Ireland's storied Cassidy clan is an ancient and noble lineage steeped in heroism, secrets, adventure, feuds and tragedy, all of which Sean Cassidy would inherit in some measure. The family's ancestral home is Cassidy Keep, a fortress-like castle in a remote, sparsely populated coastal area of County Mayo, overlooking the Atlantic Ocean from high atop the granite cliff Cassidy Crag. The Cassidy presence there predates historical records, and Cassidy Keep itself was built by Liam Cassidy over a thousand years ago to defend the Irish coast against Viking raiders. The Keep also covertly sheltered the "Families" (diminutive enchanted leprechauns or elves). In recent decades, the castle's Seneschal (steward), Eamon O'Donnell, has served as both caretaker of Cassidy Keep and guardian of the "Families". Born and raised in Cassidy Keep, Sean Cassidy had a happy boyhood there, fantasizing about fighting dragons and rescuing damsels. During adolescence, Sean discovered his mutant sonic powers but concealed them - partly because his wail spooked the superstitious elders of the community, who likened it to the ghostly banshees of Irish lore.
Sean's cousin, "Black Tom" Cassidy, became his lifelong rival. Like Sean, Tom was secretly a mutant; Tom was a gambling man who loved to live dangerously. Tom was next in line to inherit the Cassidy estate and its modest wealth, but gambled away these rights to his cousin Sean in an ill-advised wager. Despite this setback, Tom remained a suave, charming, Oxford-educated ladies' man, often making the less cultured Sean feel like a hapless country bumpkin by comparison. As young adults, the cousins competed for the affections of Maeve Rourke, whom Sean met in Northern Ireland. He had been hitchhiking home from a concert when he was harassed by corrupt policeman Sergeant McLanahan, who seemed keen on arresting Sean as a supposed terrorist, despite lacking evidence. A passing motorcyclist (Maeve) rescued Sean, but McLanahan chased them across the Irish border and sent their motorcycle off a cliff, forcing Sean to use his sonic scream to fly him and Maeve to safety. Sean took Maeve back to Cassidy Keep, where she adapted surprisingly quickly to her discovery of Sean's bizarre powers. She just as quickly grew fond of both Sean and his charming cousin Tom, each of whom was quite smitten with her. Maeve dated both cousins for months, apparently genuinely torn between the two, but chose Sean as her escort for the university ball. Badly injured when a vengeful McLanahan ran him off the road en route to the university, Sean sent Tom to escort Maeve in his place. Tom initially offered no explanation for Sean's absence, and the disappointed and puzzled Maeve accompanied Tom to the dance; but Tom, unable to lie to her, eventually confessed Sean's true circumstances. Touched by the gesture, Maeve called Tom her dearest friend, though her heart clearly belonged to Sean. Stepping aside gracefully, Tom later served as best man at their wedding; but losing Maeve to Sean accelerated Tom's gradual, secret descent into immoral, dangerous and increasingly criminal acts as he dabbled in mercenary work and various illegal ventures.
Sean, meanwhile, graduated college with a science degree and went into law enforcement. Well over two decades ago, he became an agent of the international police organization Interpol. As a rookie agent, Cassidy failed to prevent the terrorist organization Hydra's enslavement of weapons specialist Patrick Lipton, whom Cassidy subsequently believed dead. Later, Cassidy hunted down the Russian mutant serial killer Arkady Rossovich (the future Omega Red) with the aid of Magneto and Interpol colleague Magrite Deveraux (who was slain by Rossovich). He also teamed with fellow agent Daniel Peyer to infiltrate the offices of mob boss Almadavor until the mercenary Deadpool interfered, saving Sean's life but ruining the operation by killing Almadavor. While Sean was on a long-term deep cover assignment that kept him away from home with no contact for many months, Maeve gave birth to their baby daughter Theresa; but mother and child were soon caught in a terrorist bombing and apparently killed. Sean had not known Maeve was pregnant, and the few who knew about the newborn baby decided it was better to conceal her existence from Sean, lest it cause him further pain. Theresa secretly survived, rescued and adopted by her "uncle" Tom. When Sean finally returned and learned of Maeve's death, Tom considered telling his cousin the truth about Theresa; but a grief-crazed Sean lashed out at Tom, blaming him for not protecting Maeve in his absence, and accidentally gave Tom a crippling leg injury. Further embittered, Tom decided to raise Theresa himself in secret. Sean became an obsessive workaholic and, perhaps hoping distance would help ease the pain, he briefly transferred to the NYPD in New York City, where he once aided young mutant telepath Emma Frost (though she mind-wiped his memory of the incident). He soon rejoined Interpol, and encountered the mutant brawler Logan (later Wolverine) while apprehending criminals in Calgary, Alberta. After years of international assignments, Sean came home and led an undercover operation targeting his cousin, Black Tom, by now a major criminal. Tom was soon sentenced to life in prison, but Theresa remained unaware of her true parentage since Tom had long since sent her to a distant private school to shield her from his darker dealings.
