A COMPREHENSIVE HISTORY WARHAMMER 40000 UNIVERSE
Sources include Lexicanum and the Black Library.
Chapter One: The Old Ones.
The Old Ones were the eldest of all space-faring races, possessing an advanced civilization long before the development of the major races of the current age. They were the first race to achieve a galactic civilization. Only the C'tan Star Gods in their incorporeal state - born at the creation of the universe itself - surpass them in age.
The Old Ones likely were the most influential species to exist - they were instrumental in the development of many of the current sentient races, and, possessing the role of galactic-level architects at such a primal epoch, the state of the present galaxy is likely ultimately a reflection of many of the decisions made eons ago by the Old Ones.
The race remembered now only as the "Old Ones" had developed terrestrially into a sentient species like most of the galaxy's other commonplace biological species several billion years before the creation of Earth. The Old Ones possessed a slow, cold-blooded wisdom. Their science was advanced to such a point that it was indistinguishable from sorcery. Their understanding of the universe allowed them to exploit the alternate universes such as the Warp and engage in psychic engineering.
Finding themselves to be virtually alone in the galaxy, they took it upon themselves to make worlds more favorable to support life, and they seeded other worlds with new lifeforms. They visited many worlds, and using primitive species as a stock, they created new and sentient species, which they continued to nurture. Many of the habitable and inhabited worlds in existence were terraformed from often barren worlds which were developed and seeded with life by the Old Ones.
The Old Ones created many races to serve them including the Slann, Krork, and the Jokaero. The Slann were probably the foremost servants of the Old Ones and may have been one of the earliest races created. The Old Ones are also reputed to have predicted the downfall of the Eldar and the resultant creation of Slaanesh, and gave control of the Webway to the Eldar both in order to escape the Fall and to aid them in the waragainst the Necron- C'tan empire.
This brings us onto the fall of the Old ones: Wars with the Necrons.
One of the few already sentient races the Old Ones encountered were the Necrontyr, who at the time were in the midst of a slow expansion to distant planets. The lives of the Necrontyr were short and painful due to the deadly radiation of their world's sun. When they met the Old Ones, they developed a deep jealous hatred of them due to their near immortal lives. This resentment led to the First War. The Necrontyr's war was futile and they were forced back to a remote world in the outer rim of the Halo Stars. The fury of the Necrontyr cooled over thousands of years of imprisonment, but they turned their hatred against all life rather than just the Old Ones.
The Necrontyr had studied their star for millions of years. In their quest for a weapon to use against the Old Ones, they found the C'tan. Gradually they managed to talk with these ethereal creatures, but they knew they would never be able to understand the material universe without a material body. So the Necrontyr built the living metal bodies they are still adorned with. They were worshiped as gods, as their powers were far beyond anything the Necrontyr had seen before. As the C'tan became more manifest in their bodies, they began to enjoy certain aspects of life more, including pain, suffering and slavery.
Eventually, with the gods themselves at their sides, the Necrontyr were ready to begin their war with the Old Ones anew. The C'tan offered the Necrontyr escape from the curse of their short lives. Their minds would be transferred into living metal bodies, much like the C'tan themselves. Whether the Necrontyr knew what they would lose by doing this will never be known, but the process destroyed their race. They gained immortality but lost their free-will. Their minds were dulled and only a few retained any independent thought, but this was still only a fraction of what it had been. With the Necrontyr destroyed and the Necron born, the C'tan began the war against the Old Ones with their legions of undying warriors.
The Old Ones were sent reeling. They could not match the cold science of the Necrons and were pushed back. Millions were slaughtered and eventually the Necrons dominated the galaxy. The last bastions of the Old Ones power were besieged and many of the younger races were completely destroyed.
The Old Ones eventually came to the conclusion that they needed to create warrior and psychic races with the specific purpose of combating the Necrons. These races are thought to have included the Eldar, the Rashan, and the K'nib.
The introduction of these warlike and often psychic races into the galaxy had the side effect of warping the Warp - the war, pain, and destruction of the galaxy during the conflict was reflected in the Warp, and the innocuous entities which naturally existed in the Warp were twisted into voracious and hostile predators. Ultimately this would lead to the destruction of the Old Ones.
The C'tan empire could not counter this new form of warfare. Eventually they became devoted to severing reality's relationship to the Immaterium in order to make psychic powers useless.
Eventually, with the eruption of the Enslaver Plague on the galaxy, the Old Ones were broken and scattered. It is believed they have died out, although sightings are occasionally reported.
One cannot deny the effect the Old Ones had on the galaxy; it is belived that they were responsible for the creation of the Eldar, Humans, and Orks and countless minor races.
