The Imperium was the main antagonist of the Justice League crisis, "Secret Origins" - which served as the beginning of the main series and had the team band together to stop an invasion of Earth by aliens loyal to the Imperium.

A monstrous amoeba-like alien with psychic powers and vast resources it once attacked Mars and caused the extinction of many of the Martian Manhunter's people - when it came to Earth it was delighted to find the Martian Manhunter alive so it could kill him via agonizing psychic torture as the other heroes watched haplessly.

Yet the Imperium also sensed something wrong as the Martian Manhunter resisted his torture, allowing Batman to break free (who had been hidden by the Martian Manhunter's psychic-shield from the Imperium): Batman proceeded to reverse the alien technology and undo the Imperium's planet-wide shield which exposed it and its kind to the Earth's sun, which they were extremely vulnerable to.

As its minions began to die from exposure to ultraviolet radiation the Imperium writhed in pain, allowing the Martian Manhunter to escape from its form and hold it in place as the others fought against its remaining forces.

However the Imperium was too strong for the Martian Manhunter to hold and it escaped into its vessel, leaving its own followers to die as it began to make its escape from the planet: however before it could succeed in doing so Wonder Woman and Hawkgirl teamed up to bring the vessel crashing back down: killing the Imperium in the process.