- "Your future, the future of our race ends here!"
- —Dr. Mercer talking about the Necromorphs
Doctor Challus Mercer was a scientist aboard the USG Ishimura and a devout Unitologist to the point of zealotry. Mercer became fanatically obsessed with the Necromorphs after the Ishimura was compromised and the infection spread. Mercer believed that the Necromorph infection was the next stage of the human race and their ability to reanimate necrotic flesh was life after death. He is the secondary antagonist of Dead Space.
Challus Mercer was one of the numerous Unitilogists aboard the USG Ishimura, presumably introduced in the massive turnover caused by the Church of Unitology to grant Captain Matthius a majority of devouts composing the crew.  Even before the Necromorph infection began, Mercer was already showing signs of insanity, making fellow doctor Terrence Kyne distrust him. Before succumbing to the dementia himself, Mercer ironically was one of the medical officers to study the colonists suffering from it, like Brant Harris. He even advised Kyne to bring more subjects to the ship in order to find out what was happening.
While this was never stated, it's quite possible that Challus Mercer was a high-ranking member of the Church of Unitology before joining the USG Ishimura. Often, Marker relics are found near Mercer; the first is the small Black Marker in his office and the second is the slightly larger variant of this ornament on a table near the place of his death on the Crew Deck. Furthermore, there is a recording of him making a speech to fellow Unitologists in Chapter 10, and asking them to don't abandon their faith despite the current outcomes of the extraction mission.
The Second Aegis VII Incident
- "I hope one of his beloved Necromorphs finds him before I do."
- —Isaac Clarke: Chapter 5 mission log
When the Necromorph outbreak hit the colony, Mercer saw first the Hive Mind and the Necromorph transformation through recordings. He was convinced that they were the divine beings supposedly prophetised by Michael Altman. Mercer even was the one who named Hive Mind, making it one of the few named Necromorphs in-game.
When the infection reached the ship, he performed several medical experiments on the crew of the USG Ishimura, one of which resulted in the Hunter. Mercer's experiments are aimed at transmitting the infection to live subjects which caused the Hunter's unique regenerative properties. Mercer also attacked and killed numerous crew members and patients as the ship fell into chaos, as evidenced by the decapitated heads present into his office in the Medical Deck. After perfoming his experiments, Mercer became one of the responsibles for the sabotage of the USG Ishimura. He had a plan to take some frozen necromorph specimens back to Earth, despite the heavily deteriorated condition of the ship by then.
Mercer is finally encountered in Chapter 5, trying to hinder Isaac's efforts to kill the Leviathan by locking doors. When Isaac reaches the Chemistry Lab in order to get the poison capsule, Mercer presents himself through a nearby window. He tries to convince Isaac that the Necromorphs are the salvation of mankind, the next step of evolution, then he releases the Hunter to attack Isaac, locking him inside of the lab. He is later seen when Isaac is ambushed by Necromorphs at the Intensive Care Room, trying to convince Isaac to just give up and let them kill him. He leaves shortly before they enter the room, locking the door once again. When Isaac finds the required DNA sample, Mercer decompresses the entire Medical Deck and lock Kendra out of the controls, in another attempt to kill Isaac. He is last seen at the Cryogenics Lab, telling Isaac his plan to make the infection reach Earth by bringing frozen Necromorphs there. He then leaves Isaac to be killed by the Hunter, once more trapping him in the room with the creature.
While Isaac was busy killing the Leviathan, Mercer unfroze the Hunter.[note 1] After this, he is reencountered in Chapter 10, murdering the crew member Jacob Temple. His monologue while killing Temple implies that he captured him with the help of an insane Elizabeth Cross. It is hinted through the scenery that Mercer went there, to the Crew Deck, to murder any remaining infidel survivor and to advocate the mass suicide among his fellow Unitologists. He may have been there since he unfroze the Hunter.
