The Underworld War of Calth

The Underworld War of Calth was the grim aftermath of the great betrayal, a conflict fought not for worlds or glory but for simple survival in the dark. When the Word Bearers sabotaged Calth's star, the surface was rendered lethal, drowned in killing radiation, and the war that had begun above ground was driven beneath it.

Millions of survivors, Ultramarines and mortal citizens alike, retreated into the vast arcologies and subterranean vaults that honeycombed the planet. There they found that traitor forces had been stranded with them, unable to flee the very devastation they had unleashed. What followed was years of merciless underground warfare, a lightless campaign of ambush, siege and hunting through endless tunnels and cavern-cities.

The Ultramarines set themselves to the systematic extermination of every traitor left beneath Calth's crust, a duty pursued with cold and relentless purpose. The Word Bearers, for their part, sought to work dark rites in the deep places, corrupting where they could not conquer. The Underworld War became a byword for grinding, hate-filled attrition, a shadow of the Heresy that outlasted the war above and left Calth a fortress-crypt of old vengeance.