The Megacruiser is a very advanced spaceship that originally belonged to Slix Vigma.
Slix Vigma would kidnap people from their home planets and teleport them onto his ship, where they would be forced to fight in gladiator-like battles for the entertainment of the inhabitants of each Galaxy the Megacruiser went to. To keep its prisoners in line, they were outfitted with electric badges that shocked them if there was any hint of rebellion. The ship is incredibly large and capable of carrying thousands of people, including spectators and fighters.
At the conclusion of Grudge Match, Ben left Kevin on the ship while he escaped. In Back with a Vengeance, it is revealed that Kevin took control of the ship and used it for his own purposes. He located Vilgax with it, and, after a brief skirmish, the two villains made a temporary alliance against Ben and used the ship to return to Earth. It is unknown what happened to it after their defeat.