Thought to be royalty from Spain, Fabian Cortez organized the original Acolytes, who pledged themselves to Magneto and his cause, with Cortez goading and manipulating Magneto into combat with humanity and the X-Men at every turn, leading to the destruction of Magneto’s space-station, Asteroid M, and the deaths of the other Acolytes, including Cortez’s own sister Anne Marie.
Believing Magneto to be dead, and martyred for his cause, Cortez recruited a new group of Acolytes to follow him in Magneto’s name, now worshipping him as their god. They launched several strikes on humanity, ranging from attacks on a military base where new Sentinels were being built, to slaughtering helpless humans in a hospital, but Magneto eventually resurfaced, sending his chosen heir Exodus to inform the Acolytes of Cortez’s betrayal. The Acolytes welcomed Exodus as their new leader, and Cortez was left behind.
Fearing Magneto’s wrath, Cortez fled to Genosha. Still claiming to speak in Magneto’s name, he incited the nation’s mutant population into starting a civil war against the humans, the world’s first instance of the long-threatened genetic war between man and mutant. With the aid of the mutates, Cortez kidnapped Magneto’s granddaughter Luna, intending to use her as a shield to protect himself from Magneto, the X-Men, and the Avengers. Cortez was unaware that Magneto had recently been mindwiped by Charles Xavier, and Exodus came to Genosha in his place, seemingly killing Cortez.
Cortez reappeared months later, claiming to have been badly injured, yet not actually killed, despite all appearances. He manipulated Joseph into posing as Magneto, pitting him against Exodus in an attempt to regain control of the Acolytes. Despite his scheme’s failure, he was allowed to return to the team. Months later, at a time when Exodus had briefly been imprisoned, the Acolytes split into two groups, each searching for Magneto. Cortez led one group, but they abandoned him during a battle with the X-Men, upon realising how he had been using them for his own purposes.
Alone again, Cortez was surprised to be recruited by Magneto himself, whom United Nations had given control of Genosha. Seeing as how Fabian Cortez had betrayed him earlier, Magneto had only let Fabian Cortez serve him on Genosha because his powers had been reduced and Cortez’s mutant power was to heighten the abilities of other mutants. After a further betrayal by Cortez, discovered by Magneto's UN advisor Alda Huxley to be the leader of the Genoshen cabinet assisting a rebellion in Carrion Cove, Magneto was restored to full power by one of the Genegineer’s machines and as he no longer needed Cortez, he pulled him from a prison in Hammer Bay fifty miles to Carrion Cove in less than 20 seconds, the impact killing Cortez instantly.
Cortez possesses the ability to bolster another mutant’s power. He could enhance a mutant’s abilities to occasionally dangerous limits beyond their control (temporarily powering them up at the expense of burning out their bodies).
He was highly intelligent and an able military strategist. Cortez was also a skilled martial artist, and often carried a gun.
In an alternate reality Marvel Zombies, Cortez is one of the survivors of a zombie-plague which bring humanity to a brink of extinction. He lost the challenge of the leadership within the colony he established with Black Panther, and at some point he fathered a child, Malcolm, who is as ruthless and cunning as he is and continue his father's challenge with Black Panther's leadership.
In the X-Men animated series Cortez appeared in the two-part episode “Sanctuary.” As in the comics storyline, Cortez was the leader of the Acolytes and used his powers to bolster Magneto’s own abilities. However, Cortez’s extreme anti-human sentiments led him to betray Magneto and try to kill him, framing Professor X, Beast and Gambit, capturing and torturing the latter. Cortez threatened to destroy Earth but was stopped by the X-Men and Amelia Voght, who revealed Cortez’s betrayal to the other Acolytes and the entire mutant population of Asteroid M. He was trapped in Asteroid M by a vengeful Magneto, but was rescued from its destruction by Apocalypse and Deathbird. Apocalypse granted him the ability of altering the mutations of other mutants.
Cortez then appeared in the final-season episode “The Fifth Horseman,” now turned into a servant and worshipper of his savior. Cortez assembled a cult worshipping Apocalypse as well as the Hounds, a foursome of altered mutants, in an attempt to find a new body for Apocalypse—who was defeated and left bodyless in “Beyond Good and Evil.” Cortez captured Jubilee and turned Beast into a feral monster but was stopped by Caliban, one of the Hounds. After being defeated, Cortez begged Apocalypse to be forgiven for his failure, though Apocalypse was not angry at Cortez, for he still had a host body to reincarnate in, Cortez’s own.