Fabian Cortez is a member of Magneto's Acolytes. He often rebelled against him, and was finally killed by Magneto.
Cortez was bored with his life, even though he was wealthy and had high social status. He joined the Upstarts for excitement, a group of mutants organized by the Gamesmaster who were awarded points for killing other mutants. Cortez gathered a group of mutants who believed in Magneto's teachings and formed the Acolytes. He brought the group to Asteroid M, Magneto's orbital base, only to arrange for it's destruction and the death of the first wave of Acolytes along with their messiah.
Obtaining many points for the deaths of such powerful mutants, Cortez then gathered a second group of Acolytes easily by playing Magneto off as a martyr. Neophyte overheard Cortez telling the Gamesmaster about how he killed Magneto, and caused the other Acolytes to abandon him.
When Magneto returned and was mind-wiped, the Gamesmaster stripped Cortez of his points. Furious, Fabian concocted an elaborate scheme to regain them. He kidnapped Luna Maximoff, the daughter of Quicksilver and granddaughter of Magneto, and began a civil war on Genosha by encouraging mutates to kill the ruling humans. Both the X-Men and the Avengers tried to stop him, and he was apparently killed by the new Acolyte leader Exodus.
Cortez resurfaced months later when Joseph assumed control of the Acolytes. Pretending to help Joseph assume Magneto's leadership role, his schemes were again unveiled. He then tried to destroy the X-Men with another wave of Acolytes, but eventually abandoned his own allies.
Later when Magneto was returned to his full mental capacity, he gave Cortez a chance to redeem himself. Weakened by trying to manipulate the Earth's magnetic field, Magneto needed Cortez's powers to keep him rejuvenated. Magneto offered Cortez a place on the Genoshan cabinet, which he accepted. When Magneto attempted to obtain technology from Carrion Cove that would return his power levels to normal, Cortez betrayed him and helped the United Nations by augmenting rogue Acolytes to fight Magneto and his mutate army. His power levels faded during the battle for Carrion Cove, while Magneto returned to full power. Magneto finally got his revenge and used his new strength to repel Cortez 50 miles and smash him into the ground; he died on impact.
Cortez was resurrected via the Transmode Virus during the Necrosha crisis and, searching for Magneto with his fellow acolytes Delgado and Mellencamp, battled Deadpool and Loa. They were eventually, and contrary to the others "zombies" who weren't possible to kill, killed by Loa's powers (Loa was used as a projectile by Deadpool).
In the 1990's X-Men Animated Series, Cortez's story plays out in a similar arc, up to a certain point. Hunted by Magneto for his schemes and betrayal on Asteroid M, Cortez is found and captured by Apocalypse. Later, he is seen leading a cult of Apocalypse-worshippers, seeking to use the young X-Men member Jubilee as a new host body for the then-destroyed mutant leader. After Jubilee and Beast escape the cultists' temple, Cortez learns to his horror that he is to serve as the host body instead. The series ended before this plotline could be pursued any further.