Alien (Dead Space)

In the Dead Space universe, the remains of an unnamed alien species are discovered on the frozen planet of Tau Volantis. Before Tau Volantis was discovered, humanity had never found any evidence that intelligent life had ever existed anywhere apart from Earth, but numerous structures, machines and fossilized remains were found on the planet, indicating that the inhabitants were intelligent and highly advanced both technologically and socially. Among the ruins also existed evidence of the presence of a Marker, an artifact that not only generated high levels of exploitable energy, but also held the power to reanimate dead organic matter in the form of monstrous undead abominations referred to as Necromorphs. A Marker had also been uncovered on Earth in the year 2214, leading the Earth Government and Sovereign Colonies to believe that the inhabitants of Tau Volantis were responsible for creating the Markers to begin with. Further investigation of Tau Volantis centuries later by the Dead Space series' main protagonist Isaac Clarke revealed that the aliens hadn't created the Marker at all and were merely another of its victims.

History
Sometime more than two million years ago, the aliens existed on the vast water covered planet of Tau Volantis and were an advanced species both technologically and socially. Sometime in their lifetime, they discovered an ancient alien artifact on their planet, a Black Marker.

Soon the planets inhabitants began to worship the strange artifact, as it promised the aliens a new and effective source of limitless energy and gave the race a shining beacon of their future. The aliens built faux markers across the planet's surface in hopes of generating more energy for their uses. However, soon the aliens would discover the artifact was sentient and it would reveal its true intentions.

Soon, a violent outbreak spread across the colonies of the planet, killing and converting its population into monstrosities of the Marker's will. When enough had been killed and enough necrotic tissue was made available, the Marker drew power from the creatures to begin a Convergence Event, in which the collective mind of the infection began to assimilate the planet's dead into a moon so that it could continue a chain of biological destruction across the galaxy.

The aliens, however, were extremely resourceful, and in a limited amount of time constructed a massive terraforming machine that was able to flash freeze the planet, stopping the convergence and preventing the moon from continuing its chain of destruction.

Several millennia later, in the year 2314, the Sovereign Colonies Armed Forces discovered the planet and the remains of an alien specimen they called Rosetta. After studying Rosetta, Dr. Earl Serrano discovered what had happened on the planet and tried to complete what the aliens could not finish, but General Mahad received orders invoking a purge of all personnel, information, and assets on and in orbit of Tau Volantis before Serrano could get the codex to the machine.

200 years later, in 2514, an expedition led by Isaac Clarke rediscovered Rosetta, with which the knowledge of the lost species and their technology were able to finish what Dr. Serrano and the aliens attempted, killing the moon and supposedly ending the threat of the Markers forever. However, despite their efforts, the expedition was unable to stop all the other Brethren Moons as they made their way to Earth to consume its population.

Anatomy
The alien anatomy is described extensively by the studies and notes of Dr. Earl Serrano, collected from the data found within a frozen body of one of their kind and amongst calcified bodies found within the underground city that circles the terraforming machine.

The aliens are mostly fish like in nature, with gills and collapsable fins, which allowed Dr. Serrano to deduce that the planet had at one point not been covered in ice, but a vast ocean.

According to Serrano's research, the Aliens communicated with six holes within their skulls that let out harmonic noises, with a similar cranial design as human nasal passages. He deduces, however, that this was not their primary language form and that this method was developed by the aliens to allow a form of cross species communication. This pseudo-language they developed was created to convey their information to creatures and beings that found their culture long after their extinction.

The creatures were described and shown as varying in size, some being small and about the same size of a human, and others towering to the point of being as tall as a Nexus. It is unclear whether the very large specimens seen around the Machine are ossified corpses, or merely very realistic statues.