A mutant geneticist with the ability to duplicate other mutants' powers.
Dr. Nathaniel Essex approached Dr. Gail Greyson and asked her to participate as a last-minute panelist at a symposium on mitochondrial DNA he was hosting in the Philippines. Due to the time crunch, he invited her to come by his brownstone in Chelsea, Manhattan the next evening to give him her decision. After advising her fellow Avengers that she would be attending, she went to meet with Dr. Essex. Dr. Essex, pleased that she agreed, offered her a drink to celebrate, and only too late did Gail realize the drink had been poisoned, and she fell unconscious.
When Gail finally awoke, she was being rescued by the rest of the Avengers. She had been kept unconscious in a “sampling tank,” a method that Essex used to harvest DNA samples of mutants. Essex was able to convert these samples into a serum which allowed his body to duplicate that mutant’s powers. Essex, working with Dr. Karl Malus had also perfected a rapid cloning technique, allowing him to make clones of people in relatively short order.
When confronted, Essex was currently under the influence of a serum that was obtained from Deadpool, with a side effect of mild insanity. He was able to take a neutralizing compound before injecting himself with a serum that included samples from Nightingale. He demonstrated her ability to teleport, but was captured and the Avengers took him into custody. The Avengers freed all the kidnapped mutants, and Deadpool destroyed the lab and the clones. Essex was taken into custody by SHIELD, but managed to teleport away before reaching prison.
Nathaniel Essex is a mutant, born in the early 1700s, with a mutant power of cellular adaptation. With exposure to enough DNA, his cells would absorb and adapt to mimic the qualities of the source. After eating large amounts of meat, Essex found himself behaving strangely, similar to the animal he had consumed. His later experiments lead him to believe that consumption of other creatures would temporarily improve his abilities.
Essex took his research into less ethical areas, and learned that consumption of human blood would increase his longevity. He thought of himself as a vampire, and while occasionally killed people to promote his health, he hated what he thought he was, a sinister predator of man. Years passed, and he began to slowly embrace what he was, viewing it as a blessing rather than a curse. As science and medicine improved, Essex learned he could much more effectively maintain his youth and life with blood transfusions. Over decades, transfusions developed into serums made from blood.
When mutants began to become more common in the world, Essex began to hunt down and experiment with them, and learned that he could temporarily duplicate mutant abilities. Proper serums made from DNA samples could provide him with as many as five separate mutant powers at once, each lasting for up to 8 hours before serum reapplication was required. Essex began to hunt down and collect mutants to use as sources, so he could stay forever young and powerful.