The Plague Wars

Mortarion After the Heresy

Following the disastrous conclusion of the Horus Heresy and the Siege of Terra, Mortarion fled into the chaotic depths of the Eye of Terror. Dragging his shattered Legion into the warp's rotting embrace, he forged the Plague Planet—a noxious reflection of his own bitter soul. Ascending as Nurgle's First Disciple and a Daemon Primarch, the Lord of Death traded the Emperor's tyranny for the Plague God's horrific vitality. For ten millennia, Mortarion remained largely aloof from the galaxy's broader conflicts, cultivating his pestilent gardens in the immaterium.

While his subordinate lords waged scattered incursions upon worlds like Vraks and Cadia, Mortarion avoided committing his true strength to a grand crusade. He brooded on his throne, biding his time as the Imperium decayed. The tearing of the Cicatrix Maledictum at the dawn of the forty-first millennium finally provided the cataclysm he sought. The Great Rift flooded the galaxy with warp energies, fracturing Terra's domain and plunging countless sectors into terrified isolation.

Yet, it was a deeply personal grievance that truly drew Mortarion from his exile. The miraculous return of Roboute Guilliman awoke a dormant, venomous hatred within the Daemon Primarch. To Mortarion, the Avenging Son had always embodied the dictatorial arrogance of the Codex Astartes—a rigid, patrician tyrant he despised. Gathering the scattered Death Guard for the first time in millennia, Mortarion set his sights on the gleaming jewel of Ultramar.

My brother wakes to a galaxy that has rotted in his absence. Let us show him the true shape of his father's crumbling empire.

— Mortarion, addressing Typhus before the fleet's departure

The Resurrection Trigger

Roboute Guilliman's awakening on Macragge, orchestrated by Archmagos Belisarius Cawl and the Ynnari emissary Yvraine, shattered a millennial stasis that had defined the late Imperium. This singular event rippled through the warp, drawing the ire of every dark entity, but none felt the insult more keenly than Mortarion. The resurrection of his loyalist sibling disrupted a delicate cosmic balance, transforming the abstract decay of the Imperium into a tangible, ideological battlefield.

Guilliman wasted no time mourning the past, immediately launching the Indomitus Crusade to reorganize the shattered Imperium Sanctus. This sweeping military revival presented Mortarion with both an unbearable provocation and an unprecedented opportunity. The Great Rift had completely severed Astropathic communication across the Five Hundred Worlds, leaving Guilliman's realm vulnerable. Mortarion spent months on the Plague Planet meticulously calculating his brother's logistical blind spots.

For the first time since the Horus Heresy, the Daemon Primarch unified the seven Plague Companies around a single, colossal armada. His intention was never merely territorial conquest or military disruption. The impending invasion was a deeply intimate message delivered in the language of viral artillery. Mortarion intended to prove that Guilliman's stoic order was a fragile illusion, and that he, the forgotten son, had ultimately chosen the winning side of eternity.

The Invasion Begins

The true nightmare began around 002.M42, when the primary Death Guard armada translated from the warp directly into the Iax system. The choice of Iax was no strategic accident; it was a deliberate, spiteful desecration. During the Great Crusade in the thirtieth millennium, Mortarion and his Legion had personally brought Iax into Imperial Compliance. Returning to poison the very soil he had once conquered was the ultimate expression of his corrupted authority.

The timing of the assault capitalized perfectly on the psychic blackout caused by the Cicatrix Maledictum. Isolated from Macragge, Ultramar's outer bastion worlds found themselves fighting entirely in the dark. The virulent offensive spread with supernatural speed, and within months, twelve additional planets succumbed to what the Inquisition would later classify as the Blight. Entire planetary defense grids dissolved into rusted sludge under the orbital bombardment of necrotic spores.

The Imperial Navy attempted desperately to establish quarantine lines, but conventional void-warfare proved useless against Nurgle's blessings. Swarms of Foetid Bloat-Drones and lumbering Plagueburst Crawlers breached every defensive perimeter, their rusted hulls absorbing staggering amounts of las-fire. The pristine garden-agri worlds of Ultramar were rapidly transformed into festering charnel houses, their atmospheres thickening with toxic smog that choked out the light of local stars.

The Defence Mobilises

Caught in the immediate path of the devastation, Chapter Master Marneus Calgar and the Tetrarchs of Ultramar frantically marshalled their surviving forces. They established desperate bulwarks across the sector, fighting a chaotic holding action against an enemy that weaponized the very air. On Macragge, Guilliman received the fragmented reports of the invasion with grim realization. Recognizing the signature of his brother's malice, the Lord Commander suspended his wider Indomitus mandates and ordered his flagship to alter course for Iax.

"This is no random daemonic incursion," Guilliman declared to his inner circle, dismissing the counsel of his admirals who urged a cautious encirclement. "This is Mortarion. He comes for me, and I will not let my sons pay the toll for his hatred." The Avenging Son mobilized his personal retinue, drawing upon the newly awakened Primaris Space Marines. Cawl's fresh reinforcements, untouched by the fatigue of the long war, were thrust directly into the heart of the plague zone.

Despite the shattered astropathic choir, sheer stubborn defiance held the line on several fronts. Planets like Quintarn and Konor fiercely repelled the initial waves of plague-cultists, surprising the Death Guard commanders who had anticipated rapid capitulation. Local planetary defense forces fought and died in irradiated trenches, buying just enough time for the vanguard of the Indomitus Crusade to tear through the warp and cast anchor over the besieged worlds.