Apparently by now aware of Sean's mutant powers (perhaps due to an investigation of Cassidy's exploits by the Weird Happenings Organization), Interpol sent Sean back to America to infiltrate the rising mutant subversive organization Factor Three. The increasingly disenchanted Sean, still mourning Maeve, quit Interpol for good and became a freelancer, secretly dabbling in minor crimes. Ironically, this attracted the attention of Factor Three (mostly mutant criminals themselves), and their deputy leader Changeling tried to recruit Cassidy. When Sean refused, they forced him to wear a remote-controlled explosive headband which would detonate if tampered with. Thus enslaved, Cassidy (code-named Banshee) performed criminal missions for Factor Three under the supervision of their loyal agent Ogre, a gifted human machinesmith. In a bitter twist, Cassidy soon found himself on Interpol's wanted list. Banshee and Ogre were ultimately defeated by the X-Men, who freed a grateful Banshee from Factor Three's service. Cassidy later helped the X-Men bring down Factor Three and exposed its "Mutant Master" leader as an extraterrestrial, prompting the defeated villain to kill himself. Banshee kept a low profile thereafter, apart from escaping the anti-mutant Sentinel robots with the aid of the X-Men, and skirmishing with Captain America and the Falcon while fleeing the mutant-hunting Secret Empire.
A diehard country music fan, Cassidy was in Nashville, Tennessee when Charles Xavier recruited Sean and several other mutants to form a new squad of X-Men to rescue the previous X-Men from the living island Krakoa. After the successful completion of this mission, Banshee considered leaving, since he felt too old for the mostly-youthful group, but Xavier and team leader Cyclops convinced him to stay. During an early mission, Sean saw his reckless teammate Thunderbird killed in action; though haunted by this loss, Sean soon settled comfortably into life with the X-Men. Being of similar age, he sometimes served as a confidant of sorts to Xavier; and being one of the new team's more experienced and stable members, he occasionally assumed an informal leadership role in the absence of Xavier and Cyclops. The team's foremost foe, Sean's former ally Magneto, once referred to the formidable Banshee as the only worthy opponent among the new X-Men. Sean also began a long-running romance with Xavier's silent partner, geneticist Moira MacTaggert. When Sean finally inherited his family's estate, he and the X-Men were ambushed at Cassidy Keep by a vengeful Black Tom and his new ally Juggernaut, both of whom had been paid to destroy the X-Men by Shi'ar agent Eric the Red. The X-Men defeated the villains with the aid of Cassidy Keep's leprechauns. After many more adventures with the X-Men, Banshee burned out his sonic powers while saving Japan from destruction by Moses Magnum. He continued to assist the X-Men as best he could, notably aiding them against Moira's murderous mutant son, Proteus.
Realizing his injuries might be permanent and knowing Moira needed him, Sean retired from the X-Men and moved into Moira's Muir Island research complex, where he convinced a grieving Moira to abandon plans to resurrect her monstrous son. Cassidy lived a mostly quiet life for the next several years, though his lack of super-powers did not stop him from joining a successful mission to rescue Moira and other X-Men associates from Murderworld. He continued to assist the X-Men on occasion in various support capacities; through them, he finally met his daughter Theresa, now the young adventurer Siryn. Her true parentage was belatedly revealed to both Sean and Theresa by Black Tom. It was a joyful discovery for both, though Sean's complete absence from Theresa's early life has occasionally made their relationship an uneasy one. A new Thunderbird, brother of the original, mistakenly blamed the X-Men for his sibling's death and kidnapped Sean to lure the X-Men into a trap. He ultimately found himself incapable of killing and relented, parting with the X-Men on civil terms. Ironically, Proudstar and Siryn would later become close friends while serving together in X-Force.
