Seeking to unlock the secrets of genetics and genetic mutation, Dr. Nathaniel Essex was a scientist a century ahead of his peers with whom he worked in the 19th century. But when his studies brought him face-to-face with the immortal villain Apocalypse, his studies, and his life, would never be the same! Transformed into something more than a normal man and calling himself Mr. Sinister, Essex has learned much in the last 100 years. Using the X-Men's Cyclops as his prime test subject, Sinister has continued his work on mutants. But Sinister is never satisfied, and he continues to be a threat to mutants and humans everywhere!

Presented here as he looked during the X-Men crossover event Inferno in UNCANNY X-MEN #240, Mr. Sinister is poised to take over the world! Using his scientific knowledge, Sinister has cloned one of the original X-Men, Jean Grey and created an agent named Madelyne Pryor that has infiltrated the team and used them without their knowledge. Now, Madelyne Pryor and Mr. Sinister have transformed New York into a demonic nexus and ready a final spell to allow a dimension of demons access to Earth!

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