The Plague Wars were a devastating series of invasions unleashed upon the realm of Ultramar in the aftermath of the Great Rift, as the forces of decay sought to claim it as a new dominion of pestilence. They were orchestrated by the daemon primarch Mortarion, who led his Death Guard out of the warp storms to drown the Imperium's most stable region in rot and contagion.
Mortarion's legions spread engineered plagues and virulent corruption across dozens of worlds, transforming verdant planets into festering gardens of disease. The scale of the assault was immense, and for a time it seemed that even Ultramar, long a bastion of order, might succumb entirely. Populations sickened and died, defences crumbled, and the boundaries between reality and the warp grew perilously thin.
The defence was led by Roboute Guilliman, the resurrected primarch of the Ultramarines, who returned to his homeland to face his fallen brother directly. Their confrontation was as much a clash of ideals as of arms, the son of order against the lord of entropy. Through immense effort the tide of plague was at last stemmed and Mortarion driven back, though not destroyed, and Ultramar emerged scarred but unbroken. The wars proved that even the Imperium's strongest heartlands were no longer safe in the shadow of the riven heavens. For more, see the Plague Wars.