Sean creates powerful sonic waves with his voice with which he can achieve various effects, often with the assistance of limited psionic abilities that function only in unison with his sonic power. The Banshee can use his sonic powers to propel himself through the air in flight, to shatter solid objects, to place human beings temporarily into a hypnotic trance, or to cause human beings to fall temporarily unconscious.

Prior to the fall of Apocalypse, Sean had already served alongside Magneto once as an X-Man, but chose to retire claiming that he needed some time to himself, though his departure was most likely brought on by fear and helplessness. Near the end of En Sabah Nur's reign, Magneto would approach the Banshee once more and seek his aid. Times were bad and the X-Men and needed him back in their ranks. They were going to lose the war and Magneto asked him if it would be better to die on his feet or to live on his knees? It was a sales pitch that the Banshee could not refuse.

During the Sentinel Evacuation of Northern America, the Sentinels' programming was rerouted by the Brotherhood. The altered instructions had the Sentinels hunting down the X-Men instead of aiding with the escape. Through the help of Banshee's harmonics and a logic trap, the Sentinels were reprogrammed to evacuate the humans. During the conflict, a member of the Madri would escape and Banshee would take off in search of him. He would find the Cardinal Madri dead, killed for pleasure by the men responsible for dispatching the Brotherhood, Apocalypse's newest Horseman who called himself Abyss. He was holding a child hostage and wanted the Banshee to fetch Quicksilver so that he might be able to futher impress Apocalypse by doing away with Magneto's first son.

When the X-Men went to rescue Bishop from the Temple of the Madri in Quebec, it was again Banshee's harmonics that enabled Quicksilver and himself to sneak past the guards. During the mission, the two crossed paths with Abyss once more. He disregarded Banshee as had looked into the deepeset, darkest corner of the Banshee's mind during their previous meeting and discovered that he was terrified of dying and living and everything in between.

Sean admitted that he was frightened as he had watched the world turn into a hell that he was unable to prevent. He hated what the world had become and how little it offered, but was now trying to make it a better place. Now, that meant he had to sacrifice his own life so that others might have a chance to live. Sean concluded that the world needed one less Abyss more than it needed one more Banshee. Abyss underestimated the strength of Banshee and his sonic scream. Sean plunged into Abyss' void and let loose with a sonic scream that tore Abyss apart from the inside out, thus allowing Quicksilver to rescue Jamie Madrox.