ALTERNATE VERSIONS
Banshee’s fate in Days of Future Past is uncertain. His tombstone was seen in the original story, but what happened before that has two tales.
In Days of Future Present, Franklin Richards conjured a “memory” of his New Mutants team training under Sean Cassidy. Banshee had been maimed in an unknown incident, his left eye covered by a patch and his left hand missing, replaced with a bionic hand. Sean’s New Mutants 
trained in barracks underneath the demolished X-Mansion, apparently following the initial Sentinel strike. Among Cassidy's future students was their team leader, Franklin Richards, a young Rachel Summers, a horrific Doug Ramsey (consumed by Technarchy matter and calling himself the Magus), the apparent daughter of Nightcrawler named Blue and two other unfamiliar mutants named Blaze and Cudgel.
On the other hand, Days of Future Past: Doomsday showed the original New Mutants were still together during the initial Sentinel strike on the mansion, when Rachel was captured and turned into a Hound, leaving no room for a second New Mutants team. Banshee appeared in this version of the timeline with both hands and eyes, but he was cut down and killed by Sentinels on the same day Kate and Peter’s children were murdered.
Banshee also played an important part in the X.S.E. future that Bishop is from. Though the time-displaced X-Man didn’t know any details, having originating from some 80-90 years ahead in time and most records of the past having been lost, Bishop told Banshee that among the X.S.E. it had always been rumored that Banshee had been the last hope and the custodian of the next generation of mutants.
In the Age of Apocalypse, Banshee was also fighting for Xavier's dream. As one of the senior members of the team, at least in terms of age, 
Sean alluded to having been retired for a time and having only recently returned to the fold at Magneto's request when the X-Men attempted to go back in time and change the timeline. Oddly enough, he seemed to be Quicksilver's official second-in-command, as opposed to Storm, who was also on the team. After the X-Men had a few run-ins with Apocalypse’s Horseman Abyss, Banshee decided to let himself be consumed by him to use his sonic scream at him from within the void. Although it meant sacrificing his own life, Banshee took Abyss along, knowing that there was at least one less evil person in their world. It seems likely that, in this reality, Black Tom Cassidy was killed years before he could reunite Banshee and his daughter, as Sean seemed to have no idea of Theresa’s existence.
On Battleworld, a different version of the Age of Apocalypse existed. Banshee was an early member of the X-Men who died alongside Sunfire during their first encounter with Apocalypse's Horsemen.
Banshee resided in the Realm of the Dead in the Earth X reality, having died during a conflict with Black Tom. Strangely, in the afterlife, the color schemes of costumes and clothing were reversed and everyone thought that they were still alive and that the people not there (those really alive) were the ones who had died. When Mar-Vell tried to overthrow Death herself and gain control of the realm, Sean was among his earlier recruits. With Death destroyed, a new Paradise was created; Sean's fondest wish was to spend the rest of his life in peace with his wife, Maeve, and for them to together raise their daughter, Theresa. Apparently, in the world of the living, Sean’s passing was apparently not widely known, as Mephisto was able to infiltrate an X-Men reunion disguised as an elderly Sean. It wasn't until Jean informed Scott through their psychic link that Banshee was with her in the afterlife that the Devil was uncovered.
Ten to fifteen years in the future, the X-Men met their End when several of their deadliest foes banded together and attacked them simultaneously on various fronts. During an attack by a group of Warskrulls, Banshee was revealed to have been killed and replaced by one of the aliens. The imposter’s mission was not fully revealed, though he might have been sent to get close enough to X-Men’s resident telepath, Emma Frost, to take her out before suspecting an attack.
In the altered reality of the House of M, Banshee was an early member of Apocalypse’s mutant forces, who sided with Magneto when Erik opposed the External’s brutality. Banshee remained loyal to Magneto and his Brotherhood as they conquered Genosha and overthrew the United 
States government. In modern times, Banshee was part of the Marauders, a team of bounty hunters. Led by Callisto, the Marauders tried to hunt down Nocturne, as genetic scans revealed her to be related to Magneto. The Marauders even fought the Braddocks, the royal family of Great Britain, to get to their target, but they underestimated Nocturne’s powers. Using her possession talent, she took over Banshee and began attacking the remaining Marauders with Banshee’s sonic scream.
In the original timeline the Apocalypse Twins, Eimin and Uriel, succeeded in their plan to destroy the Earth and relocate all of mutant kind to another world, though Uriel was killed. On this mutant utopia, 
called Planet X, Banshee served as one of Eimin’s Horsemen of Death for years. He also served as jailor for some of the Avenger’s Unity Squad members they had captured, Wolverine and Sunfire. When the surviving Unity Squad members rescued their friends, Banshee tried to fight his brainwashing but ultimately failed, and Sunfire burnt him to a crisp. The surviving Unity Squad members were then successful in traveling back in time and erasing this future from occurring.
In Sins of Sinister, Vox Ignis remained agent to Mother Righteous as Krakoa fell to the Sinister-corrupted mutants. She sold him on the idea of creating the Reliquary Perilous, a holy spell weapon forged from other items of magic and importance. Banshee found a path towards assembling the Reliquary when he discovered the Scream of Change could free Chimera mutants from Sinister’s influence if they contained Nightcrawler’s DNA. The Nightkin became a holy order of thieves, seeking the means of breaching Sinister’s lost lab, hidden away in the Worldfarm by Orbis Stellaris. Mother Righteous manipulated them into a religion feeding her their faith, even sacrificing themselves in suicide teleports smashing against the Worldfarm’s shielding.
Decades, then centuries passed. While Sean occasionally had bouts of conscience over how Mother manipulated the Nightkin, her influence over his soul kept him in line. Eventually, Vox Ignis discovered the lost Spirits of Vengeance from Earth, united as a hive mind possessing Galactus himself as a steed. They were little more than a distraction to Mother Righteous finally accessing the Worldfarm. She revealed her Reliquary was actually a magic virus intended to infest the Moira Engine of Sinister so that, when she died and rebooted the timeline, all the knowledge of this universe would transport back in time to Mother as well. Mother Righteous revealed the real reason she kept Vox Ignis around when her Reliquary consumed the Spirit of Variance as fuel for its spell. An ancient Sean Cassidy and the apostate Wagnerine killed Mother Righteous in the end, but Moira X emerged from the shadows. She shot Banshee and used the spell to spite Sinister and escape this disaster of a timeline.
In What If? (2nd series) #74, Mister Sinister retained control of Scott and Alex Summers at his orphanage, allowing him to raise Cyclops and Havok as members of his own, more vicious incarnation of the X-Men. In turn, Charles Xavier recruited Banshee and Mimic as founding members of his X-Men, who eventually faced Sinister’s in combat.
In Days of Future Now, Banshee led a roster of X-Force that included mainstays like Domino, Cannonball, and Boom Boom, but also Omega Red and Sean’s cousin Black Tom. There was apparently some bad blood between them and Wolverine’s X-Men when Magneto gathered all the mutant factions. Banshee survived until the final push against the Weapon X facility, where he died fighting a Sentinel.