They nurtured many potential warrior species, among which are believed to be the earliest members of the Aeldari species and many other xenos races, including the Rashan, the K'nib, the Krork and many others. Terran millennia passed as the Old Ones' creations and experiments finally bore fruit even as the C'tan and their Necron servants continued to extinguish life across the galaxy.
The Old Ones' psychically-empowered servant species spread across the galaxy, battling the advanced Necron technology with the psychic power of their Warp-spawned "magic." Facing this new onslaught, the C'tan's empire was shattered, as the psychic forces of the Immaterium were anathema to soulless entities whose existence was wholly contained within purely physical patterns of electromagnetic force. For all the destruction they could unleash, they were unable to stop the Old Ones and the younger races' relentless advance across the stars.
The remaining C'tan, unified by this great threat for the first time in millions of Terran years, sought a way to defeat the soul-fuelled energies of the younger species. They initiated a great warding, a plan to forever defeat the psychic sorceries of the Old Ones by sealing off the material universe from the Warp, a plan whose first fruits could once be found on the Imperial Fortress World of Cadia in the form of the great pylons that littered the surface of that world in intricate networks and create the area of space-time stability near the Eye of Terror known as the Cadian Gate.
With their god-like powers, it was only a matter of time until the C'tan succeeded. But before their great work was complete, the seeds of destruction the Old Ones had planted millennia before brought about an unforeseen cataclysm. The growing pains and collective psychic flaws of the younger species threw the untapped, psychically-reactive energies of the Immaterium into disorder.
War, pain and destruction were mirrored in the bottomless depth of the Sea of Souls that was the Warp. The maelstrom of souls unleashed into the Immaterium by the carnage of the War in Heaven coalesced in the previously formless energies of the Warp. Older entities that had existed within the Immaterium transformed into terrifying psychic predators, tearing at the souls of vulnerable psykers as their own environment was torn apart and reforged into the Realm of Chaos.
The denizens of the Warp, stirred into a frenzy by the conflict in the Materium, clustered voraciously at the cracks between the Immaterium and the material universe, seeking new ways to enter the physical realm. The Old Ones brought forth new genetically-engineered warrior species to defend their last strongholds, including the technology-mimicking Jokaero and the formidable, green-skinned Krork who were the ancestors of the present day Orks, but it was already too late. The Old Ones' intergalactic Webway network was breached from the Immaterium and lost to them, several of their Warp Gates were destroyed by their own hands to prevent the foul entities of the Warp from spreading to uncorrupted worlds and the Old Ones' greatest works and places of power were overrun by the horrors their own creations had unleashed.
The most terrifying of these horrors were the Enslavers, Warp entities whose ability to dominate the minds of the younger species and create their own portals into the material realm using mutated, possessed psykers brought them forth in ever-greater numbers. For the Old Ones, this was the final disaster as the Enslavers took control of their servants. The Pandora's Box unleashed by the creation of the younger species finally scattered the last of the Old Ones and broke their power over the galaxy once and for all. Life had stood at the edge of an apocalypse during the War in Heaven between the Old Ones and the C'tan. Now as the Enslavers breached the Materium in epidemic proportions, the survivors looked doomed.
Ultimately, beset by the implacable onset of the C'tan and the calamitous Warp-spawned perils they had themselves mistakenly unleashed, the Old Ones were defeated, scattered and finally destroyed. Whether the species went extinct or simply fled the galaxy to seek a new haven elsewhere is unknown.
Throughout the final stages of the War in Heaven, Szarekh bided his time, waiting for the moment in which the C'tan would prove most vulnerable.
Though the entire undying Necron species was now his to command, the Silent King could not hope to oppose the C'tan at the height of their power, and even if he did and met with success, the Necrons would then have to finish the War in Heaven against the Old Ones and their increasingly potent allied warrior species alone. No, the Old Ones had to be completely and utterly defeated before the C'tan could be brought to account for the horror they had wrought among the Necrontyr.
And so, when the C'tan finally won their great war, their triumph was short-lived. With one hated enemy finally defeated, and the other spent from hard-fought victory, the Silent King at last led the Necrons in revolt. In their arrogance, the C'tan did not realise their danger until it was too late.
The Necrons focused the unimaginable energies of the living universe into weapons, god-killing hypercannons devised by the finest Crypteks in the galaxy, too mighty for even the Star Gods to endure. Not even the great overlords of the Necron crownworlds well remember the battles against the Star Gods, for causality itself was damaged by the forces unleashed to dismember the C'tan, and the Silent King was wont to remove the knowledge of the dreadful weapons employed from his warriors after the fact in fear of what might later be done with them.
Alas, the C'tan were immortal star-spawn, part of the fundamental fabric of reality and therefore nigh-impossible to destroy. So was each C'tan instead sundered into thousands of smaller and less powerful energetic fragments each with the personality of the whole entity, yet this outcome was sufficient to the Silent King's goals.
Indeed, he had known the C'tan's ultimate destruction to be impossible and had drawn his plans accordingly; each newborn "C'tan Shard" was bound within a multidimensional Tesseract Labyrinth, as tramelled and secured as a legendary Terran djinn trapped in a bottle. Though the cost of victory was high -- millions of Necrons had been destroyed as a consequence of their rebellion, including all of the members of the Triarch save the Silent King himself -- the Necrons were once more in command of their own destiny.
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 species. 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 intelligent 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 Aeldari, one of the psychically-potent species 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 Aeldari 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 Aeldari would eventually pass, as it must pass for all those beings still cloaked in flesh. It would take millions of Terran years for the Aeldari's power to fade, but what mattered is that the Necrons would be there to take advantage of it.
So it was that the Silent King ordered the remaining Necron cities to be transformed into great tomb complexes threaded with stasis-crypts. Let the Aeldari shape the galaxy for a time -- they were but ephemeral, whilst the Necrons were undying and eternal. The Silent King's final command to his people was that they must sleep for the equivalent of 60 million standard years but awake ready to rebuild all that they had lost, to restore the Necron dynasties to their former glory.
This was the Silent King's final order, and as the last Tomb World sealed its subterranean vaults, Szarekh destroyed the engrammatic command protocols by which he had controlled his people for so long, for he had failed them utterly. Without a backward glance, Szarekh, the last of the Silent Kings of the Triarch, took ship into the starless void of intergalactic space, there to find whatever measure of solace or penance he could.