Armed with weapons of god-like power and starships that could cross the galaxy in the blink of an eye through the use of quantum phase technology, the Necrontyr stood ready to begin their war against the Old Ones anew. But the C'tan had another gift for their mortal subjects. They offered the Necrontyr a path to immortality and the physical stability their species had always craved. Their diseased flesh would be replaced with the living metal of necrodermis that made up their Star Gods' own physical forms. Their discarded organic husks would be consumed and their cold, metal forms would then be free to pursue their great vengeance against the Old Ones and the rest of a hateful universe, freed forever from the weaknesses of their hated flesh.
With the pact between Necrontyr and C'tan sealed, the Star Gods revealed the form that immortality would take for the Necrontyr, and the great biotransference process began. Vast bio-foundries were constructed, and into these the Silent King Szarekh's peoples marched according to the terms of the pact he had made with the C'tan. What blasphemous procedures the Necrontyr were subjected to within the raging bio-furnaces cannot be known, but certainly, each was stripped of flesh and of soul, their body replaced by a shell of living metal necrodermis much like the C'tan themselves animated by what remained of their guttering self.
Above each furnace swooped and dove the ethereal true-forms of the C'tan as they glutted themselves on the spiritual and electromagnetic detritus of an entire species. It was only when the Silent King himself emerged from the biotransference process and looked upon what had become of his people that he saw the awful truth of the pact he had made. Though immortality and nigh-godlike strength and vigour were his, it had come at the cost of his soul, the aetheric remains of which had already been sucked down the gullet of a circling C'tan.
As Szarekh watched the C'tan feast on the life essence of his people, he realised the terrible depth of his mistake. In many ways he felt better than he had in Necrontyr decades, the countless aches and uncertainties of organic life now behind him. His new machine body was far mightier than the frail form he had tolerated for so long, and his thoughts were swifter and clearer than they had ever been.
Yet there was a terrible emptiness gnawing at his mind, an inexpressible hollowness of spirit that defied rational explanation. In that moment, he knew with cold certainty that the price of physical immortality had been the loss of his soul. With great sorrow the Silent King beheld the fate he had brought upon his people: the Necrontyr were now but a memory, and the soulless, undying Necrons had been reborn in their place.
Yet if the price had been steep, biotransference had fulfilled all of the promises that the C'tan had made. Even the lowliest of the Necrontyr was now blessed with immortality -- age and hard ionising radiation could little erode their new mechanical bodies, and only the most terrible of injuries could destroy them utterly.
The biotransference process had embedded engrammatic command protocols in every Necron mind, granting Szarekh the unswerving loyalty of his subjects. At first, the Silent King embraced this unanimity, for it was a welcome reprieve from the chaos and fractiousness that had consumed the Necrontyr Empire in recent years. The entire species was now his to command, and so it fell upon the Necrons to honour their side of their terrible bargain. However, as time wore on Szarekh grew weary of his burden but dared not sever the command protocols, lest his subjects turn on him seeking vengeance for the terrible curse he had visited upon them.
Biotransference had left behind only the ghostly echoes of the Necrontyr's consciousnesses. Only a few of the most strong-willed Necrontyr among the nobility and the military retained their intellect and self-awareness and even they were but shadows of their former selves. They had been purged of so much of what had made them unique individuals. But unlike the Silent King, most of the Necrons at first cared not at all for their loss; all that mattered to them was that they would live forever without disease or death as their Star Gods had promised.
Renewed and empowered as never before by their devouring of the souls of an entire species, the C'tan were now unstoppable, and with the undying legions of the Necrons marching in their wake, the Old Ones were doomed. Only one thing truly remained of the old Necrontyr -- their burning hatred for all the other living, intelligent species of the universe.
Legions of the undying living metal warriors set out into the galaxy in their Tomb Ships and the stars burned in their wake. The Old Ones' mastery of the Warp was now countered by the C'tan's supremacy over the physical universe and the ancient enemies of the Necrons suffered greatly in the interstellar slaughter that followed.
