Xul

The Xul, also called the Hunters of the Dawn and known to themselves simply as We Who Are, are an ancient and hostile race that dominated the Milky Way galaxy for a period of ten million years. They ensured their dominance by killing off any other species that had even the remotest chance of challenging them, and were responsible for the near-destruction of the Ahannu, the N'mah, and the Eulers, as well as the extinction of numerous other species. They are the overarching antagonists of all three trilogies of the Galactic Marines saga.

Description
The Xul have long since abandoned physical bodies, and it is unknown what they originally looked like. Instead, they exist solely as digitized minds uploaded into the hardware of their technology, consisting of trinary code. Consequently, they cannot really said to be "alive," and do not feed or reproduce in the conventional sense. The computer systems that support them are powered by vacuum energy, and a Xul program can copy itself as needed whenever more Xul are required. Xul are thus effectively immortal, and can only die if the hardware hosting them is destroyed.

Psychologically, the Xul are profoundly alien. They are strongly implied to have lost their individual sapience, or at least are so immersed in their virtual worlds that it is difficult for outsiders to tell the difference. The Xul instead form group minds, also called metaminds, consisting of thousands or millions of Xul bouncing thoughts and ideas off each other as a collective. These metaminds in turn can join together in larger and larger group minds, forming a nested and hierarchical structure. Only in these vast collectives do the Xul exhibit conscious behavior, although they do seem to be individually intelligent. This is an example of a complex adaptive system, similar to an ant or termite hive.

The Xul are most notable, though, for their violent and aggressive Darwinian survival instinct. They are instinctively driven to attack and destroy any being or civilization that could pose a threat to them, either in the present or in the future. They do not consider the possibility of peaceful coexistence or communication, and in fact appear to be incapable of doing so.

Technology
The Xul are the most advanced species in the Milky Way galaxy. They use advanced spacecraft and robotics to interact with the material world, and build vast megastructures to host the majority of their population. Most notably, they are able to interact with and manipulate the so-called Quantum Sea, the "base state of reality." This allows them to alter reality to a limited degree, generally by "downloading" physical objects or manipulating their inertia to move or destroy them.