Like the homeworlds of all the Traitor Legions, it was ultimately destroyed by an Exterminatus action carried out by Cyclonic Torpedoes just before the end of the Horus Heresy by the Dark Angels Legion because of the sheer levels of Chaos corruption that prevailed upon its surface.
The Poisonous World
Of all the worlds upon which the Primarchs were scattered, few were as terrible or forlorn a place as damned Barbarus. This Feral World orbited near a dim yellow sun in the Segmentum Tempestus, which created a thick, miasmic atmosphere of toxic chemicals. The most virulent of these gases rose through Barbarus' perpetual clouds towards the heat of its star, making the world beneath a dismal place of night, unbroken by starlight and possessed of short, shadowy days.
Very little can be said for certain about this remote and baleful planet, or the Primarch Mortarion's formative years spent there as it appears even at the time of his finding the truth was carefully edited and obscured by the hand of the Emperor Himself. What consistent information can be gleaned now of Barbarus and the Death Guard Primarch's story have a singe source, the so-called Stygian Scrolls of Lackland Thorn, authored by the famed and disreputable Remembrancer. He was a historian attached to the Crusade Fleet that breached the dark nebula that surrounded Barbarus.
The world on which the young Mortarion fell was the very epitome of the terrors which had befallen Mankind during the long night of the Age of Strife -- a domain of savage, alien overlords who ruled over an entrapped and preyed-upon human population as cruel as terrible gods. The only atmosphere breathable by humans existed only in the lowest elevations, on flat moors and in the valley basins of the jagged, stony mountains which spined the world. These unknown alien overlords were immune to the toxic soup of the planet's upper atmosphere, building great keeps of grey stone in the mountain fastnesses.
When humans first settled Barbarus, the horrific environmental conditions from which they had to eke out survival quickly reduced them to a pre-feudal state. The higher beings' incomprehensible Warp-based powers, their ability to survive where men could not, and above all their hunger to prey upon and experiment with humankind, caused the settlers to ascribe to those beings a medieval supernaturalism and an evil animus. The true nature of these dark overlords, beyond their obvious connection to the dark entities of the Empyrean, will likely never be known.
The Coming of Mortarion
After the Emperor of Mankind's 20 Primarchs were scattered across the galaxy from Terra through the Warp still in their gestation capsules by the actions of the Chaos Gods, they came to rest on many different human-inhabited colony worlds. The Stygian Scrolls record the fate of one of these Primarchs, Mortarion:
"There was one who came to rest on a bleak moor, strewn with the dead and scattered with the carnage of battle for leagues in every direction. This was the world of Barbarus. A planet perpetually shrouded in poisonous fog, whose mountainous crags were ruled by warlords with fantastic powers and horrific appetites, and whose human settlers, stranded there millennia before, were crowded into the lowest valleys, beneath the choking mists. They lived lives of unrelenting terror, eking out a peasant's existence by day beneath a dim sun which never burned completely through the fog, and cowering by firelight after dark from the terrible beings which moved unseen above."
The greatest of these overlords stood in triumph on the battlefield, reveling in his massacre until the silence was shattered by a child's cry. Legend tells that the warlord walked the sea of corpses for a day and a night in his creaking battle armour, drawn by the wail of the infant. For an instant, he considered ending its young life; but no mere human ought to be able to breathe the poisonous miasma of the heights of Barbarus, much less cry out as this child did.
For long moments he contemplated this thing which appeared to be human but was clearly much more; then he gathered up the infant and carried him from the carnage. For all his dark power, until that moment he had not had what this child now promised: a son and heir. Born of death, upon a field of death, the warlord christened the infant Mortarion: the child of death.
When Mortarion was taken by the overlord, his master tested the infant's limits. When he had determined precisely how high into the toxic clouds of Barbarus's peaks the child could survive, he erected a stony keep and fenced it behind black iron. Then he moved his own manse beyond, to the highest crag, where the atmosphere was deadly even to the nascent Primarch.
Mortarion grew to adolescence in such a world, of citadels of weeping grey stone and cast-iron fences, where the very air was death, and the sun never more than a distant smudge. It was a world of constant war, against opposing lords who came with golem armies of stitched-together dead one day, then tormented shapeshifters, more monsters than men, the next.
The Primarch was shaped by his grim environs, but he was a child of the Emperor for all that - superhumanly resilient to the poisonous air around him and superhumanly strong even in the absence of sufficient sunlight or nourishment. Mortarion possessed an intellect which was highly keen and which asked questions his lord was not wont to answer.
Increasingly, the questions centred around the fragile things in the valleys below, which the warlords preyed upon for their corpses to reanimate, or victims to accurse. His master kept Mortarion as distant from the human settlements as he could, but his very act of denial fed the maturing Primarch's obsession. The day finally came when Mortarion would be denied no longer. Mortarion slipped through the dungeons from his keep.
