The Brother Moons (or Brethren Moons) are literally Moon sized Necromorph ceatures that live to consume and absorb all life in the galaxy. They are the true manipulators of the Necromorph outbreaks and the power behind the mysterious Markers. The Brother Moons are revealed to be the main antagonists of the Dead Space series.
The Brethren Moons, though their origins are unknown, are a series of moon-sized creatures throughout the galaxy that consume all organic life that they encounter through the use of their Markers, which are imbued with their alien DNA. Through the beginning of a Convergance event, a Moon begins the process of assimilating all organic and living tissue within its radius, thus, making it whole. It is unknown if the Brethren Moons actually constructed the original Markers or merely used them to spread the Necromorphs infection .
A Moon begins its growth once a Marker begins a Convergance event, usually brought on by a mass of Necromorphs assembling within its area. Once the Convergence begins, the necrotic flesh of both Necromorphs and dead tissue within its reach are flung into the stratosphere where they begin to form together, assembling the Moon from the tissue matter.
The creature is only finalized once the makers of the Marker are absorbed along with the planets entire biosphere. Once the makers have been absorbed as well as a large portion of the biosphere, the Marker itself gravitates to the central heart of the moon, so that it can broadcast telepathically to the rest of the formation as well as release broadcasts to other nearby Markers. It is currently unknown what happens to the Moon once it has absorbed all life on its birth site, but it is assumed that the mass travels to the nearest source of organic life to continue feeding.
It is revealed during the events of Dead Space 3 that it is likely that all life between the origin of the Moons (wherever in the galaxy that may be) and humanity's home system has been extinguished and that the Moons are responsible for the destruction of countless species in their quest to absorb all organic tissue within the universe, leaving much of our galaxy in a state of dead space. It is also revealed that the Moons form a network spanning the entire galaxy and that this network enables them to communicate with each other. The Markers influence their target races to build more Markers, which in turn trap and assimilate other species, expanding this Brethren Moon network.
Dead Space 3
During the events of Dead Space 3, Isaac Clarke comes to realize that the Marker signals that Markers receive are a distress call from the incomplete moon of Tau Volantis, which was forced into a hibernation state when the alien race it was attempting to lure and assimilate built a machine to freeze over the entire planet, preventing the monstrosity from becoming whole and continuing on in the galaxy. As a result, the Moon began deploying distress signals through its alien Markers, hoping to bring new organic beings to disable the machine preventing its growth, which would allow it to continue its assimilation of the galaxy.
The faux Markers that Isaac had been facing were attempting to continue the "Brethren Chain" of Moons by circumventing the hibernating Tau Volantis Moon. He also realizes that if the moon is freed from its frozen hibernation, any location within the galaxy with a still active Marker will begin the ultimate Convergence, continuing the growth of the Tau Volantis Moon.
The Moon's completion would then awaken the other completed Brethren Moons which would subsequently head for the Marker locations and devour all organic life present, resulting in the inevitable destruction of humanity and possibly all life in the galaxy.
In the final chapter, the Unitologist Terrorist Jacob Danik shuts down the machine keeping the moon frozen and effectively awakens the slumbering creature. However Isaac manages to reconfigure the alien machine and destroy the Tau Volantis Moon by using the machine to pull the massive Necromorph into the planet's surface. The Moon's destruction causes all the Markers to cease broadcasting their signals, ending the danger they pose to the rest of the galaxy. However it is hinted that there are more Brethren Moons still active in the galaxy.