Dr. Challus Mercer was an antagonistic character from the video game Dead Space. He was a scientist aboard the planet-cracking vessel USG Ishimura and a devotee of the Church of Unitology. Mercer became fanatically obsessed with the Necromorphs after the Ishimura was compromised and the infection spread; Mercer believed the Necromorph infection to be the next stage of human evolution, and their ability to reanimate necrotic flesh was life after death.
Challus Mercer performed several medical experiments on the crew of the USG Ishimura, one of which resulted in the Hunter. While ordinary Necromorphs are usually produced by the infection of corpses, audio logs suggest that Mercer's experiments were aimed at transmitting the infection to live subjects, which may have caused the Hunter's unique regenerative properties. When Isaac Clarke encountered Mercer, the doctor appeared psychologically unstable; he advocated mass suicide among the crew and kept severed human body parts in his office. He viewed the human race as obsolete in the face of the Necromorphs, even to the extent of assisting the infection by way of deliberate sabotage to ship systems. He viewed Isaac Clarke's struggle to survive as admirable but misguided, and attempted to kill him on several occasions by both having the Hunter attack Isaac and shutting off the life support system in sections of the Ishimura that Isaac travelled through. Mercer was also responsible for murdering fellow crew members Jacob Temple and Elizabeth Cross.
After Isaac slew the Hunter by incinerating it with a shuttle's engines, Mercer allowed himself to be killed by an Infector, thereby becoming a Necromorph himself —an Enhanced Slasher. When Isaac passed through the Commons Lobby on his way to the Tram Station on the Crew Deck, he encountered the changed Mercer and euthanized him.
Before his descent into madness, Mercer was a loyal and hardworking crew member aboard the Ishimura. After the Necromorph outbreak, Mercer became insane and attempted to aid the Necromorphs in their assault on the ship, believing that they were sent by God to destroy humanity. He sided with Captain Mathius in the belief that the Red Marker was a holy artifact, but could tell that Dr. Terrence Kyne wanted the Marker away from the Ishimura. In an abandoned log tape, Mercer revealed that, while Dr. Kyne was studying the Marker to find a means of ending the infection, Mercer himself was experimenting on live crew members to create stronger Necromorphs.
Mercer showed no interest or sympathy in the survival of the Ishimura's crew. He praised Isaac Clarke for surviving snare after snare, but believed that all attempts to survive on Clarke's part were pointless. Mercer also had very little humanity, as he performed trepanation on a conscious crew member in order to directly infect them while they lived. He also bored holes into the skulls of dead crew members to ensure easy infection.