Sean Cassidy was born the heir to Cassidy Keep in Ireland, along with its estate and a modest fortune. After earning a Bachelor of Science degree from Trinity College, Dublin, he joined Interpol, the international law enforcement agency, and steadily advanced through its ranks. By the time he married Maeve Rourke, Cassidy had attained the position of Inspector. Although he had discovered his mutant abilities during adolescence, he kept their existence secret, even from his colleagues at Interpol.
Cassidy’s powers were known, however, to his cousin Black Tom Cassidy, a disreputable figure who had also discovered his own mutant abilities. The two men had long been rivals, particularly over their mutual affection for Maeve Rourke, who ultimately chose to marry Sean.
Interpol later assigned Sean Cassidy to a highly classified mission that required him to sever contact with his family for an extended period. When he departed, Maeve was in the early stages of pregnancy, though neither she nor Black Tom was aware of it. During Sean’s absence, Maeve gave birth to their daughter, Theresa. Maeve later traveled with the infant to visit relatives in Armagh, Northern Ireland. While there, she was killed in a terrorist bombing in which she was an innocent bystander. No trace of Theresa was found, and authorities assumed the child had also perished. In reality, Black Tom Cassidy had been present at the scene and secretly abducted Theresa. Suspecting that she might develop mutant powers, he raised her in secrecy, intending to exploit her abilities for his own purposes.
When Sean Cassidy returned from his mission, he was informed only of Maeve’s death. Those who knew of Theresa’s birth chose not to tell him, believing the knowledge would deepen his grief. Initially, Cassidy attempted to cope by immersing himself in his work at Interpol. Over time, however, his melancholy led him to resign and become a freelance operative, a path that eventually drew him into criminal activity. Although law enforcement never uncovered his crimes, the mutant shapeshifter known as Changeling did, using the advanced technology of the covert mutant organization called Factor Three. Identified as a mutant himself, Cassidy was approached and invited to join the group. Appalled by Factor Three’s goals, he refused. Nevertheless, Changeling and his superior, the Mutant Master, deemed Cassidy’s powers and law enforcement expertise too valuable to lose. He was forcibly captured and fitted with an explosive headband that could be detonated remotely or would explode if tampered with. Under the codename Banshee, Cassidy was placed under the supervision of the organization’s human enforcer, the Ogre, and compelled to carry out criminal missions.
While on assignment in New York City, Banshee clashed with the original X-Men, who captured both him and the Ogre. Professor Charles Xavier disabled the headband’s detonation mechanism with a psionic attack, allowing Cassidy to remove it safely. Once freed, Cassidy revealed everything he knew about Factor Three. Although he was later recaptured after locating the group’s European headquarters, he ultimately participated in the X-Men’s final confrontation with the Mutant Master. It was Banshee’s powers that exposed the Mutant Master as an extraterrestrial.
Cassidy returned to an honest life, experiencing few major incidents thereafter aside from brief captivity by the Sentinels and by the second Secret Empire. Professor Xavier later recruited him into the X-Men during the battle against Krakoa. During his tenure with the team, Cassidy met Moira MacTaggert, Xavier’s research partner and a geneticist based on Muir Island, Scotland. The two fell in love, though their work kept them largely apart.
While in Japan, the X-Men confronted the criminal Moses Magnum, whose psionic powers allowed him to generate devastating earthquakes. Magnum attempted to trigger a seismic chain reaction that would destroy the country. Banshee countered this threat by generating vibratory sonic waves to neutralize Magnum’s forces. Although he succeeded in limiting the destruction to uninhabited islands, the strain severely damaged his vocal cords. Cassidy eventually regained his ability to speak, but his mutant sonic powers appeared to be lost.
Cassidy later accompanied the X-Men to Muir Island to confront Proteus, Moira MacTaggert’s mutant son, who briefly took her captive. After Proteus was defeated, the experience convinced Cassidy that he wanted to remain by Moira’s side. He retired from the X-Men, dividing his time between Cassidy Keep and Muir Island, though he briefly returned to assist the team during an emergency involving Arcade and a Doombot created by Doctor Doom.
Some time later, the X-Men clashed with Black Tom Cassidy, the Juggernaut, and Theresa, who had developed sonic abilities of her own and operated under the name Siryn. Loyal to Black Tom, who had raised her, Theresa aided him in his crimes. After their defeat, Black Tom gave Theresa a letter revealing her true parentage. Sean and Theresa were reunited at Xavier’s mansion.
Sean Cassidy, once known as Banshee, now serves as co-headmaster of the Massachusetts Academy, where he trains young mutants alongside Emma Frost.
