Damnos

Damnos was once an unremarkable Imperial ice world, its glacial wastes riddled with the mining-shafts that drove ever deeper in search of ore. In their greed the colonists dug too far, and their drills breached the sealed vaults of a slumbering Necrons tomb, rousing a dynasty of the ancient star-gods from their aeons-long death-sleep.

What followed was massacre. The unliving legions rose from beneath the ice in tides of gleaming metal, their gauss-fire flaying the flesh from screaming defenders and their reanimating warriors rising again and again no matter how they were cut down. The world's garrisons were overwhelmed, its cities entombed in frost and slaughter, until the Space Marines intervened to stem the mechanical tide.

Even the Adeptus Astartes could not hold Damnos. Though they won bitter victories and slew the enemy in droves, the Necrons simply endured, patient and remorseless, and the world was ultimately abandoned to the dead. Later crusades returned to shatter the tomb legions in vengeance, but Damnos remains a byword for the horror that sleeps beneath so many worlds, a frozen grave where humanity learned that some things are far older and far more terrible than the Imperium itself.