Sean Cassidy was born as the heir to the castle and estate of Cassidy Keep, Ireland, as well as a small fortune. After graduating from Trinity College, Dublin, with the degree of Bachelor of Science, Cassidy became a detective at Interpol, the international law enforcement organization. By the time he married Maeve Rourke, Cassidy had risen to the rank of Inspector at Interpol. Although Cassidy had discovered his mutant powers in his adolescence, he kept them secret, even from the rest of Interpol.
However, Cassidy's powers were known to his disreputable cousin Black Tom Cassidy, who had discovered that he himself was a superhumanly powerful mutant. Sean and Black Tom had long been rivals, especially over the love of Maeve Rourke, before she ultimately married Sean.
Sean Cassidy was assigned by Interpol to a top secret mission which required that he stay out of touch with his family for a considerable amount of time. When he left on the mission, Maeve was in her first month of pregnancy, although neither Tom nor Maeve knew it at the time. Their daughter, Theresa, was born during Sean's absence. Maeve took the infant Theresa with her on a visit to her relatives in Armagh in Northern Ireland. While she was there, Maeve, an innocent bystander, was killed by an explosion caused by terrorists. No trace of Theresa was found by the authorities, and they and Maeve's relatives assumed that Theresa had also been killed in the explosion. In fact, however, Black Tom Cassidy had been present at the scene of the explosion and had secretly carried Theresa off with him. Black Tom suspected that Theresa might develop superhuman mutant powers as she grew older, and therefore he intended to raise her secretly so that he might exploit her powers himself.
On returning from his mission, Sean Cassidy was informed that his wife was dead. Those who knew that Theresa had been born decided not to inform Sean Cassidy that Maeve had had a daughter in order to spare him further grief. At first Cassidy attempted to escape his despair over this news by throwing himself into his work at Interpol. Eventually, however, the still melancholy Cassidy left Interpol to become a freelance operative, and as time went on, he found himself engaged in criminal activities. The legal authorities never discovered that Cassidy was guilty of crimes, but the mutant known as the Changeling did, thanks to the technology of the subversive organization of superhumanly powerful mutants and their underlings known as Factor Three. The Changeling contacted Cassidy, whom Factor Three's technology had also identified as a superhumanly powerful mutant, and invited him to join the organization. Cassidy was appalled upon learning of Factor Three's goals and adamantly refused. However, the Changeling and his superior, the so-called Mutant Master, agreed that Cassidy's powers and knowledge of the workings of law enforcement agencies were too valuable for Factor Three to lose, and so they sent their agents to capture Cassidy. Factor Three had a headband containing explosives placed around Cassidy's head. If Cassidy defied their orders, the headband could be detonated by remote control; it would also detonate if Cassidy attempted to remove the headband himself. Furthermore, Factor Three put Cassidy, who went by the code name of Banshee, under the supervision of one of their trusted human agents, the Ogre. The Banshee thus felt forced to obey Factor Three's commands, and, accompanied by the Ogre, performed various criminal missions for the organization.
While in New York City on a mission for Factor Three, the Banshee clashed with the original X-Men, who captured both him and the Ogre. The X-Men's leader, Professor Charles Xavier, used a psionic 'mental bolt' on the Banshee's headband which prevented the detonation mechanism from functioning so that the Banshee could remove it. Once freed of the headband, the Banshee told the X-Men all that he knew about Factor Three. Later, the Banshee discovered Factor Three's secret European headquarters, only to be recaptured. But the Banshee participated in the X-Men's battle with the Mutant Master, and it was the Banshee's powers which exposed the Mutant Master as an extraterrestrial.
Powers & Abilities
Banshee's superhumanly powerful lungs, throat and vocal cords could produce a sonic scream for various effects, in concert with limited, reflexive psionic powers which directed his sonic vibrations. He could hover or fly at the speed of sound, and could carry at least one passenger. He could overwhelm listeners with deafening noise, stun them with tight-focus low-frequency sonic blasts (effective even against shielded ears by penetrating the skull via bone conduction), plunge them into a hypnotic trance, disorient them, nauseate them, or simply render them unconscious. Using sonic waves, he could rapidly vibrate himself or other masses at will. He could generate sonic blasts which struck with tremendous concussive force, liquefying or outright disintegrating targets at his highest levels of power. By radiating sound waves outward and reading the feedback, he could locate and analyze unseen objects in a sonar-like fashion. By modulating his scream's harmonics, he could confuse most scanning equipment. He could instinctively analyze, replicate, and block sonic waves or vibrations from other sources.
Banshee generated a psionic field which protected him from the detrimental effects of his sonic vibrations, though his sonic powers could still injure him when pushed beyond safe limits. His physiology seemed fully vulnerable to conventional injury when his sonic powers were not engaged. Banshee had selective hearing, enabling him to focus upon, enhance, or totally block out any given sound in his environment; this shielded him from the deafening sound of his own screams, and made him a superhumanly acute eavesdropper in surveillance situations. Sean and his cousin Black Tom were immune to each other's natural mutant energy powers, though Sean's immunity did not extend to the new powers Tom later developed via artificial mutations.