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.

History
The Xul's distant ancestors evolved a billion years ago on a planet much like Earth. It is unclear if they were intelligent at this point, but this is when they developed their extreme Darwinian survival instinct.

The Xul reached space around ten million years ago and encountered the Children of the Night, then the Milky Way's dominant species. They waged war against the Children for two million years and ultimately exterminated them, along with any other spacefaring societies they encountered during this time. During or after this period, they abandoned their physical bodies and constructed an enormous Dyson swarm around the Milky Way's central black hole as a new "homeworld."

500,000 years ago, they attacked the Builders, destroying most of their civilization. During this war, the Xul entered the Sol System for the first time, in order to destroy the Builder outposts there. They destroyed the Builder settlement on Mars, killing the Builders and thousands of early humans there, but did not attack the Earth itself. One Xul huntership, heavily damaged during the battle, crash-landed on Europa and sank into the moon's ocean, where it would remain for the next half a million years.

Around 18,000 BCE, the Xul discovered the N'mah and wiped out their interstellar empire, killing the vast majority of the population. A small remnant, numbering only a few thousand, escaped the genocide and hid within the Stargate located at Sirius. Other N'mah outposts may have survived, but they do not have any contact with each other. The term "Xul" derives from the N'mah word for their destroyers.

Ten thousand years later, the Xul attacked the empire of the Ahannu. They devastated the Ahannu homeworld and many of their colony worlds. One of these colonies was Earth, where the Ahannu had enslaved much of the human race. The Xul wiped out the Ahannu there by dropping an asteroid into the Arabian Sea, which also devastated the human population and gave rise to the myth of the Great Flood. However, the Xul overlooked the colony of Ishtar, as it was well outside Lalande 21185's habitable zone, so the Ahannu and humans there survived.

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.

Spacecraft
The Xul travel using large spacecraft called hunterships, ranging from one to twenty kilometers in length depending on model. Hunterships are coated in a metal-ceramic mixture harder than diamond, colored either gold or black. A huntership's interior contains virtually no crew space, as it is essentially a single Xul entity; instead, its internal structure consists of multiple channels allowing its drones to move around the ship, performing repairs or killing intruders. The ships are capable of going from a dead stop to near-c velocities and back again almost instantaneously, and achieve faster-than-light travel by making brief jumps into the Quantum Sea and back out again.

Weaponry
Xul drones designed for anti-personnel combat use low-power lasers and particle beams. Xul hunterships wield much more powerful lasers and plasma weapons, and also fire small black holes that deal additional damage when they decay inside a target. Hunterships are also capable of offensively manipulating the Quantum Sea, allowing to destroy a target by altering its inertia or patterning it.

Quantum Sea manipulation
The Xul can reach down into the Quantum Sea of virtual particles "beneath" the universe and alter the standing waves in that sea, which form the basis for the matter and energy of the material universe. The Xul use this technology to modify the inertia of objects, allowing them to change the vector and velocity of objects however they choose, or to crush them into singularities. The Xul can also use it to pattern objects, downloading their information from the Quantum Sea. This process destroys the object and captures its information in the Xul's computer systems. Any living creature the Xul pattern remains fully conscious, perpetually enduring the pain of its disintegration.

Robotics
The Xul use robotics for small-scale interaction with the physical world. Their combat machines, generally used to defend against attacks on spacecraft or terrestrial bases, are pear-shaped black robots about two meters tall, equipped with a seemingly random arrangement of tentacles and eyes. These machines use built-in lasers and particle beams as weapons. Xul combat machines are generally not directly "inhabited" by Xul minds, and instead operate under their remote control. The Xul also have smaller robots, some on the scale of nanotechnology, which they use for building and repairs.