Born into a wealthy family, Fabian Cortez joined the Upstarts, formed by the enigmatic omnipath Gamesmaster, playing a game where killing mutants earned points. Seeking to win the game, Cortez targeted Magneto. Traveling to Magneto’s orbital base Avalon, Cortez and a group of mutants pledged their allegiance to Magneto, becoming his Acolytes and battling the X-Men. Cortez used his guile to bring himself into Magneto’s trust, then secretly betrayed him and seemingly killed him. Cortez took over the Acolytes and captured geneticist Moira MacTaggert. The X-Men defeated the Acolytes and freed MacTaggert; Cortez was imprisoned, but freed soon after and informed that Magneto was still alive. Seeking to maintain control of the Acolytes, Cortez sought to recruit Magneto’s son, Quicksilver, but was opposed by his teammates in X-Factor. The Acolytes turned on Cortez after learning of his betrayal of their lord.
After Professor X wiped Magneto’s mind, Cortez fled to the island nation of Genosha and fueled the mutant population’s revolt against the oppressive government. He took a hostage in Luna, the daughter of Quicksilver, but was seemingly slain by the nigh-immortal Acolyte Exodus.
Later resurfacing, Cortez tricked the Magneto-clone Joseph into confronting Exodus, but his plan went awry when the Acolytes came to believe Joseph truly was Magneto. Joseph ordered the Acolytes to disband, however it was not long before Exodus reformed them, including Cortez, to oppose the enigmatic geneticist known as the High Evolutionary. Exodus sought the rare element Isotope E from the Evolutionary, but was opposed by the Evolutionary’s Knights and their then-leader, Quicksilver.
Cortez then led a group of Acolytes against the X-Men, but fled when the Acolytes, whose powers he was abusing during the battle, turned against him. Soon after, Magneto was ceded control over Genosha by the United Nations and offered Cortez a chance to redeem himself. Ever the schemer, Cortez accepted and became one of Magneto’s advisors. After Magneto was restored to full power, Cortez found his services no longer required and was magnetically pulled by Magneto from a prison in Hammer Bay fifty miles to Carrion Cove in less than twenty seconds. The impact killed Cortez instantly.
Recently, Cortez was revived with an alien techno organic virus and Selene's mystical powers.