When the robotic Master Mold mind-controlled Moira into creating the mutant-killing Retribution Virus, Sean was afflicted by it; however, an antidote created by Moira not only saved his life, but also restored his powers to some extent, and he helped Cyclops defeat Master Mold. Later, when Sean was shot during the cyborg Reavers' assault on Muir Island, the Morlock Healer healed Sean's wound, also restoring his powers to their peak. After helping repel the Reavers alongside a makeshift roster of substitute X-Men, Banshee teamed with Forge to search the world for the core X-Men roster, which were missing and mistakenly presumed dead. During this period, the Irish government's new super-agents, the Kinsmen, came to Cassidy Keep to recruit Banshee into their ranks and force him to submit to a long-overdue debriefing; but Siryn single-handedly sent the unfriendly visitors packing. Thanks in part to Forge and Banshee, the X-Men gradually reassembled. Sean rejoined the group, notably helping liberate Muir Island and its inhabitants from the Shadow King. Sidelined by a broken jaw, Banshee sought out a distraught Moira, who was coping with guilt over her long-ago genetic alterations of Magneto. After a healed Sean rejoined the team, becoming concerned about the welfare of largely unsupervised junior member Jubilee, he helped secure treatment for a mutating Black Tom, aided Deadpool in dodging the vendetta of vindictive Interpol agent Daniel Peyer, and learned of Patrick Lipton's survival just in time to attend Lipton's deathbed.
Banshee soon teamed with Jubilee, Emma Frost and a reluctant Sabretooth to rescue a group of mutant teens from the extraterrestrial Phalanx. One of the teens, Blink, sacrificed herself to make the victory possible. The survivors, Jubilee and various other young mutants all enrolled in Xavier's new youth education program at the Massachusetts Academy, with Sean and a newly reformed Emma as co-headmasters. Cassidy and Frost worked together uneasily at first, and experienced several severe fallings-out, but gradually forged a mutually respectful working relationship and a strong bond with their students, Generation X. Family troubles would continue to haunt Sean, though - Generation X once rescued Cassidy Keep from displacement in another dimension, and Black Tom attacked the group several times over. Banshee's lover Moira placed herself in quarantine after contracting the Legacy Virus, and she eventually died when Mystique bombed Muir Island. Sean took the news hard and began drinking heavily. This alcoholic downslide, combined with Emma's wavering morality, funding troubles, and other factors, led to the disbanding of the Generation X program.
Sean retreated back to Ireland, quit drinking and channeled his grief into an obsessive new mission: creating and leading the international paramilitary mutant police force known as the X-Corps. Using funds from a forgotten Factor Three account and various criminal accounts frozen by Interpol, and basing the group in an abandoned A.I.M. (Advanced Idea Mechanics) facility stocked with highly advanced equipment, Banshee recruited various volunteer mutant adventurers (including former Generation X students) and pressed criminal mutants into X-Corps service through the mental powers of captive criminal mutant mentalist Mastermind. Designed to self-police the mutant community, the X-Corps rapidly became a respected and feared presence in Europe sanctioned by various authorities, but the X-Men voiced concerns regarding the group's sometimes questionable methods and Banshee's uncharacteristically ruthless attitude (possibly influenced by Mastermind, who was secretly undermining the group on behalf of Mystique). The X-Men's fears proved well-founded when Mystique infiltrated the group, freeing the criminal members from Banshee's control, slitting Banshee's throat, and trying to take over the Corps. The X-Men and the benevolent members of X-Corps soon halted the freed criminals' rampage, and a wounded Banshee managed to defeat Mystique with the aid of Abyss. Unfortunately, several X-Corps members were apparently slain, the city of Paris was devastated, and Banshee was again stripped of his sonic power by his injuries. After a lengthy hospital stay, Sean went into seclusion and the remnants of his X-Corps were taken over by Xavier to form the search-and-rescue X-Corporation.
Sean had barely begun to heal, both emotionally and physically, when he uncovered vital information regarding the recent disappearance of Charles Xavier. Unwilling to entrust this information to electronic communication, he flew back to America via commercial aircraft, but the X-Men's mysterious new foe Vulcan seized control of the X-Men's Blackbird jet and set it on a collision course with Sean's plane. Healed enough to fly short distances under his own power, Banshee exited the plane and could have flown to safety, but instead tried to save his fellow passengers by sonically deflecting the Blackbird. However, the injured Banshee lacked the power to accomplish this feat, and was caught between the two planes in a fiery mid-air explosion, sacrificing himself to protect innocent human lives.
Recently, Banshee was revived with an alien techno organic virus.