Only the blackest of hearts would dare delve into the dark magic of the Elder Gods, but 13th Century sorcerer Belasco was of just such a heart. He promised to help these evil gods return to Earth in exchange for vast mystical power and nigh immortality. The Gods agreed, but their power left a mark. Belasco became like unto a demon himself, and the father of a race of new demons. The ritual to bring the Elder Gods to Earth involved five mystical bloodstones, but it must have never been completed or was disrupted-- the actual details remain lost to the ravages of time.
Belasco has suggested that it was he who kidnapped Beatrice, the lover of the poet Danté, in order to perpetuate his demon race. Danté's pursuit for his love took him through an ancient Atlantean outpost that became the inspiration of his poem Inferno. In the final showdown with Belasco, Danté caused the villain to break an overhead pipe, releasing a freezing liquid on himself that placed him into suspended animation for centuries. It wasn't until more recent years that a nearby volcano released him from his icy animation (although this story fails to account for the loss of his right arm, a fact which still remains a mystery.)
Returning to his original mission, Belasco tried to again summon the Elder Gods to Earth, while at the same time he kidnapped Shanna The She-Devil as the next mother of his new race of demons. Ka-Zar thwarted his schemes, of course, and Belasco found himself banished into a Limbo dimension by the Elder Gods as punishment.
Belasco eventually became absolute master of the Limbo dimension. However, his rule was threatened when the young Illyana Rasputin, Magik, found that her teleportation power enabled her to enter Limbo. Belasco sensed the opportunity to gain a powerful ally, and he taught Magik true magic abilities, even unleashing her "Darkchilde" form. Ultimately, Magik rebelled, deposing Belasco and claiming the leadership of Limbo for herself.
When Illyana Rasputin died, Belasco began an elaborate plot to reclaim his title as Master of Limbo, and stole Illyana's Soulsword by way of the heroes Amanda Sefton and Nightcrawler and their mother, Margali Szardos. Sefton ultimately defeated Belasco and claimed the Soulsword, and thus the rule of Limbo, for herself.
Almost immediately, Belasco reappeared, having his right arm restored by the Elder Gods, and formed an alliance with the demons known as the N'Garai. The N'Garai and Belasco wanted to used Limbo as a launching pad for a demonic attack in order to regain their lost kingdoms. To accomplish this, Belasco stole the Shadowcat's soul and the N'Garai switched their souls for those of five other X-Men. Shadowcat and Sefton, know calling herself Magik, defeated Belasco and the N'Garai, restoring the X-Men's souls.
Illyana Rasputin was briefly brought back to life during a re-shaping of reality by the Scarlet Witch. Even from Limbo, Belasco sensed this reappearance, and was curious when Illyana disappeared again (when the Scarlet Witch restored reality to normal.) He forcibly seized control of Limbo from Amanda Sefton in a quick, furious coup. His next targets were the people with whom Illyana was last "seen," the students of the Xavier Institute! He reached out and drew the entire Institute into Limbo, where he could confront the young mutants in his own domain.