History
Years ago Charles Xavier battled another telepath hosting the Shadow King; however, he lost this battle and the Shadow King possessed him, turning his nature dark.
Later, Charles assembled a group of young mutants and trained them in the use of their powers. Charles or rather the Shadow King working through him, turned each of his students into killers, naming them Shadow X.
Somehow Shadow X slipped through the cracks of the multiverse created when the Scarlet Witch created the events of M-Day and found themselves on Earth-616. There, Professor X and the Shadow X team battled the group called New Excalibur but they were defeated by them and sent to prison.[3]
A short time later (a believed to be brain dead) Professor X awoke and asked to see Psylocke and revealed that he was possessed by the Shadow King and wanted revenge on her from an earlier encounter.[4]
This time the body of Professor X apparently died. However, the Shadow King revived it once more and battled New Excalibur.[5]