Primarchs

The Primarchs are an ancient species and the predecessors of the Old Race. Their culture flourished from between 160 trillion and 90 trillion years ago before they began to recede. This gradual decline was what ultimately led to the rise of the Old Race who waged war with the Primarchs, afraid of their power. The Primarchs were officially listed as extinct by the Old Race approximately 85 trillion years ago.

Physical Description
Based one a single, incomplete strand of Primarch DNA located by the Alinteans, a partial reconstruction of the physical appearance of a Primarch was completed. From the strand, it was gleaned that the Primarchs were a type of fiber-optic luxite-based species with one thousand arms, all of them deriving from a spherical, malleable center (Dartimien mentions them probably looking more like a long-spined sea urchin or a living plasma node than anything else). As for any other portions of their physical anatomy, such as distinguishing features or further physical information, more individual DNA strands are required.

Weaponry and Technology
Primarch technology is exceedingly rare, as the Old Race is responsible for having destroyed almost all of it in their quest to break The Cycle. What is known, however, about the few remaining artifacts is that the Primarchs were one of the most advanced species to have ever appeared in the Galaxy, and that their technology focused around altering the fundamental physics of the universe itself. They created small, geometric objects from an unknown material, into which they transcribed massive equations. When these objects were then exposed to an energy source of a great enough magnitude, the equation inside was activated.

In  "The Consultant" , Martin Sinclair helps the Senate recover one of these artifacts, called the Kelari Cube, after it falls into the hands of the Constellation. According to Alyymia, the Kelari Cube is actually part of the most powerful Primarch equation ever discovered - the Kelari Formula - which is the equation which controls life and death by altering the presence of the spark of life or the spark of sentience.

In  "Ancient Horror" , Edge and Klaara see similar technology in the Primarch Ruins on Naarama, along with a still-active (albeit damaged) Primarch AI and several types of defense mechanisms, some of them anchored in other dimensions or universes entirely. Proving the technological superiority of the Primarchs once again, the AI seemed to be capable of reading Edge and Klaara's minds. The exact mechanism by which the Primarchs construct these equations and their relevant technology is unknown.

Culture and Beliefs
The only known fact about the Primarchs is that they saw The Cycle as necessary for the continuation of life, and therefore did nothing to halt their regression. Although they did fight back against the Old Race during the war, this was not to prevent their extinction, but to allow them to finish the preparation of several time capsules, such as the one hidden in the ruins on Naraama, whcih would have contained teachings, technology and gifts for the next race. During the war, they deemed the Old Race unfit to receive these gifts, and were preparing the capsules to hide their technologies for whoever came next. The Old Race destroyed many of these capsules, but a few remain. Their locations, however, or a method to locate them, are unknown.

Military
There are no surviving records about Primarch military tactics, weapons, etc.

Personality
The only clue to a Primarch's individual personality is the AI on Naarama, Xivxita. Using her as a template, the Primarchs were a calm, almost cold species. They were concerned with ver little, including their own extinction, as they saw no point in fighting against the forces of the universe. If Xivxita is any indicator, the Primarchs also had an unusally mentor-esque personality, as she attempts to educate Edge in their teachings, as well as counseling him about his internal thoughts and struggles (something he rebelled against).