The last thing he heard was the voice of the overlord, the only father he had known, roaring in the miasmic darkness from the high battlements as Mortarion descended from the mountain, renouncing the Primarch for his betrayal, warning Mortarion that to return would mean death.
Descending beneath the mists was a revelation to Mortarion; his lungs were filled with air free of poisons for the first time. He smelled aromas of food being prepared, of crops freshly harvested, heard voices unmuffled by fog and, for the first time, heard laughter. The young Primarch realised that he was among his own kind, that the "fragile prey" of the warlords were his own people. And with the realisation came rage. He was determined to bring them the justice denied them by the dark powers which moved above.
Mortarion's acceptance amongst the human settlers of Barbarus was no simple thing. However like them he felt himself to be, to them he was little different from the monsters above. Towering over even the tallest of his fellow Barbarans, gaunt and pallid, with hollow, haunted eyes which betrayed the horrors he had seen, Mortarion terrified most of the settlers.
They looked upon him with suspicion and fear. It stung the young Primarch, but he bided his time, using his great strength to work the fields for their meager harvest, knowing that his opportunity to prove himself would come. When it did in the twilight hours, he was ready.
Savior of Barbarus
The village was eventually attacked by a lesser lord and his corpse-thralls. The villagers fought back with their farmer's tools and torches but were unable to prevent the shambling creatures from carrying out their master's dark purposes. Mortarion strode forth, a large two-handed starveling scythe in his hands, and charged into the ranks of the enemy and annihilated them with his Primarch-borne rage.
Their dark lord smiled at Mortarion as the Primarch neared, withdrawing into the poisonous heights where he thought this rebellious human could never reach him. He was still wearing his contemptuous smile when Mortarion caught up with him on the mountainside and exacted his vengeance for the 'fragile prey' below. After that night, Mortarion's place among the settlers was never in doubt.
As he matured, Mortarion taught the settlers of Barbarus what he knew of warfare. Word of his exploits spread, and many others made the perilous journey to learn. Slowly, villages became strongpoints, and the villagers more effective defenders. Eventually, Mortarion went amongst the people, traveling from settlement to settlement, teaching, building and, when occasion demanded, defending them. Always, however, his ultimate justice was denied; the dark powers could always retreat into the impregnable bulwark of their poisonous mists. His people could only fight in defense. That had to change.
Mortarion recruited the toughest, most resilient of Barbarus' population, forming them into small units which he drilled himself, teaching them not only defence but also attack. He turned blacksmiths from tool-working to weapons-making when time allowed, and crafters to the shaping of armour. And, with the best artificers he could find, he bent his formidable intellect to the problem of the poisonous air.
When one of the overlords descended from the poisonous fog blanketing the mountainous crags, the villagers not only managed to mount a successful defence, but were led by Mortarion into the poisonous fog where the overlord's army thought it would escape the marauding humans. Masked with crude filtering hoses and breathing apparatus, the Primarch led his retinue to bring death into the realm of death.
They killed the warlord and massacred his army. The Primarch continued to improve the breathing apparatus of his "Death Guard," as his retinue of warriors came to be known, campaigning ever higher into the dark overlord's domain. Encountering ever more virulent pestilence, the constant exposure to the higher doses of toxins toughened the Death Guard. These traits would eventually prove to be transferable to each new iteration of the Death Guard, building up their legendary resilience to the poisonous toxins and studiedly harsher environments that they encountered.
Mortarion and his Death Guard warred for solar months across the poisonous spine of Barbarus, until only one grim, but very familiar manse stood against them. The concentration of poisonous toxins was such that they threatened to overcome the Death Guard and the Primarch himself, forcing him to withdrew. Upon his return, however, his world was destined to once again spin out of his control.
The Emperor Arrives
Mortarion and his brethren arrived to find the village alive unlike he had ever known it. On everyone's lips was word of the arrival of a stranger, a great benefactor who brought promise of salvation. The Primarch's mood darkened; this day of deliverance was one he had worked for all his life, and he found himself altogether unhappy to see it co-opted by the arrival of some newcomer of uncertain agenda.
The Primarch entered the hall and saw the stranger, who was opposite in every imaginable way, seated at the banquet table. The stranger was robust with bronzed flesh and possessed an utterly perfect physique, whereas the people of Barbarus were gaunt, unhealthy-looking and pale. Despite the affect wrought upon him by Barbarus' poisons, the connection between the new benefactor and their defender was nevertheless plain to them all.
