Mountjoy comes from Bishop's alternate future. He was taught by the Emplates to possess the bodies of others. He came back in time through Trevor Fitzroy's while possessing Bantam. He left Bantam afterwards, but hunted him down once Fitzroy was dead, because Bantam was the only person left who knew of Moutjoy's existence.
While hunting Bantam, he fought and defeated Bishop by possessing Storm and using her as a hostage. Mountjoy escaped.bishop tracked him to a NYPD station. Mountjoy absorbed Bishop and used his body to get into the X-Mansion and attack the X-Men. He absorbed Archangel, Gambit and Psylocke, but Bishop used a hologram of his sister Shard to lure Mountjoy into the Danger Room, where Mountjoy tried to absorb the hologram. While Mountjoy was confused, Bishop blasted him, forcing him to release the tree X-Men.
Mountjoy then infiltrated the London branch of the Hellfire Club, by possessing Scribe. He and Emma Steed ended up fighting Captain Britain, but they were defeated and put in jail.
Sebastian Shaw bailed Mountjoy out of jail, and had him duel Madelyne Pryor. He nearly won, but Pryor psionically pulled him out of Scribe's body, and Mountjoy lost.
He has not been seen since. It is unknown he retained his powers after M-Day.
Mountjoy was able to merge with others and absorb their memories. If he merged with a mutant, he can use their powers at will. He can either ride along with his victims, controlling their actions, or and can absorb their bodies into himself using their powers as his own.
He can also project a "Basilisk" field, which slows his opponents' reaction times within a five yard radius.