the necrons are a ancient race of metal conquerers who are out to reclaim their territorys of old after their sixty million year slumber.
The Necrons were dormant in their stasis-tombs for close to 60-million human years. Necrons can destroy enemies at a molecular level with their Gauss weapons, all the while taking untold amounts of punishment with their metallic skeletons that are powered by their souls.
When it comes to technology, the Necrons can only be barely matched by the Eldar, and maybe even the Tau. A few phalanxes can be defeated in battle, but when a whole tomb world is mustered it's as good as unstoppable. Even if the Imperium could halt the Necron assault, it would cost them more than even they can afford.