The Imperium was a civilization of space-faring, shapeshifting conquerors who are apparently ruled 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, ensuring they remained in stasis.
However, they eventually were released by unwitting astronauts from Earth while he was hibernating, with one of their ranks taking the form and identity of one of the now-deceased astronauts, J. Allen Carter. The Imperium then made their move to invade Earth, infiltrating the highest ranking positions to convince Earth to disarm its nuclear weaponry, leaving the planet vulnerable to attack. They installed factories to pump out an ionized gas to block out the sun and shroud the Earth in eternal darkness.
Six heroes of varying origin joined forces with J'onn to take out one of their factories, but all save one were captured. The Imperium leader then descended in his ship to greet the captured J'onn, and eradicate the last Martian. However, Batman sabotaged the ion chamber as J'onn had telepathically shielded him, reversing the ion charge to dispel the dark clouds in Earth's atmosphere and bring back sunlight. The Imperium tried to escape, but was dragged into the sun by J'onn, then slammed by Hawkgirl back into the factory, destroying the escape craft and The Imperium along with it. The Imperium flagship then fled, leaving their soldiers to die on Earth.
The Imperium are amorphous entities whose typically white bodies are mottled with red and black. Their normal form is vaguely bipedal, with no discernible sensory organs. Their leader, the Imperium, has different anatomy from its subordinates, with a bulbous, purple-black body and multiple tentacles. They feed off of psychic energy from other beings, and can assimilate the traits of other species; after their encounter with the Martians, they gained their ability to shapeshift. The Imperium are nocturnal, but this makes them extremely sensitive to the rays of Earth's sun, which can incinerate their pale bodies in seconds. The Imperium leader is somewhat more resistant than his subordinates and was able to survive sunlight for a prolonged period of time.
The Imperium possess advanced, apparently organic technology, which appears to be primarily made from the same substance as their own bodies. In addition to their smog-producing factories, their forces consist of starships, roughly triangular flying craft, and tripodal walkers.
The Imperium bear a similarity to the White Martians, a violent genetic offshoot of the Green Martians that J'onn J'onzz belongs to.