Challus Mercer is the secondary antagonist of the video game Dead Space.
Doctor Challus Mercer was a scientist aboard the USG Ishimura, and was a devout Unitologist to the point of zealotry. Mercer became fanatically obsessed with the Necromorphs after the ship was compromised and the infection spread; he believed them to be the successors to the human race and that their ability to reanimate necrotic flesh was life after death.
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 Dr. Mercer, Mercer appeared psychologically unstable; he advocated mass suicide amongst 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 life support in sections of the Ishimura Isaac traveled through. Mercer also murdered Jacob Temple and Elizabeth Cross, the former in front of Isaac.
After Isaac finally destroyed the Hunter, Mercer allowed himself to be killed by an Infector, thereby becoming a Necromorph - an Enhanced Slasher. Isaac Clarke encountered the changed Mercer and euthanized him.
The player can kill Mercer before the transformation is complete, which would be his ultimate defeat as he would be denied his "rebirth" as a necromorph.