Sean Cassidy, a member of the X-Men, until he died trying to save the lives of innocents. Sean was the heir of Cassidy Keep and began as a member of Interpol, until his wife was killed in an explosion. Sean later found himself engaged in criminal activities and joined the subversive organization of superhumanly powerful mutants, known as Factor Three, where he used his mutant ability of Audiokinesis to terrorize and encountered the X-Men. Professor Xavier succeeded in releasing Factor Three's hold on Sean and, after his original team was captured, Sean joined the X-Men. Sean later met Prof. Xavier's silent partner, the Scottish geneticist Dr. Moira MacTaggert, and soon fell in love. After leaving the X-Men, Sean was asked to guide the new generation of X-Men, Generation X, with the former villain, Emma Frost. After Dr. McTaggert's death, of the Legacy Virus, Sean went to alcoholism and founded the militia organization, X-Corps, that used primarily mind-controlled, former terrorist mutants to protect mutant kind at large. After Mystique slit his throat, Sean was inactive for a while, until he died trying to save the lives of innocent humans on an airplane, set up to die by Vulcan. Theresa Cassidy, Sean's daughter, is currently a member of X-Factor and uses her Audiokinesis and the codename Banshee in honor and remembrance of her deceased father, stating that her father will come back soon or late, as the X-Men always does. Indeed, Sean has been briefly resurrected recently, during Necrosha, and was shown, as well, during the Dead X-Men storyline, but both times was returned to the grave.
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