Phantom Zone by Superman.
Later on, when Brainac was defeated by Superman on Earth and New Krypton established, Zod, now general of kryptonian military guild, once again freed Jax-Ur and sent him to Earth to serve as his sleeper agent.
Jax-Ur complied, posing as a famed geneticist on Earth, and abducting people for his experiments. He became aware of Flamebird and Nightwing capturing Zod's sleeper agents, but was surprised when they actually came to him for help, unknowingly of course.
Nightwing, also known as Chris Kent, went through another growth spurt due to his Phantom Zone physiology, and ended up in a body of an eighty year old. To save him, Thara Ak-Var, aka Flamebird and Doctor Light called upon Jax-Ur's human alter ego of Doctor Phillings to help them.
Jax-Ur was able to do so, but would not do it without a price. So through Chris, he induced Thara's Flamebird persona and posed an ultimatum: He would save Chris if she would give him a sample of her blood. Flamebird agreed and Jax-Ur saved Chris. The two left none the wiser, and he was left with a sample of Flamebird's blood to experiment on.
Recently, it was revealed that, like Chris Kent and Thara Ak-Var are avatars of Nightwing and Flamebird, respectively, so is Jax-Ur an avatar of their enemy, Vohc the breaker. Continuing his age old vendetta against the two demi-gods, Jax-Ur/Vohc uses the sample from Flamebird and his own ingenuity to create an artificial avatar of the Kryptonian chief god, Rao, a terrifying weapon which he then unleashes upon the world.