- "Your time has come. No need to be frightened. No reason to fight. Many have gone before us, and now it's time for us to take the voyage together, to transcend death... and let the future take its course. Join me, as I gaze upon the face of God!"
After Isaac defeated the Hunter, Mercer allowed himself to be killed by an Infector, becoming a Necromorph himself —an Enhanced Slasher. Isaac Clarke encountered the changed Mercer and euthanized him.[note 2]
Before his descent into madness, Mercer was a loyal and hardworking crew member aboard the USG Ishimura despite being a devout member of the Church of Unitology. After the Necromorph outbreak, Mercer became insane and attempted to aid the Necromorphs in their assault on the ship, believing that they are 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. 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.
It is interesting to note that, although Mercer was insane, he was still able to point out those who were under the influence of the Marker, as he did when detailing how Dr. Kyne mentioned speaking to his wife—noting that "Amelia Kyne has been dead for several years". It's uncertain how much Mercer was acting due his dementia and due to his pre-established fanatism. 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 are 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 the dead crew members to ensure easy infection.
- Iranian-American actor Navid Negahban provides the voice and likeness of Challus Mercer.
- It is a mystery as to how Mercer managed to survive for so long after the infection spread to the ship, let alone how he managed to move around the ship without being attacked by the Necromorphs. Though it is speculated that Necromorphs do not attack those who are under the influence of the Marker, and since he was actively helping them as a human, the Necromorphs didn't see a reason to turn him until he recognized that he had became useless to the creatures and literally asked for it.
- This theory is backed by the death scene of Hanford Carthusia, who is killed by a Slasher only after he calmly finishes a monologue, and a scene that shows Natalia Deshyanov being spotted by numerous Necromorphs that refrain from attacking her.
- The same theory can be used to explain Nolan Stross' case in Dead Space 2, and the unnamed Cult Leader in Dead Space 3: Awakened. While the Necromorphs still attack the Circle members, this can be explained by the fact that the Circle members were not being completely controlled by the Markers, with some of them even defecting like Randall Carr.
- Notably, Mercer was not featured in any of the prequels for the first Dead Space, even through he is one of the main characters like Nicole Brennan, Terrence Kyne and Benjamin Mathius. It's unknown why Mercer was not featured on the prequels.
- When the player first enter Mercer's office, a short, strident segue of music is played to denote the event. Another strident segue of music is heard if the player kills Mercer following infection.
- If the player acts fast enough, the Infector Mercer submits himself to can be killed before fully infecting him, both preventing the need to fight him as an Infector and ultimately robbing him of his final wish, though this has no impact on the game's story one way or another.
- Strangely, in the last video log, it is seen that Mercer is perfectly fine when he gets killed by an Infector. However, as Isaac approaches the Infector and Mercer, the Necromorph is just getting ready to infect the corpse. He should've already been transformed by the time Isaac gets there.
- If the Infector is killed before it can convert him into a Necromorph, Mercer's body is totally mutilated, bloody with bone and muscles visible as well as his head missing (this could be interpreted as a nod to Mercer's "blind faith"), even though in the earlier log it was clearly seen that Mercer was totally fine. The model is in fact merely a generic Ishimura crew model.
- Like Captain Mathias, the resulting Enchanced Slasher from Mercer's body is in no way special from the others encountered in the game.
- A book entitled "Field Medicine" with the logo of the USM Valor, can be found in one of Mercer's laboratories on the Medical Deck.
- During Chapter 7, the Hunter can be heard and briefly seen. When the player is first going between Floor B and Floor D, while Floor C is still locked, the player can watch the locked Floor C go by and vaguely see the shadow of the revived Hunter snarling to itself and sharpening it's claws. The claw sharpening is a signature movement of the Hunter; no other Necromorph does this.
- If the player is fast enough, the Infector can be killed before the transformation can be completed. It’s possible to decide to just let Mercer turn into a Necromorph and leave him be and give him what he wants, but when the Marker is destroyed the signal is stopped thus killing the infected Mercer and then he collapses and becomes sludge.