Sharding of the C'tan

VICTORY AND BETRAYAL
"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 near unstoppable, and unleashed forces beyond comprehension. Planets were razed, suns extinguished and whole systems devoured by black holes called into being by the reality-warping powers of the star gods. Necron legions finally broached the webway and assailed the Old Ones in every corner of the galaxy. They brought under siege the fortresses of the Old Ones’ allies, harvesting the life force of the defenders to feed their masters. Ultimately, beset by the implacable advance of the C’tan and the calamitous warp-spawned perils they had themselves mistakenly released, the Old Ones were defeated once and for all.

Throughout the final stages of the War in Heaven, Szarekh bided his time, waiting for the moment in which the C’tan would be vulnerable. Though the entire Necron race was his to command, he could not hope to oppose the C’tan at the height of their power. Even if he did, and somehow met with success, the Necrons would still then have to finish the War in Heaven alone. No, the Old Ones had to be defeated before the C’tan could be brought to account for the horror they had wrought. And so it was that, 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 against the star gods.

In their arrogance, the C’tan did not realize their danger until it was too late. The Necrons focused the unimaginable energies of the living universe into weapons too powerful for even the C’tan to endure. Alas, the C’tan were immortal star-spawn, part of the fundamental fabric of actuality and impossible to destroy entirely. So was each C’tan instead sundered into thousands of fragments. Yet this was sufficient to the Silent King’s goals. Indeed, he had known the C’tan’s utter annihilation to be unachievable and had drawn his plans accordingly: each C’tan Shard was bound securely within the extra-dimensional space of a tesseract labyrinth, unable to escape. Though the cost of victory was high – millions had been killed as a consequence of rebellion, including all of the Triarch save the Silent King himself – the Necrons were once more in command of their own destiny.

Had it not been glutted to the point of hubris on the essences of its fellow star gods, the Nightbringer might not have fallen. After the last of the Old Ones were banished, the C’tan was brought low by those it thought beneath its notice. Its essence was shattered by god-killing hypercannons devised by the finest Crypteks in the galaxy – weapons the Nightbringer was convinced were built out of tribute, but which were turned upon it as it feasted. It was bound within an array of necrodermis containment units and trammelled under the rule of those it once called slaves."

-Necrons 8th edition Codex