Sean Cassidy, the hero Banshee, was stricken with several crises at once: his lover, Moira MacTaggert fell victim to the lethal Legacy Virus while attempting to create a vaccine for it. His protégé, Synch, was killed in a battle that led to the closure of the Massachusetts Academy and the dissolution of Generation X. And he was forced to recognize a drinking problem he had developed.
Cassidy soon founded a mutant team under his own leadership, the X-Corps, finding funding from a forgotten Factor Three Swiss bank account and outfitting them via through an abandoned A.I.M. facility. The team was envisioned as a paramilitary force that could police the mutant community. Despite the recruitment of several known villains, such as Avalanche and Blob, the X-Corps was given official sanction from many countries in Europe. Unbeknownst to Banshee, his former students Jubilee, Husk, and M joined the X-Corps in order to keep an eye on him. They were wary of the fact that they were working side by side with ruthless criminals, and vowed to try their best to keep the group from getting into trouble.
When the X-Men learned of the team's existence, they warned Cassidy to abandon the idea. Unknown to them, Cassidy had, in fact, imprisioned the second Mastermind and was using her psychic powers to dull the violent impulses of the "reformed" villains on his team. Eventually, Mystique infiltrated the X-Corps under the guise of a mutant named Surge, and freed Mastermind and the other villains. The villains embarked on a rampage through Paris until the X-Men were able to defeat them. During the battle, Mystique ambushed Cassidy, slitting his throat. Cassidy's teammates managed to save his life, but Cassidy appeared to have lost his mutant powers. With its ranks split and its leader remanded to the hospital, the X-Corps was disbanded.