Imperiex the destroyer/devourer of galaxies called itself a force of nature, and was a name feared in many galaxies in the DC comics Universe.
Mongul's son Mongul Jr. was thrown to Earth when his home planet Warworld was destroyed by Imperiex (or so he thought). Mongul Jr. managed to steer himself (with such precision I might add) right in front of the Daily Planet building. He came to Earth asking for Superman's help, because he could not face Imperiex on his own.
Superman wasn't able to take on Imperiex in his current state either, and allowed Mongul Jr. to train him to fight more efficiently in outer space, and to use his own powers effectively. When they were ready, they decided to confront Imperiex somewhere near Saturn. Superman and Mongul Jr. managed to destroy Imperiex's construct (which was used to "hollow" galaxies) and believed that they had stopped Imperiex.
What they had done, was slow down an Imperiex probe's progress, and incurred the wrath of Imperiex Prime. In the Our Worlds At War (OWAW) storyline, it was revealed that Imperiex would be the force to destroy one universe, and start a new one in the old one's place. Earth was a focal point in the universe and the universe would have collapsed if Earth fell to Imperiex.
Imperiex unleashed his probes on earth, the first one landing in Topeka, Kansas. Superman defeated that probe, and its remains MAY have been the remains used to create the Entropy Aegis armor on Apokalips.
During the course of OWAW, Superman, Strange Visitor and Sgt.Rock were able to breach Imperiex Prime's armor, but Brainiac 13 siphoned off the Imperiex energy and used it to strengthen Warworld (that he was now in control of).
At the end of OWAW, when it seemed that nothing could stop B13, Superman flew into the sun, and let his cells soak up tremendous amounts of solar energy. Then this.. super.. Superman flew directly into Warworld and decided to tear B13 and Warworld apart piece by piece. There he discovered that Imperiex existed as pure energy, and that by destroying Warworld, he would be unleashing the Entropy force, and destroying the universe anyway.
In a final desperate move, Superman had the Martian Manhunter telepathically relay his plan to Darkseid, the other heroes and Lex Luthor. In this massive coordinated effort, they combined B13 tech from Metropolis to bend time, and Apokaliptian tech to create a boomtube (and bend space) and combined this energy in the Aegis Armor (worn by the recently revived John Henry Irons) to create a time traveling boom tube. Superman used the extra energy from the sun to push the Warworld through the boomtube. The plan was to go back in time to just slightly after the first big bang, and allow Imperiex to release his energy there, allowing both big bangs to overlap and create a new universe. In this process B13 was scattered all over the universe, and spread too thin to be any threat.