A bioengineer for Oscorp's New Orleans subsidiary, Nathaniel Essex developed techniques to create a super-soldier who could evade any form of detection and hypnotically persuade others. When his supervisors refused to allow him to test his procedures on human subjects, Essex experimented upon himself.
Much like Oscorp's self-mutated president Norman Osborn, Essex developed superhuman abilities accompanied by schizophrenic traits. One of these voices identified itself as Lord Apocalypse, a powerful mutant who intended to usher in "the Final Age" of humanity and, presumably, bring about either mutant rule or the end of the world entirely. It is unclear if Lord Apocalypse was real or hallucinatory, but regardless, Essex was fired from Oscorp for his interaction with his hallucinatory voices.
Calling himself "Sinister" and elaborately tattooed to that effect, Essex relocated to New York, following "Lord Apocalypse's" instructions to slay ten "innocent" mutants, after which Lord Apocalypse would help complete Essex's transformation. Essex murdered four mutant youths but a fifth victim, Jean-Paul Beaubier, survived thanks to his superhuman speed. Under Marvel Girl's telepathic counsel, Beaubier provided the X-Men with information about Sinister. X-Men members Cyclops, Colossus, Storm, Wolverine, Marvel Girl and Nightcrawler dispersed throughout New York in search of Sinister Having anticipated this response, Sinister snuck into X-Men headquarters, intending to slay the mutants who had remained behind. Sinister threw Professor Xavier down a flight of stairs, then fought Iceman, Angel, Dazzler and Shadowcat to a standstill before Rogue caught him and flew into the air with him. Rogue intended to drop Sinister to his death, but a returning Storm dissuaded her.
Taken into S.H.I.E.L.D. custody, Sinister broke down completely, suffering from sensory disorientation. Lord Apocalypse (or the hallucination thereof) appeared before him in his cell, declaring Sinister unworthy of being his Horseman, and then ordered Sinister to choke himself. Sinister apparently tried, but failed, to kill himself, and is now kept restrained and under suicide watch.
But during an intermission in which he was not being watched. He stuffed his shirt in his mouth and his throat, causing him to sufficate and die. His body was found later in his cell.