The Plague Wars

The Plague Wars were the great trial of the resurrected Roboute Guilliman, fought in the long shadow of the Great Rift when his own cherished realm was plunged into a living nightmare of pestilence. Out of the churning warp storm poured the Death Guard of the daemon primarch Mortarion, intent on transforming the jewelled and ordered worlds of Ultramar into a rotting paradise dedicated to the glory of Nurgle.

Mortarion sought a bitter and deliberate symmetry: to defile utterly the perfect kingdom his loyal brother had raised, spreading contagion from world to world until the whole of Ultramar became a garden of decay. World after world was engulfed by the dreaded Plague of Unbelief, their populations dying in their millions as the Death Guard's poxes withered the crops in the fields, fouled the water in the wells and turned the newly dead into shambling hosts of further disease.

Against this creeping horror stood Guilliman himself at the head of the Ultramarines and the fresh strength of his Indomitus Crusade. The war became a grinding campaign of siege, containment and quarantine fought across dozens of worlds at once, the Primarch matching his brother's ancient malice with cold and precise strategic brilliance while his warriors fought and died to hold the line against an enemy that could not truly be slain. Every world painstakingly cleansed of the contagion seemed to sicken and fester once more the moment the fleets moved on.

At the last Guilliman confronted Mortarion in person and drove him from the realm, though only with the intervention of powers far beyond mortal reckoning. Ultramar survived the ordeal, but scarred and diminished, and the war left its mark upon all who had endured it. The full history is told in the chronicle of the Plague Wars.