Entities and the War in Heaven.
Billions of years before mankind had evolved, before the mighty Imperium of man was born, there was a war that raged in the stars.
The Old Ones were at war with a species that had suffered oh so much, and had grown tainted with resentment and hate.
They called themselves the Necrons, and they were a clever species. Clever, but unwise. If they were wise, they'd have not fought this war. If they were wise, they'd be happy that they left the hellish home planet they came from. If they were wise, they'd have left the Entities alone.
The Old Ones had a scourge that plagued their gates and the Immaterium. A Species that feasted on worlds filled with life, granting the inhabitants with great power only to collect the accrued debt in blood thousandfold. The Entities. They sailed amongst the stars, large enough that one plane of existence could not hold these beasts.
The Necrons fought amongst themselves, and then in a bid for unity, The Triarch had them fight the Old Ones.
The War's initial campaigns were a disaster, but it brought attention from the Entities. They saw technology both new and amazing, and they too held a grudge. Seeing the Necrons and what they could learn, the Entities made an offer. How this communication happened, The Adeptus Mechanicus fails to comprehend.
The Entities exploded above the planets, and rained themselves down like hellfire, infesting and infecting Necrons throughout the Galaxy. Pain and hate and resentment became fuel for the fire, and secretly the entities ate away at the secrets of Necron technology. The Entities themselves took the forms of the forgotten ancient gods of Necron lore and crafted nigh indestructible beasts that they used to harbour the end of so many worlds - The Endbringers.
Before the Silent King, one of these Entities stood beside Szarekh. The Deceiver. All Entities knew different skills, and sometimes hid from others, sharing only with siblings or partners. The Deceiver was one such Entity. It feasted on some of the smartest and many selfish races. It had been found out again and again, and it was one who could infect nearly any planet with it's shards.
The Necrons didn't know the true nature of the Entities, or perhaps they were blinded by hate, and then the infestation. It promised the King both victory and immortality.
The Necrons said yes to the alliance, which was a good thing for them at the moment. They delayed their fate, and whilst those words were spoken as lies they became true in many ways. If they refused, it would have been so easy to rip the shards from the still living aliens. Their lands would explode, and the Entities would continue to feast elsewhere.
The powers of the Entities were strange and godlike. They granted access to the Immaterium, they granted weaponry and ideas beyond the wildest dreams. They granted near immortality to soldiers and generals.
They loaned out a debt of blood and watched as the Necrons slaughtered their way through the Old Ones.
As Expected, the test began. Shards began to return to the Entities, and they collected and refined themselves in the galactic scale war. They improved themselves, Stronger, faster, better powers. If they were godlie now, the growth by the war's end was unstoppable.
K'nib, Eldar, Krork and more were thrown at the Necrons from the old Ones in a desperate bid to survive. Worlds were sundered and soon enough the relentless fighting turned inwards.
After all, the Entities cared not for the Necrons. Only what the Necrons would teach them. Abaddon, Metatron, Azreal, The Deceiver, The Thinker and the Warrior, The Seven Deadly Sins... Wars waged amongst themselves as new upstart races saw themselves controlled and destroyed by the Entities. But the Necrons soon fought back. They learned many things too. They learned where the Entities hid, and how to fight them. They bound Mamon in the Immaterium and used it to kill others. They crashed meteors into the giants and killed those who were infected.
The War in Heaven became chaos. Warp spawned beings attacked the Entities and distracted them, as their extra dimensional mass turned against them. The Necrons cast aside their traitor masters and as everything came to a climax, the Warp ripped its way through one Entity known as The Terror. The Eye of Terror was born. The Entities, weakened from their plans failing utterly and from the attacks by the beings of the Warp, began to hide. The Eldar hunted whom they could within the Webway, killing the hibernating beasts as they fled.
The remaining Entities were split. Some are said to have fled the Galaxy, biding time before returning again. Some have focused entirely on the Warp, fighting and feasting on that realm instead. Some continued to study and consume planets to this day, hoping to find what their ancestors were after. Some had left that existence behind them, killing others of their kind in atonement. The last begged for forgiveness and protection from the Warp. They wished to be safe and in sadistic glee, the Necrons shattered them into many pieces and hid their souls within. The Entities were now dormant hosts to the Necrons themselves, and those who awakened those shards would be consumed by the immortal race.