Imperiex was a vastly powerful DC villain and godlike entity that sought to destroy the current universe in order to make a new one - making him a major antagonist to any heroes he came across.
The embodiment of entropy, Imperiex takes the form of pure energy contained inside a humanoid set of armor, colossal in size. He was first seen when Mongul, the son of the deceased villain of the same name, arrived on Earth stating that Imperiex had destroyed Warworld and was heading for Earth. Mongul convinced Superman to help him fight Imperiex, and the two managed to defeat it. However, it transpired that the "Imperiex" they had encountered was no more than a probe, whereas Imperiex Prime, leader of the Imperieces, was himself a much larger and more powerful being. He had detected imperfections in the fabric of the universe, and his ultimate plan was to destroy it and create a new, perfect one. To do so, Imperiex Prime headed for Earth, the planet which held the universe together, in order to destroy it and therefore induce a new Big Bang.
Before arriving to Earth, Imperiex obliterated countless other planets. Kalanor, Karna, and Daxam are three known planets destroyed in his cataclysmic campaign. Whole galaxies were also targeted for demolition as Imperiex Prime sought to fulfill his destructive destiny, obliterating one universe so that a new one would be born. After destroying Karna, Imperiex arrived at Almerac, the home of Maxima, and not only destroyed Almerac but "hollowed" the entire galaxy. After this, the survivors of the dead worlds, Brainiac 13's Warworld, Apokolips, and Earth formed a coalition, with Darkseid as its commander, to fight against Imperiex Prime and his probes. Finally arriving at the Milky Way Galaxy, Imperiex began sending in numerous probes. On Earth, several of Imperiex's machine-colony "Hollowers" destroyed Topeka, Kansas (as well as seven other places) when they began to dig into Earth to ready it for Imperiex's final demolition.
President Lex Luthor rallied the super-heroes of Earth, the U.S. military, and other countries, such as Pokolistan, together. Furthermore, Luthor had decided that Superman alone did not have enough power to lead a strike force, arranging for Doomsday to be released from captivity and brainwashed to see all Imperiex probes as Superman. They fought the probes and managed to destroy several until Imperiex Prime himself was finally drawn to them. Doomsday was outmatched and vaporized, only his skeleton remaining, though Darkseid saved Superman from a similar fate. Thanks to the sacrifice of Strange Visitor and General Sam Lane, Earth's forces managed to crack Imperiex's armor, intending that Darkseid would subsequently use Boom Tubes to transfer Imperiex's energy back to the galaxies he had destroyed. However, Brainiac-13 appeared on the battleground with Warworld, absorbing the Imperiex energies and vowing to use them to rule everything.
In a desperate gambit, Superman dove into the heart of the sun, thus gaining a massive power boost that enhanced his strength significantly. Rapidly realizing that Warworld couldn't be destroyed without releasing Imperiex and triggering another Big Bang, Superman and the Martian Manhunter formed a brief telepathic link to explain their new plan. With Darkseid's powers weakened, he used Tempest as a magical focus for his abilities, which in turn was empowered by the faith and strength of the Amazons, focusing the energy through Steel's new 'Entropy Aegis' armor (which was created from a burned-out Imperiex probe). With Lex Luthor activating a temporal displacement weapon, Superman subsequently pushed Warworld through a temporal Boom Tube, sending both Imperiex Primes and Brainiac's consciousness back fourteen billion years to the Big Bang, destroying both villains through a combined effort.
In his final moments, Imperiex Prime realized, in an ironic twist, that the imperfection he had detected in the universe was himself. The Imperiex War was not won without cost. At least 8 million people on Earth died during the war. The total number dead is stated to be countless. Several heroes also died, including Queen Hippolyta, Maxima, and General Sam Lane, Lois Lane's father. Aquaman, Doomsday, Guy Gardner, Steel, and General Sam Lane were all presumed dead, but later turned up alive for different reasons. Hippolyta was likewise resurrected many years later.
Imperiex appears as the secondary antagonist of "Legion of Superheroes". He was originally from the planet Apokolips where he worked his way up and became one of the planet's greatest warriors and gladiators. He was also modified mechanically to be the ultimate warrior. In the 41st century he conquered the universe and only Superman's clone Kell-El is left to stop him. In the 31st century Kell-El returns to stop Imperiex before he conquers the universe and the Legion helps him. Imperiex however frees the Fatal Five and the Legion of Super-Villains from prison and recruits other villains as well.
He also hacks the Legion's computers to find out information on their skills and abilities. In a later episode it is revealed that his cybernetic parts were built by a man named Abel and when Abel as a child is targeted by Terra-Man he and Superman team up to stop him. In another episode he and his army of robots invade the Fortress of Solitude to steal the bottled city of Kandor. It is later revealed that he had been working for Brainiac the whole time. In the season final Brainiac now done with Imperiex impales and kills him.