It was just another day in the grimdarkness of the far future. A Chapter of Space Marines called the Dark Paladins launched a force to investigate the silence that had gripped the world of Jarrman Prime. There, the Marines encountered a strange xenos species that had attacked the Imperial world. Upon hearing that these xenos were not easily killed, the Chapter Master declared that the entire chapter be deployed and that these xenos would not take more ground. Even though the Dark Paladins fought valiantly, they were annihilated down to the last man. Not only were the ground forces destroyed but the fleet in orbit was also eliminated. Only by piecing together the scattered remains and reports did the Imperium know of what truly happened.
Okay, what really happened was that a Necron tomb awoke on Jarrman Prime, killed all the living creatures on it then were attacked by the Dark Paladins. It was even reported that the xenos were mechanical and did not die. Not only did these robots not die, but the Techmarines were stunned. This event is the event that has given rise to the idea that the Priesthood of Mars would gladly bob a few "Cron's" if you get my drift rather than, oh I don't know, DESTROY MACHINES THAT CAN THINK FOR THEMSELVES LIKE IT SAYS SO IN THEIR RELIGION! So much for religion causing people to be stupid, Big E, because the AdMech is stupid because it doesn't follow it's religion.
There's always a few Marines left behind to continue recruitment or something, though (or Marines sent to Honor Companies), so maybe they rebuilt. Or maybe they truly were so stupid they sent their entirety to attack a Necron Tomb World (not knowing isn't good enough when it's clear the planet itself is murderizing your fleet) instead of evacuating as many people as possible and getting reinforcements.
We all seem to have missed something important, though. The Imperium pieced together reports and remains to find out what happened. Remains. Which would be above and on the Tomb World. So... Where did the Necrons go?