While he's rarely been the team's marquee member, Banshee is a classic member of the X-Men. Sean Cassidy stands among the likes of Storm, Colossus and Nightcrawler, among others, in shaping some of the X-Men's most formative years. But for the better part of a decade, Banshee was trapped in his own personal limbo after becoming a Horseman of Apocalypse.
Banshee remained dead for years following 2006's X-Men: Deadly Genesis #2, by Ed Brubaker, Trevor Hairsine, Scott Hanna, Mike Perkins, Val Staples and Dave Lanphear. Since then, Sean had been only temporarily resurrected on a couple of occasions. It wasn't until 2013's Uncanny Avengers #9, by Rick Remender, Daniel Acuna and Chris Eliopoulos, however, that Banshee's return finally stuck.
Archangel's children, the Apocalypse Twins, used the Life Seed to bring Banshee back, while using the Death Seed to corrupt him into their horseman. Sean was recruited alongside the Sentry, Daken and the Grim Reaper. This move may have resurrected Banshee, but it also trapped him as a minion of the Apocalypse Twins, keeping Sean from fully returning.
While serving the Apocalypse Twins, Sean was forced to fight old friends such as Havok, Wolverine and Sunfire on the Avengers Unity Squad. At one point, when Sunfire began burning Banshee alive, Sean pleaded for mercy, confirming that he couldn't control himself in this state. Fortunately, the Apocalypse Twins were defeated, and Banshee was taken in by the Avengers.
In 2014's Uncanny Avengers #23, by Rick Remender, Sanford Greene, Dean White and VC's Clayton Cowles, Beast began studying Banshee. According to Beast, it would take years to remove the Death Seed's cosmic energies from Sean. For this reason, Banshee was kept in a coma, and off-panel, for several years. Banshee didn't awaken until 2018's Astonishing X-Men #13, by Matthew Rosenberg, Greg Land, Jay Leisten, Frank D'Armata and VC's Clayton Cowles. When Hank McCoy's lab was attacked by the Reavers, Banshee was released to help the X-Men.
Of course, Banshee was not the same as he once was. The X-Man was zombie-like in appearance, following a nanotech treatment to remove the Death Seed energy from his system. Additionally, Banshee would only use his voice to scream, refusing to speak for a while. Sean would occasionally help his fellow X-Man, but he would often fly away and abandon his teammates as well. Eventually, Banshee began to speak again, before deciding to fly away and figure out his new condition for himself.
By 2019's Uncanny X-Men #15, by Matthew Rosenberg, Salvador Larroca, Guru-eFX and VC's Joe Caramagna, Banshee's zombified state seemed worse than ever. Sean had joined the Mutant Liberation Front, looking physically drained of life and rarely speaking, if ever.
Banshee would rejoin the X-Men briefly, before facing his end at the hands of a giant Sentinel. Of course, by 2019's House of X #1, by Jonathan Hickman, Pepe Larraz, Marte Gracia and VC's Clayton Cowles, Banshee was finally back in full health. This was all thanks to the Krakoan resurrection protocols, which revitalized a character who had been physically ailing for years.
For most of the past decade, Banshee wasn't the hero and X-Man he once was. As a Horseman of Apocalypse, Banshee was clearly not in control of any of his actions. Banshee spent years recovering from this ordeal, which kept him off-panel for several major storylines and eras of X-Men. But after nearly a decade in limbo, Banshee finally returned to take his place alongside Marvel's mightiest mutants.