Subject X was a mutant agent from an alternate reality that had been rendered uninhabitable by the use of Ghost boxes. His mission was to find alternate realities suitable for annexation.

As his superiors originally targeted Earth-616, he insisted on performing the mission on Earth-889. He was sent there and was followed in his travels by an unknown levitating mutant which he killed. He regretted his choice, and reacted promptly when the mutant X-Society tracked him, after having read the mutant's notebook, which stated that the murderer was going to take a Zeppelin at Sir Francis Drake Airport.