The First War for Armageddon was a daemonic invasion of the great hive world Armageddon, the opening chapter in that planet's long and bloody saga. It began when the daemon primarch Angron, once a loyal son turned champion of the Blood God, led a host of World Eaters and Khornate daemons in a sudden, savage assault.
The slaughter was immense, for the invaders sought not conquest but carnage, drowning the world in blood to open the way for a greater manifestation. Standing against them came the Grey Knights, the secret Chaos-hunting brotherhood, alongside Space Wolves and the beleaguered planetary defenders. In brutal fighting the daemonic tide was at last banished and Angron cast back into the Warp, but victory came at a terrible price in lives.
The war's aftermath proved as grim as the conflict itself. The Inquisition, determined that no knowledge of daemons or the Grey Knights should spread, moved to silence those who had witnessed the horror; many survivors and civilians were taken away, never to be seen again. This ruthless secrecy nearly brought the Space Wolves to open conflict with the Inquisition, sowing a bitterness that would linger for centuries. For the embattled hive world, it was only the first of three great wars, a prelude to the orkish invasions still to come.