During a period when Brainiac 5 suffered a nervous breakdown, he used the Miracle Machine to summon up all the petty hatreds from the known galaxies. He combined them all into one supremely powerful entity he dubbed Omega. Omega's soul purpose was to journey to Earth and destroy the Legion of Super-Heroes. He had managed to defeat the entire Legion and destroyed their headquarters on Earth. It wasn't until Matter-Eater Lad managed to eat the Miracle Machine, and thus destroy Omega's power source, that the monster was eventually stopped.
Unfortunately, digesting all of the alien energies inside of the Miracle Machine drove Matter-Eater Lad insane in the process.
Omega was a being of immeasurable strength far greater than that of either Mon-El or Superboy and durability so great the combined power of the entire Legion of Super-Heroes couldn't cause it significant harm.