In ancient times, the beings now known as Necrons were the mechanical soldiers of a long-dead interplanetary empire, sentient robots created for the sole purpose of fighting that empire's enemies across space. The Necrons were exposed to eons of terrible warfare across hundreds of worlds, seeing all its atrocities and horrors, witnessing the cruelty each living race was willing to inflict on another for its own gain. With time, they became embittered and despairing, lamenting the abject suffering of all their conquered worlds and the death and destruction they were forced to spread, and that inflicted on one another by thousands of other species.

Eventually, this sorrow crystallized into a powerful belief; life was devoid of hope or meaning, full of violence, sorrow, murder and pain. Only in death was there true peace. Overcome with compassion for other lifeforms, they turned against their creators and shattered the empire they had been created to serve from within.

The Necrons abandoned the terror weapons their masters had equipped them with, taking up vaporizing gauss weaponry that could kill instantly and without pain. They began to make their way across the worlds within the empire's territories, putting down all living things to save them from the suffering of being alive in an uncaring universe.

The modern Necron army is composed almost entirely of soulless robots, manufactured by their fellow Necrons to replace those who had been destroyed in battle over the ages, designed with only the basic AI needed for combat. Of the entire force labeled "Necron", only about 1% are actually sentient, these now being the Necron Lords who lead their mindless robot armies to euthanize a galaxy in agony.

As time has passed into the age of the Imperium, the Necrons have developed methods and rituals for their euthanic mission. When a planet has been cleared of all intelligent life, a vast funeral service is held on the surface by the commanding Necron Lord and any others with him, mourning the lives and deaths of the population and of the world itself. Following this, builder drones begin constructing vast pillars adorned with solemn skeletal statuary, fountains built into the empty eye sockets, tall bells attached to their sides to chime miserably in the wind. Upon the very tops of these kilometers-tall pillars are constructed the foundations of a low-orbit Dyson sphere, a global casket which will bury the planet and starve its ecosystem of light, dooming it to slowly die out. When the worldwide coffin is complete, teleportation-based mining operations on its interior surface begin extracting minerals and other materials from the planet, slowly "rotting" it and providing the materials needed by the factories on the exterior surface for new robot soldiers and battleships. From there, new planets are chosen to be purged and transformed into Necron grave-worlds.