The Imperium was a civilization of space-faring, parasitic aliens who are apparently controlled by a single, bulbous mass.
According to J'onn J'onzz, the Imperium first entered the Solar system roughly 1,000 Earth years ago. They destroyed nearly all life on Mars, absorbing and duplicating the Martians' shapeshifting ability and telepathy. However, a small resistance movement entered their underground network and was able to use a nerve gas to paralyze the Imperium about 500 years later but J'onn was the only Martian left. He kept an eye on the Imperium, keeping them in stasis but they eventually were released by astronauts from Earth while he was hibernating. The Imperium then made their move to invade Earth, infiltrating the highest ranking positions to have Earth disarm its nuclear weaponry, leaving the planet open to attack. They installed factories to pump out an ionized gas to cover the Earth in eternal darkness. The Imperium leader, then comes to Earth to kill J'onn to fully eradicate the Martians, however, Batman sabotaged the ion chamber as J'onn had telepathically shielded him, reversing the polarity to bring back sunlight. The Imperium tried to escape but was slammed by Hawkgirl back into the factory, destroying the escape craft and The Imperium along with it. The Imperium flagship then left, leaving their soldiers to die on Earth.
The Imperium are nocturnal, and can be severely injured or killed by exposure to direct sunlight. They possess advanced, apparently organic technology including starships. However, unlike his troops, he is able to withstand sunlight for a prolonged period of time whereas his soldiers died within seconds.
The Imperium bear a similarity to the White Martians, a violent genetic offshoot of the Green Martians that J'onn J'onzz belongs to.