The Dead Space Marker - specifically the Red Marker - is at the heart of the Dead Space story and drives the vast majority of the plot. We'll cover exactly what the Marker is and what it's doing in the story in our explainer below, how it differs from a Black Marker, and where it even came from. However, because the Dead Space Marker is so central to the story, obviously we should clarify: massive spoilers for Dead Space and the Dead Space Remake to follow!
The Dead Space Marker explained
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The Red Marker in Dead Space that Isaac Clarke encounters on the USG Ishimura is one of many Markers, though this one is actually an artificial recreation of a "Black Marker" found on Earth. These obelisks are big alien statues of mysterious origin that, in the original games' lore, are eventually discovered to be scattered around the galaxy. Part of the function of a Black Marker is to inspire an urge to recreate it in any intelligent species that discovers one. The colour difference is thought to be from differences in composition, as Black Markers features several unknown elements.
Either type of Marker largely does the same thing: generates a signal that has two effects: firstly, it affects the minds of human beings nearby, starting with insomnia and depression, and escalating over time to full-on homicidal or suicidal behaviour, usually driven by hallucinations that make this behaviour seem rational to those affected.
Secondly, the Marker mutates any dead tissue nearby into the monsters known as Necromorphs, undead aliens whose form shifts and changes over time, even having them merge together or split apart. So once the mind-controlled humans above start killing each other, those people are likely to come back as gloopy, spiky monsters. Eek.
What is Convergence?
Eventually, when the Necromorph outbreak is sufficiently huge, the Marker triggers Convergence - aka, the creation of a Necromorph Moon, sometimes called a Brethren Moon. All the Necromorphs gather around the Marker and literally form into a small planetoid made of meat with the Marker at its center. It's effectively one giant Necromorph, which then flies through space and infects other worlds.
Markers seem to have some intelligence - they want Necromorphs and outbreaks to happen, and will often induce hallucinations specifically to further their goals and manipulate humans into helping them or protecting them.
Where did the Marker come from?
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The Black Markers' origins are unknown - nobody knows where they came from, but they seem pretty alien and have appeared on different worlds either in falling asteroids or buried underground. In fact, in Dead Space lore, a Black Marker in an asteroid is what supposedly wiped out the dinosaurs.
Red Markers, as we mentioned, were made by human beings, specifically by government researchers who found a Black Marker in the 2200s (about 300 years before Dead Space is set), and tried to recreate them, as they produced huge amounts of energy that they hoped could be harnessed. A scientist named Altman recognized the danger and sank the Black Marker into the sea, but based on his data three Red Markers were constructed anyway, and all of them caused outbreaks because… of course they did. The whole affair was declared a massive failure and swept under the rug by the World Government, before the Red Markers were left abandoned on distant planets.
In the 2500s, a megacorporation called the CEC was mining across space and found one of the Red Markers on the world of Aegis VII - and that's where the plot of Dead Space 1 kicks in! Having uncovered the Red Marker, the CEC mining ship known as the USG Ishimura is hit by a Necromorph outbreak as explained above.