The Blighting of Iax

The planetary assault culminated when Mortarion himself made planetfall upon Iax. Accompanied by his elite Deathshroud terminators, the Daemon Primarch initiated a massive biotic ritual designed to drag the world fully into Nurgle's embrace. The once-pristine jewel of Ultramar shuddered as its surface was rapidly rewritten. Verdant forests dissolved into pools of green-yellow rot, and the atmosphere grew so toxic that unhelmed mortals dissolved from the inside out within seconds of drawing breath.

The defenders of Iax, anchored by the stalwart veterans of the Ultramarines 2nd Company and millions of local militia, mounted a heroic but doomed resistance. They held the final bastions around the planetary capital, fighting thigh-deep in necrotic mud. Every fallen guardsman soon rose again, their corpses hijacked by fungal blooms to attack their former comrades. The defenders were slowly ground into dust, vastly outmatched by the unnatural resilience of the Plague Marines.

Dominating the battlefield, Mortarion swung his colossal scythe, Silence, unleashing waves of entropic destruction that shattered bunker networks and armored columns alike. The ritual reached its zenith, permanently tethering the world to the warp. Iax was irrevocably changed, scarred beyond the restorative capabilities of the Adeptus Mechanicus. It earned the grim designation of a Blighted World, a weeping sore on the pristine flesh of the Ultramar empire that remains infected to this day.

The Duel at Iax

The climax of the campaign unfolded upon the ruined central platform of Iax's capital, where two demigods of the old era finally clashed. Guilliman arrived clad in the Armour of Fate, the Emperor's Sword blazing with golden fire in his grasp. Mortarion towered over him, a terrifying amalgamation of moth-eaten wings and rusted armor, wielding Silence and exhaling clouds of armor-eating smog. They did not exchange pleasantries; thousands of years of bitter ideological hatred erupted into immediate, world-shaking violence.

The duel lasted for agonizing hours, reducing the surrounding plaza to a cratered wasteland. Silence carved deep, rending gouges into Guilliman's life-sustaining armor, flooding his system with necrotic phages that would have instantly melted a lesser being. In return, the Avenging Son drove the Hand of Dominion into Mortarion's horrific visage, unleashing torrents of sanctified promethium and psychic fire. Both brothers suffered grievous, mortal wounds, their divine blood staining the corrupted earth.

Ultimately, neither combatant could secure a definitive physical victory. As his physical manifestation began to destabilize under the Emperor's fire, Mortarion executed a symbolic withdrawal, teleporting his ruined form back to the Plague Planet. Guilliman collapsed amidst the ruins, surviving the toxic overload only through the absolute final emergency protocols of Cawl's armor. The Primarch of the Ultramarines had held his ground, but the cost was nearly his life.

You died ten thousand years ago on a rusted ship, creature. I have no brother here.

— Roboute Guilliman, rejecting Mortarion's final taunt

The Withdrawal and the Aftermath

Following their master's sudden disappearance, the primary Death Guard armada broke orbit and retreated toward the Eye of Terror, leaving behind a ruined theater of war. The cost of the defense was astronomical. Ultramar mourned the total loss of thirteen planetary systems, and the jewel of Iax was permanently quarantined as a Blighted World. The scars inflicted upon the realm of Macragge would require centuries of grueling reconstruction and spiritual purification to even partially erase.

Guilliman was immediately evacuated to Macragge, where he spent agonizing months secluded in medical stasis, purging the lingering daemonic toxins from his bloodstream. Imperial scribes officially declared the Plague Wars concluded in 005.M42, painting the campaign as a decisive victory for the Avenging Son. The remaining Ultramarines and Indomitus assets began the slow, grim work of scouring the lingering plague-cults from the surviving hive cities.

However, this official end date was little more than political propaganda. Almost immediately, the so-called "Plague Episodes" began. Smaller, vicious raiding fleets led by subordinate Plague Captains continued to harass the outer fringes of the Five Hundred Worlds. While Guilliman publicly declared that his brother had perished in spirit on Iax, every veteran of the campaign understood the grim truth. The Daemon Primarch was immortal, deeply patient, and utterly guaranteed to return.

The Argument That Did Not End

The historical assertion that the Plague Wars ended in 005.M42 remains one of the Adeptus Terra's most hollow lies. The conflict never truly ceased; it merely fractured into a grueling war of attrition. Throughout the late-Indomitus era, extending well past 010.M42, the Plague Episodes have bled the Ultramarines' garrison forces. Small, lethal bands of Plague Marines periodically breach the quarantine lines, striking hard enough to ensure no defender in the sector ever truly sleeps.

For Mortarion, this campaign represents his most intimate endeavor since the Heresy. It transcends mere military objective. Every year, on the anniversary of the duel at Iax, the grand bells of Macragge toll in solemn mourning. Simultaneously, across the ruined continents of the Blighted World, the wind abruptly changes direction, carrying the scent of blooming rot—a phenomenon widely believed to be Nurgle's mocking reply to the Imperial grief.

Ultimately, the war for Ultramar is the physical manifestation of an ancient, unresolved sibling rivalry. It is a fundamental argument regarding order versus decay, discipline versus entropy. This is a bitter discourse that can only be concluded by the true death of one of the Emperor's sons. And since neither managed to slay the other during the apocalyptic fires of the Heresy, the galaxy itself must burn while they continue to debate.

The Plague Wars did not end. They merely paused to draw breath, waiting for the bells to toll again.

— Journals of Saint Celestine