The Battle of Calth

The Battle of Calth was the surprise dagger the Word Bearers drove into the heart of Ultramar, a treacherous stroke meant to cripple the largest and most powerful of all the loyal Legions before it could march to the Emperor's aid. Under the cover of a joint muster and a feigned campaign, the sons of Lorgar fell upon their unsuspecting cousins in the void and on the ground alike.

The betrayal began in orbit, where the Word Bearers ambushed the assembled fleets and unleashed daemonic sorcery to poison Calth's own star. As the system's sun flared into a lethal storm of radiation, the surface of the world became a killing ground exposed to a slow and searing death. What had been a proud staging world for the Ultramarines collapsed into desperate, fragmentary battles fought amid burning arcologies, downed warships and streets choked with the dying.

Roberte Guilliman rallied his scattered companies with the cold efficiency that defined him, but the cost was staggering and the strategic damage worse. To survive the poisoned star, the surviving population and Astartes were driven beneath the planet's crust into its vast network of arcology-tunnels, there to wage a lightless subterranean war against the Word Bearers and the daemons they had summoned into the dark. The fighting in those depths would grind on for years after the wider Heresy had moved elsewhere. Kor Phaeron and the Dark Apostle Erebus drove the treachery onward, glorying in the ruin they had wrought upon a loyal and unsuspecting world.

Calth endured, and Guilliman's Legion was not destroyed as Lorgar had hoped, denying the traitors their great prize. Yet a jewel of Ultramar had been rendered a scorched and poisoned tomb, its billions driven underground for generations to come. The deeper account is told in the chronicle of the Battle of Calth.