Nathaniel Essex/Mr. Sinister is a rogue geneticist with mutant abilities given to him by Apocalypse that have made him immortal. He is on the quest for a perfect host for his soul, which has led him to extract genetic material from countless mutants in order to create the ultimate mutant, for which he is still searching.
Nathaniel Essex was a biologist and geneticist in Victorian England. He was considered the greatest mind of his time, a peer of Charles Darwin. Essex believed that morals had no place in science, and that they limited it. His son, Adam, died at the age of four, and Essex buried an empty casket. His wife, Rebecca, found Adam's body in an incubation tube in Nathaniel's lab. She dug up the grave to confirm her suspicions and fainted upon finding it empty. It was then that Nathaniel was approached by En Sabah Nur, who was interested in his work.
Nur (Apocalypse) promised an end to Nathaniel's human life, and Nathaniel agreed to become a monster to expand his knowledge and further science, as opposed to remaining human and being shunned by Rebecca. Nathaniel became Mr. Sinister. He is immortal in that he can transfer his consciousness into a new body when his current one dies. Thus, he creates clones of himself, ever-enhanced with the abilities of mutants he gathers in experiments. He believes that the most powerful mutant would incorporate the DNA of Cyclops and Phoenix.
Recently, he has discovered the wizarding population, and he is intrigued by their mutation. He has secretly joined forces with the Hellfire Club to begin gathering subjects. With Sebastian and Shinobi Shaw, he orchestrated the kidnapping of Lila Malfoy, unbeknownst to others in even the Club's Inner Circle. Soon, he will reveal himself to the others, and their plans will move forward. He plans to send Madelyne Pryor to infiltrate the X-Men, posing as her genetic template, Jean Grey.