Krypton and the Fortress of Solitude

The Eradicator was created by Superman's ancestor, Kem-L to preserve Krypton's purity (or at least his version of it) from being corrupted by alien influences. One such alien, the Cleric, took the device and left Krypton with his followers. The Cleric's followers died shortly after leaving as they were Kryptonian and bound to their homeworld.

The Cleric kept the Eradicator as he travelled the universe until a chance meeting with Superman on Warworld 200,000 years later. The Cleric feared for Superman's life as he had seen visions of his death at the hands of Mongul and used the device to escape Warworld. He noticed the device protected the last son of Krypton and healed his wounds. He eventually gave the device to Superman and passed away.

On Earth, the Eradicator caused a number of bizarre events in it's efforts to change Earth into a new Krypton. Superman threw the device into Antarctica, where it altered the ice to create the Fortress of Solitude, and used two scientists to create a portal to the Phantom Zone, where it drew Kryptonian artifacts to the Fortress. When Superman recovered the Eradicator, he discovered it's xenophobic origin and it's plan to transform Earth. It then erased his memory when he tried to stop it. Professor Emil Hamilton, who had previously examined the Eradicator, managed to restor Superman's memory. Superman then went into the Phantom Zone to stop the Eradicator's plan. There he encountered a projection of Kem-L and discovered that only Kem-L's descendants, the House of El, can control the Eradicator. First though, he had to undergo a rite of passage. When Superman completed the rite, he commanded the Eradicator to shut down and it returned to it's dormant state.

Kryptonman

While on Earth, in the process of transforming the landscape to that more Kryptonian. The Eradicator took on human (kryptonian) form and was also known within the story arc as the Kryptonman. The Kryptonman had all the powers of Superman but none of his emotions.

As the Kryptonman he wore the black and red kryptonian garb and used the power of flight standing vertically.

Death of Superman

When the "Man of Steel" died during his battle with Doomsday, the Eradicator placed Superman's body in a Kryptonian rejuvenation chamber. He then took over the body of Doctor David Conners who was dying of cancer. Dr. Conners was now a human/Kryptonian hybrid but could not control the annihilation program and killed his wife and abandoned his children. Superman eventually awoke, without his powers, to fight the team of Cyborg and Mongul, the Eradicator protected Superman by shielding him from a Kryptonite pulse that would have killed The Man Of Steel for good and used it to restore his powers.

Fortress Eradictor

Dr.Conner's realized that somehow the Eradicator had split in two, one halve stayed with the hybrid and the other merged with the ash from the Fortress of Solitude becoming a Kryptonian warsuit. Conner, having allowed himself to be absorbed and briefly gaining control, blasted himself into space vowing never to return, a vow he has failed to keep.

The Eradictor/Conners gestalt returned, greatly changed, to warn Earth's heroes about the approach of Imperiex.