- "Let them flee beneath cover of their virus bomb. By the Emperor, they shall be repaid tenfold for the evils they have wrought upon Ultramar."
- —Roboute Guilliman, as the Plague Wars end
The Plague Wars were an attempt by the daemonic and Heretic Astartes forces of the Chaos God Nurgle, including the Daemon Primarch Mortarion and his Death Guard Traitor Legion, to conquer the Realm of Ultramar and add it to the Plague God's growing realm in realspace. The Plague Wars began at some point after the birth of the Great Rift and the onset of the Noctis Aeterna and ended in ca. 111.M42, after the Primarch Roboute Guilliman successfully ended his Indomitus Crusade and brought Imperial reinforcements to defend Ultramar.
To the galactic north of Ultramar, the followers of the Chaos God Nurgle first established dominion in the Scourge Stars soon after the success of the 13th Black Crusade in precipitating the fall of Cadia. From this hive of corruption, armies of Nurglite daemons poured forth, accompanied by the traitorous Death Guard Legion along with Renegades and Chaos Cultists beyond count. Three loathsome spearheads pushed into Ultramar, attacking along a hundred fronts and bringing with them unnatural pestilence. The defenders of Ultramar -- Astartes and mortal alike -- fought bravely, but quickly lost ground. Ultramarine Primaris Space Marines of the Ultima Founding arrived not long after the successful completion of the resurrected Primarch Roboute Guilliman's Terran Crusade from Terra aboard Fleet Avenger to reinforce their brethren, but even these transhuman reinforcements could only slow the attackers' progress.
By ca. 111.M42, over a hundred standard years after the start of the Indomitus Crusade, the Imperial defenders all across Ultramar were depleted in dozens of ground campaigns, while a Plague Fleet systematically destroyed the realm's Ultramar Defence Fleet and star fortresses. Guilliman returned from the Indomitus Crusade after over a standard century of campaigning to stabilise the Imperium, and his deft and defensive manoeuvres bought time to launch what became known as the "Spear of Espandor" counterattack. The combined plague armies were eventually fought to a standstill amongst the ruins of Iax, before the Death Guard Daemon Primarch Mortarion escaped with his forces back to the Scourge Stars under cover of a Virus Bomb attack, both because of his brother's staunch defence and because Nurgle's realm in realspace had come under assault by the forces of the Blood God Khorne.
In a brief respite from the work of safeguarding the Emperor's realm after Mortarion's defeat, Guilliman ordered the rebuilding and decontamination of Ultramar, as well as the establishing of new procedures for creating further Ultramarines. It was not long before new Imperial Crusades called the Lord Commander of the Imperium away from Ultramar and back out into the dark galaxy. The Ultramarines then began the work of preparing their vengeance against the servants of the Dark Gods.
- Codex Adeptus Astartes - Space Marines (8th Edition), pp. 24, 29