With the C'tan and the Necrons fighting as one, the Old Ones were now doomed to defeat. Glutted on the life force of the Necrontyr, the empowered C'tan were able to unleash forces beyond comprehension. Planets were razed, suns extinguished and whole star systems devoured by black holes called into being by the reality-warping powers of the Star Gods. Necron legions finally breached the Webway and assailed the Old Ones in every corner of the galaxy. They brought under siege the fortresses of the Old Ones' many allies amongst the younger intelligent species of the galaxy, harvesting the life force of the defenders to feed their voracious C'tan masters.
In the closing years of the War in Heaven, one of the primary factors that led to the Necrons' ascendancy was their ability to finally gain access to the Old Ones' Webway. The C'tan known as Nyadra'zath, the Burning One, had long desired to carry his eldritch fires into that space beyond space, and so showed the Necrons how to breach its extradimensional boundaries. Through a series of living stone portals known as the Dolmen Gates, the Necrons were finally able to turn the Old Ones' greatest weapon against them, vastly accelerating the ultimate end of the War in Heaven.
The portals offered by the Dolmen Gates are neither so stable, nor so controllable as the naturally occurring entrances to the Webway scattered across the galaxy. Indeed, in some curious fashion, the Webway can detect when its environs have been breached by a Dolmen Gate and its arcane mechanisms swiftly attempt to seal off the infected spur from the rest of the Labyrinth Dimension until the danger to its integrity has passed. Thus, Necrons entering the Webway must reach their intended destination through its shifting extradimensional corridors quickly, lest the network itself bring about their destruction.
Of course, in the present age, aeons have passed since the Necrons used the Dolmen Gates to assault their archenemies. The Old Ones are gone, and the Webway itself has become a tangled and broken labyrinth. Many Dolmen Gates were lost or abandoned during the time of the Necrons' Great Sleep, and many more were destroyed by the Aeldari, the Old Ones' successors as the guardians of the Webway. Those that remain grant access to but a small portion of the immense maze that is the Webway, much of that voluntarily sealed off by the Aeldari to prevent further contamination.
Yet the Webway is immeasurably vast, and even these sundered skeins allow the Necrons a mode of travel that far outpaces those of the younger races. It is well that this is so. As a species bereft of psykers as a result of the loss of their souls during the biotransference process, the Necrons are also incapable of Warp travel, and without access to the Webway, they would be forced to rely once more on slow-voyaging stasis-ships, dooming them to interstellar isolation.
In the wake of these victories, the C'tan and their undying Necron servants now dominated the galaxy. The last planetary bastions of the Old Ones were besieged and the intelligent species they had once nurtured became cattle for the obscene hunger of the C'tan. To the younger intelligent species of the galaxy, the Necrons and their Star Gods were cruel masters, callously harvesting their populations at will to feed the C'tan's ceaseless hunger for life energies. The C'tan were figures of terror who demanded mortals' adoration and fear in equal measure.
For unknown reasons, but probably because their individual hungers for mortal life energies knew no bounds, the C'tan ultimately began to fight amongst themselves for both sport and out of spite. Among the Aeldari, an ancient myth holds that their Laughing God tricked the C'tan known as "the Outsider" into turning on its brothers and beginning their long war for ascendancy.
In the course of the C'tan's struggle against one another, whole planets were razed, stars were extinguished and entire solar systems were devoured by unleashed black holes. New cities were built by the efforts of millions and then smashed down once more. As the "red harvests" of the C'tan and their Necron servants grew thin, C'tan eventually devoured C'tan, until only a few were left in the universe and they competed amongst themselves for a long age.
Eventually, even the Old Ones, who had once been defined by their patience and unstoppable will, became desperate in the face of the Necron assault. They used their great scientific skills to genetically engineer intelligent beings with an even stronger psychic link to the Warp, hoping to create servants with the capability of channeling psychic power to defend themselves.