Sean Cassidy, also known as the former X-Men and Generation X member Banshee, had fallen into a rough part of his life. Moira MacTaggert, his close friend and lover, had recently died from the Legacy Virus and his team of superpowers mutant teenagers, Generation X, had disbanded in the wake of the death of one of his students, Synch. Using funding from his old, now defunct, team ( Factor Three), Banshee created a group of super-powered mutants, mostly reformed villains, in order to act as a military force to police the mutant community. Members of the team included villains Avalanche, Blob, and Fever Pitch and former Generation X members Jubilee, M, and Husk.
When the X-Men got wind of Banshee's new endeavor, they confronted him and attempted to convince him to disband the team. Banshee refused, mostly due to the fact that he had imprisoned Mastermind and forced her to suppress the violent and evil behavior of the villains on the team.
Mystique, in disguise as "Surge", infiltrated the team and freed Mastermind and the other former villains. In the ensuing battle, Sunpyre was killed and Banshee has his throat slit by Mystique, effectively quieting both his voice and, therefore, his mutant powers.
Note: This group should not be mistaken for the X-Corporation, an international organization set up by Charles Xavier.