WARNING: The following contains spoilers for Way of X #5, on sale now.
Since his introduction in X-Men #1 by Chris Claremont and Jim Lee, Fabian Cortez has proven to be a consistently craven and cruel mutant, always working for his own benefit. Every chance before at redeeming him has just been a ploy. But Nightcrawler just made more progress than anyone else, only for it all to be potentially for naught.
Fabian Cortez just gained a key moment of growth in Way of X #5 by Si Spurrier, Bob Quinn, Java Tartaglia and VC's Clayton Cowles -- only for Onslaught to render him effectively braindead and unable to continue his growth as Nightcrawler's surprising messenger.
Fabian Cortez has positioned himself in an interesting light in the current era of Krakoa. Although the former Acolyte and villain was given the chance to make a difference on Krakoa as a member of the Six -- the off-shoot of S.W.O.R.D. responsible for the mining of Mysterium -- his attitude and clear attempts to earn respect and power led Abigail Brand to strike a deal with the Arakko Mutants to recruit one of their number, Khora of the Burning Heart, to their group. Cortez was further humiliated in front of the Quiet Council when he argued for mutant superiority over humanity and was left behind by those he sought to impress. He's spent the ensuing days walking Krakoa, causing chaos and insulting various mutants.
Most recently, he set himself up as the opposite of Nightcrawler. The ever-optimistic mutant has been struggling with discovering the key to the mutant religion, and Cortez proved to be a genuine challenge to his philosophy -- trapping him in an ethical test over Gorgon. Nightcrawler eventually solved it and forced Cortez to face his own mistakes in a bid to help him grow by making him confront Lost, a mutant who lost her parents to Cortez years earlier. Cortez initially resisted, but finally began to open up -- just in time for Onslaught (the genuine threat Nightcrawler is working to help Krakoa overcome) to take control of Cortez and murder Lost by overloading her powers. The act even brought the moon of Planet Arakko down onto the planet -- risking the lives of everyone there.
Cortez quickly gives in to despair and was is further shocked when Nightcrawler orders him to overload his powers, giving him the chance to actually teleport the moon back into place and save everyone planet-side. Amidst Cortez's self-pitying sobbing and concerns, Nightcrawler's arguments that Krakoa's "Sacred Land" is the mutant population itself finally starts to get through to him. Nightcrawler even makes Cortez realize that he so plainly despises himself that he can only want others to love him instead of empathizing with them, leading him to make choices that illicit harsh feelings from everyone else. Nightcrawler convinces him to empower him despite the risk it poses to Cortez if the plan fails and the blame falls at his feet.
Nightcrawler saves Cortez so that he can survive and later remind Nightcrawler of the epiphany he's had for Krakoa to flourish. Nightcrawler even refers to Cortez as his "messenger," the one person who can impart the knowledge Nightcrawler needs to know so that he can help save the soul of Krakoa. It's a major moment of reckoning for Cortez, and one he seems to embrace -- only for the world to not be fair in the end. By the time Nightcrawler is restored by the Five, without the memory of his final moments on Arakko and his talk with Cortez, Xavier informs him that Cortez's mind has been shut off by the strain on his own body. Even if he could be revived, Xavier has decided Cortez is too much of a risk and will be moved to the bottom of the resurrection queue.
Considering Cortez was awake and conscious when Nightcrawler saved him, it's clear that a psychic attack is what felled Nightcrawler's latest convert. Formerly described by Doctor Nemesis as the greatest threat to Nightcrawler's work, Cortez seeing the light could have been a major boon for Nightcrawler and helped finally redeem the villain. But Onslaught's efforts have left him justifiably hated by people unaware of the potentially massive growth he'd just gone through. The still catatonic Cortez might still be the key to unlocking Nightcrawler's religion and fighting back against Onslaught. But thanks to Onslaught, that redemption might have just been prevented.