The Imperium are the race of aliens responsible for the destruction of Martian civilization, and appear in the first episode of the Justice League cartoon.
 
They’re a race of shapeshifting invaders (though they copied the shapeshifting ability from the Martians) that imprison their victims within biologically engineered cocoons in order to feed off of their psychic energy.
 
They also can’t survive in strong sunlight, so they build massive smoke generators on Earth in order to block the sun’s light, effectively making the night last forever.