Sternum is one of the leaders of the Prison Planet, which is divided into two factions. He was once Emperor of Bone, but was exiled to the Prison Planet after his 'dear' brother siezed his throne (with the help of some poison, of course).
Sternum's arch-nemesis is the other ruler of the Prison Planet, Jewelia. Sternum has since forgiven his brother for his treachery and has put all his effort into stopping Jewelia's mad reign.