"There is a terrible darkness descending upon the galaxy, and we shall not see it ended in our lifetimes." — Inquisitor Bronislaw Czevak at the Conclave of Har

The Necrons are a mysterious race of robotic skeletal warriors that have lain dormant in their stasis-tombs for more than 60 million Terran years and who are the soulless creations and former servants of the ancient C'tan, the terrible Star Gods of Eldar myth. The Necrons are ancient beyond reckoning, predating even the birth of the Eldar. At long last, however, they are beginning to awaken from their Tomb Worlds, for the galaxy is ripe for conquest and the restoration of the Necron Empire since the disappearance of the Old Ones more than 60 million standard years ago. The Necrons are a completely robotic humanoid species whose technological prowess is probably unmatched by any of the other intelligent species of the galaxy. Yet out of a desire for vengeance against the more fortunate long-lived ancient xenos race called the Old Ones, and the trickery of the godlike intelligences known as the C'tan, the Necrons shed their original organic forms and lost all forms of compassion and empathy, becoming ruthless, undying killing machines who are determined to exert their mastery over the galaxy once more.

Across the galaxy, an ancient and terrible race is stirring back to life. Entombed in stasis-crypts for millions of Terran years, they have slumbered through the aeons, waiting for the galaxy to heal from the wounds of a long and bloody war. Now, after sixty million years of dormancy, a great purpose begins. On desolate worlds thought long-bereft of all life, ancient machineries wake into grim purpose, commencing the slow process of revivification that will see those entombed within freed to stride across the stars once again. The unstoppable, undying Necron legions are rising. Let the galaxy beware.

The Necrons had been vindicated in their pursuit only of science and control over the material realm and certainly took pleasure in seeing the Old Ones' civilisation collapse as a result of their over-indulgence of psychic power, and the end of the C'tan's domination over their race. Yet even with the defeat of the Old Ones and the C'tan alike, the Silent King saw that the time of the Necrons in the galaxy was over -- for the moment, at least. They would allow the Enslavers to take what was left of the sentient life in the galaxy and let it become an interstellar wasteland; the psyker swarm would then die away and in time the galaxy would evolve new lifeforms who would be less sophisticated and easier to dominate. In addition, the Necrons understood that the mantle of galactic dominion was soon to pass to the Eldar, one of the psychically-potent races that had fought alongside the Old Ones throughout the War in Heaven and had thus come to hate the Necrons and all their works with the burning passion that is the defining characteristic of that species. The Eldar had survived where the Old Ones had not, and the Necrons, weakened by their expenditure of lives and resources in overthrowing the rule of the C'tan, could not stand against them. Yet the Silent King knew that the time of the Eldar would eventually pass, as it must pass for all those beings still cloaked in the flesh. It would take millions of Terran years for the Eldar's power to fade, but what mattered is that the Necrons would be there to take advantage of it.

Just as the stars gave birth to their children so the planets of the newborn galaxy eventually gave birth to lifeforms composed of matter which began the long evolutionary climb to self-awareness. The first sentient beings of the Milky Way Galaxy known to have developed a civilisation technologically advanced enough to cross the stars was a reptilian race of beings called the Old Ones by the Eldar, who knew them best. They possessed a slow, cold-blooded, but deep wisdom; having long studied the stars and raised astronomy and physics to such a level that their science and technology would appear to humanity like an arcane art. Their understanding of the workings of the universe were such that they could manipulate alternate dimensions and undertake great works of psychic engineering. Their science allowed them to cross the vast gulfs of space with only a single step via the myriad of Warp Gates they had built to connect the worlds of the galaxy in a vast network, much like the Eldar Webway of today; though on a much larger scale. The Old Ones had spread their spawn to many places in the galaxy, but they also knew that all life was precious. Where they passed, they seeded new intelligent species and reshaped thousands of worlds to make them their own according to their predetermined environmental and geographic criteria. It is believed by some in the Adeptus Mechanicus that even Terra felt the Old Ones' touch long before humanity's rise to self-awareness, though this notion is considered heretical at best by the Ecclesiarchy, as the Imperial Creed teaches that Mankind was made in the image of the God-Emperor before His spirit was incarnated in physical flesh millennia ago

Of all the galaxy's great powers, only the Eldar see the Necrons for the threat they truly are to all of the other sentient species -- and even they cannot be sure how many Tomb Worlds slumber in the darkness. After the War in Heaven, the Eldar took up a silent watch for any sign of Necron reemergence, and set watch on worlds they suspected of nurturing hidden stasis tombs. Many such worlds were seeded with life and adopted as homes by Eldar outcasts and Exodites, whose descendants would maintain the vigil. Where this was not possible, suspected Tomb Worlds were marked on a great crystal map so that their locations would not be lost as the millennia passed. Yet, as the ages of the galaxy passed, the Eldar became distracted by their own plights and thus forgot the duty they had sworn to uphold for their lost patrons, the Old Ones. By the time of the Fall of the Eldar in the 30th Millennium -- the terrible birth of the Chaos God Slaanesh -- the slumbering Necrons had been all but forgotten. Only in the Black Library and amongst a few outspoken segments of Eldar society did the vigil continue.