As plain as father and son. However, Mortarion was oblivious to any connection. He greeted the stranger with barely masked hostility, which quickly turned to outright anger at the stranger's utter unflappability. Angry, Mortarion declared that he and his Death Guard needed no help to finish their quest for justice.
It is said that the benefactor quietly challenged the stormy young Primarch's assertion, pointing out the Death Guard's failure to reach the last high citadel, and then threw down a gauntlet. If Mortarion could defeat the high overlord alone, he would withdraw and leave Barbarus to its own means. But if he failed, they would join his Imperium of Man and Mortarion would swear total fealty and allegiance to him.
Over the protests of his Death Guard, he spun on his heel and struck out alone for the last manse standing against him, the keep of the overlord he had called father. Mortarion climbed ever higher, driven by the inevitability of the imminent conflict with his once master, driven by his desire to bring final justice for the people of his world. However he was mostly motivated by a compulsion to prove himself to the stranger below.
The confrontation, when it finally came, was mercilessly brief. Mortarion, choking in air so toxic that the hoses of his protective breathing gear began to rot away, struggled to the very gates of the overlord's citadel, calling out his defiance. The last thing he saw as he fell to his knees, the world turning grey as he was overcome, was the Overlord of Barbarus coming for him, to fulfill the promise he had made generations before. Then the mighty stranger stepped between them, defying the death-fog, and felled the overlord with a single blow of his gleaming Power Sword.
The Death Guard
Mortarion was true to his oath. When he recovered, he bent his knee to the stranger and swore himself and the Death Guard to his service. Only then did the Emperor of Man reveal Himself as the young Primarch's true father, and the destiny such service would bring: command of the XIV Legion of the Legiones Astartes, the Space Marines.
The XIV Legion were already known as the "Dusk Raiders." The Space Marines that had been created from his genetic material had already developed their signature strategy; attacking their foes at nightfall, thus earning them this moniker. The Dusk Raiders' Power Armour was storm-grey in colour whilst their right arm and one or both shoulder plates were painted crimson. This was done with the intent to symbolically show their enemies what they truly were -- the Emperor's red right hand, relentless and unstoppable. Many enemies would throw down their weapons the moment the sun dipped beneath the horizon, rather than dare to fight them.
Mortarion changed the name of the Legion to the Death Guard after the name of the military force he had raised on his homeworld of Barbarus to overthrow the rule of that feudal world's necromantic warlords. He began to enlarge the Legion's ranks with men taken from the population of Barbarus. Upon first seeing them he told them: "You are my unbroken blades. You are the Death Guard." The Legion's name was then changed in accordance with this decree.
The Fate of Barbarus
Barbarus was destroyed by the Dark Angels Legion in an Exterminatus action carried out with Cyclonic Torpedoes shortly before the start of the Battle of Terra. When Mortarion learned of his homeworld's death, he showed little emotion, though the destruction of their homeworld led many Astartes of the Death Guard Legion to hope to carry out a similar extermination upon the Dark Angels' world of Caliban following their expected victory over the Emperor.
The Plague Planet
Within the Eye of Terror, Mortarion claimed the world which would become known as the 'Plague Planet' as his own; its location near the fabric of reality was ideal for launching new strikes into the Imperium and across the galaxy. He shaped it so satisfactorily and defended it with his Plague Marines so well that his patron, Nurgle the Unclean, elevated the Primarch to daemonhood and gave Mortarion what the Emperor had denied him, and what Horus had not been able to provide: a world of his own. Mortarion became the overlord of a world of poison, horror and misery. He had come home.
Since his elevation to daemonhood, Mortarion has, consciously or not, remade the Plague Planet very much in Barbarus' image. Its citizens cower in festering villages on the planet's surface, serving their supreme masters, Mortarion's champions and other daemonic chosen of Nurgle who reside in mighty fortress-citadels high above them. Diseased things which should be dead, yet are not, roam the landscape, and skeletal Mortarion rules over all, enthroned upon the highest peak of the world.
In the novel Lords of Silence it is said that Barbarus was destroyed after the Horus Heresy by virus bombs dropped by the Imperial forces as they scoured all the Traitor Legion homeworlds from existence. However the novella Dreadwing and the novel The Buried Dagger explained that Barbarus had been destroyed before the end of the Heresy by the actions of the Dark Angels as described above.
- Codex: Space Marines (3rd Edition)
- Index Astartes III, "The Lost and the Damned - The Death Guard Legion"
- Lords of Silence (Novel) by Chris Wraight, Ch. 5
- The Horus Heresy - Book One: Betrayal (Forge World Series) by Alan Bligh, pp. 122-127
- The Flight of the Eisenstein (Novel) by James Swallow
- Dreadwing (Novella) by David Guymer
- The Buried Dagger (Novel) by James Swallow, Interval I