"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.
There is some debate amongst Imperial circles as to precisely when the Plague Wars began, or indeed which war zones they encompass. As with all the great works of Nurgle, this sprawling conflict has waxed and waned, undergoing periods of festering incubation and temporary remission before flaring to life again just as the warriors of the Imperium of Man believed that victory was finally theirs.
Even the origins of the Plague Wars are debated by the historitors of Ultramar, with the earliest accounts of hostilities claimed to have taken place even before the Great Rift split the stars.
Whatever the truth, it is known that the Plague Wars as the Imperium understands them have stretched across not only Ultramar -- where the forces of Nurgle have assailed the sons of Guilliman time and again -- but also the star systems to the galactic north of the Ultramarines' stellar realm now known as the "Scourge Stars."
It was amongst these formerly prosperous worlds that diseases such as the Crawling Pustulence, Eyerot, the Plague That Walks and the Oozing Pox erupted in riotous profusion after the Great Rift opened. Imperial defence lines were overrun by wave upon wave of trudging plague daemons and fanatical Chaos Cultists.
Corrupt demagogues spewed filth and falsehoods that saw Planetary Defence Force regiments turn upon their former comrades before joining the pestilent throng. Everywhere, the droning tally of misery was felt. Corpse mountains burned as Imperial authorities strove in vain to stem the tide of corruption that was drowning their worlds, until at last each planet in turn fell wholly into the grip of Nurgle.
It was these pox-riddled systems that Mortarion, Daemon Primarch of the Death Guard, claimed as his staging post for the invasion of Ultramar, and from there that he launched his initial veiled sorties before his final offensive commenced.
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.
War of Flies
The Plague Wars also encompass the earlier conflict known as the "War of Flies." This was the stop-start war of attrition that the Death Guard and their allies fought against the worlds of Ultramar in the wake of the Noctis Aeterna.
From Talassar to Iax, Circe to Laphis, the Plague Marines launched their strikes. Some were little more than squad-level actions intended to seed contaminants onto Loyalist worlds or enact ritual sacrifices to destabilise the Imperial defences.
Others were full-blown offensives by entire vectoriums of Death Guard warriors, instigated to erode the defences of Ultramar and spread destruction and dismay.
Against all these actions the Ultramarines stood strong, rallying swiftly to battle the minions of Nurgle wherever they raised their horned heads. Often outnumbered, typically fighting alongside beleaguered battalions of the Ultramar Auxilia, the scions of Guilliman threw back one attack after another and burned out the corruption of Chaos wherever they found it.
- Codex Adeptus Astartes - Space Marines (8th Edition), pp. 24, 29
- Codex Supplement: Ultramarines (8th Edition), pg. 35