Sean Cassidy is the heir to both a small fortune and a castle in Cassidy Keep, Ireland, where he was born. In his youth, he marries Maeve Rourke and takes a job with Interpol as an Inspector. While Cassidy is away on a long mission, his wife discovers she is pregnant and gives birth to their daughter, Theresa Cassidy. Not much later, Maeve dies in an IRA bombing. With no means to contact Sean, his cousin Tom takes care of Theresa. When Sean returns to learn of his wife’s death, he is devastated. Before Tom can even tell him of the existence of his daughter, Sean lashes out at Tom with his sonic scream for not having taken better care of Maeve. While Sean flies away in anger, Tom falls into a chasm, breaking his leg as a result of the attack, which leaves him with a limp. Angrily, Tom swears to make Sean pay and vows to never tell him about his daughter, raising her himself instead.
Cassidy leaves Interpol (later retconned as being due to Deadpool unintentionally botching one of Sean's missions), and becomes a freelance detective. The villainous Changeling discovers him through the group Factor Three and invites him to join the organization. Cassidy is appalled upon learning Factor Three's goals and adamantly refuses. However, Factor Three, with the Ogre, captures him and places a headband containing explosives around his head to force him to obey them. Code-named after the banshee, a spirit from Irish mythology, Cassidy is forced to obey Factor Three's commands and performs various criminal missions for the organization. On a mission in New York City, Banshee encounters the mutant team of superheroes called the X-Men. Professor Charles Xavier is able to use his telepathy to disarm the headband and remove it, allowing Banshee to help the X-Men defeat Factor Three. Later, Factor Three again capture him but he helps the X-Men defeat Factor Three's ally, the Mutant Master. The Sentinels capture him, but he is released from their captivity. While on the run from the Secret Empire, who were capturing mutants in order to harness their powers, he fights Captain America and the Falcon, mistaking their then fugitive status for a link to his pursuers.
The Banshee returned to an honest life, and time passed without major incident for him, except for his brief captivities by the Sentinels, and by the second Secret Empire. Professor Xavier later invited the Banshee to join the X-Men when he was recruiting new members to battle the menace of Krakoa. After most of the original team left for a time, Cassidy, along with fellow new recruits Storm, Colossus, Wolverine, and Nightcrawler, stayed as members of the new X-Men. He remained with the X-Men for a time, and encountered adversaries such as the N'Garai, Magneto, the Juggernaut, and his own cousin Black Tom. It was during this time that he met Xavier's silent partner, the Scots geneticist Moira MacTaggart. Cassidy and MacTaggart soon fell in love, but while Cassidy was based at Xavier's mansion, MacTaggart was based at her mutant research center on Muir Island off the coast of Scotland.
While in Japan the X-Men found themselves in battle with Moses Magnum, a criminal mastermind who had been endowed with psionic power to cause earthquakes. Magnum unleashed tremendous psionic energy in an attempt to create seismic waves great enough to cause a chain reaction that would destroy Japan. The Banshee simultaneously used his powers to create waves of vibratory force that would cancel out the waves of force that Magnum was creating. The Banshee succeeded to the extent that only small uninhabited islands in the vicinity of Magnum's base were destroyed. However, the tremendous strain that the Banshee had placed upon his powers in performing this heroic feat seriously damaged his vocal cords. Cassidy soon regained his ability to speak, but his superhuman sonic powers, which were dependent on his vocal cords, appeared to have been destroyed.
Cassidy accompanied the X-Men to Muir Island where they battled the menace of Proteus, Moira MacTaggart's mutant son, who at one point took his mother prisoner. Proteus was defeated, and the experience of seeing the woman he loved in danger made Cassidy realize that he wanted to stay by her side. Therefore, Cassidy retired from the X-Men, and he and Moira MacTaggart divided their time between Cassidy Keep and Muir Island. Cassidy briefly returned to aid the X-Men against operatives of Arcade during an emergency situation when most of the X-Men were simultaneously engaged in conflict with an android which Doctor Doom had created in his own image.
Later, the X-Men battled Black Tom Cassidy, his partner the Juggernaut, and Theresa, who had developed sonic powers of her own, which she used under the alias of Siryn. Theresa felt obliged by her duty toward Black Tom, the man who had raised her (and whom she called "uncle" out of respect, although they are actually cousins), to assist him in his crimes. After they were defeated by the X-Men, Black Tom gave Theresa a letter to Sean, explaining that she was his daughter. Sean and Theresa were joyfully united at Xavier's mansion.
After rejoining the X-Men as an active member, Banshee suddenly realized the Phalanx's kidnapping of the X-Men. He and Emma Frost worked to save some young mutants targeted by the Phalanx. Banshee later became the co-head master of the Massachusetts branch of the Xavier institute along with Emma Frost, the branch that housed Generation X.
Generation X disbanded after Chamber's graduation, shortly after the death of Sean's lover, Moira MacTaggart. Sean became inactive, until He turned back to his alcoholism for a brief period, and then his bitterness towards the mutants who were like those who killed her got to him.
Sean founded X-Corp, a superhuman militia organization that used primarily mind-controlled formerly terrorist mutants to protect mutant kind at large. Mystique infiltrated the organization in disguise, and slit Cassidy's throat. Luckily, he managed to capture Mystique by releasing Abyss, a mutant who could open an other-dimensional portal.
He did little more without his powers for a long time, until he was asked by the X-Men to go to Muir Island after M-day to look for any notes that might help find Professor X. Sean uncovered some startling information, and quickly arranged a flight to Westchester.
Death of a Legend
He was supposed to meet Nightcrawler and Wolverine, but noticed that the Blackbird seemed to be heading towards the plane on a collision course. He quickly used his powers to fly and attempted to use his sonic scream to stop the plane from hurting the passengers, Sadly, it was not enough to stop the Blackbird and it rammed into him as well as the plane filled with innocent passengers. Banshee's body was found by Kurt and Logan. When the X-Men were blamed for the situation early on, Emma Frost told O.N.E. that the attack also killed the X-Men's best friend.
Cyclops delivered the news of Banshee's death to his daughter, Siryn. Siryn was given a video tape that Banshee had made for the purpose of delivering a message to his daughter when he died. In the message, Banshee stated that he felt he had done more good than bad in his life, and that he hoped St. Peter would allow him entrance into Heaven. He expressed the hope that he would there be re-united with Siryn's mother. Siryn refused to believe that Banshee was really dead, citing all of the X-Men in the past who have been presumed dead but later returned. Sean willed Siryn Cassidy Keep, as well as his pipe.
He was buried at the Xavier Institute, along with the empty graves of Sway and Petra Kristensen. Charles Xavier, Quicksilver and Siryn were notable guests at the funeral.
Under unclear circumstances at the moment, Sean was resurrected as part of Selene's plan to attack the X-Men. Sean at first obeyed, but upon seeing his old teammates, he hesitated to attack them. His ultimate fate is yet to be revealed.
Powers and Abilities
Banshee is a Mega-level mutant
Super Sonic Screaming
owers to create powerful sonic waves with his voice with which he can achieve various effects, often with the assistance of limited psionic abilities which function only in unison with his sonic power.
Sonic Scream: ability to shatter solid objects with a sonic scream.
Movement: his supersonic scream propels him off the floor
Sonar: can use his sonic powers as a form of sonar (Sound Navigation and Ranging). By sending out a precise, pure note and listening to the returning, altered wavefront of the note, he can evaluate his surroundings in total darkness with varying degrees of resolution.
Sonic Shield: can tighten sound waves around himself or others near him, making a barrier against outside attack.
Vocal Disorientation: ability to change someone’s balance, by affecting their equilibrium using his sonic scream. Vocal Trance: ability to subtly influence people's subconscious mind by changing the tones and vibrations of his voice using sonics.
Vocal Unconsciousness: ability to affect the fluid in someone’s ear causing the person to go unconscious.
Sound Immunity: Banshee's hearing and equilibrium are unaffected by his power or other similar abilities. Superhuman Vocal Stamina: possesses superhuman stamina in his lungs, throat, and vocal cords
A gifted detective, seasoned undercover operative, and unarmed combatant, Sean was also well-versed in combat strategy and tactics. He was an excellent marksman, amateur machinesmith, educator, organizer and lobbyist. His interests included American country music and playing the piano.
Conventional firearms, explosive "micro-bombs".