Mortarion, also known as the Death Lord or the Prince of Decay after he turned to Chaos, was one of the original twenty Imperial Primarchs created by the Emperor of Mankind. He was given command of the Death Guard Space Marine Legion on the arrival of the Emperor to his homeworld of Barbarus, but he turned to the Forces of Chaos during the Horus Heresy. At present, Mortarion is the greatest Daemon Prince of Nurgle and the Daemon Primarch of the Death Guard Chaos Space Marines.
When the twenty Primarchs of the Space Marine Legions were scattered across the galaxy in a mysterious accident, one came to rest on the planet Barbarus, a world wreathed in poisonous fog. The population of the world was split into two groups: the controlling warlords; necromancers with fantastic powers along with the human settlers, who had been trapped on the planet millennia before and were now forced to eke out an existence in the poison-free valleys of the planet, fearing the wrath of the warlords and their creations.
The Primarch-child was taken in by the most powerful of the warlords, who found him amongst the corpses of a battlefield, screaming and wailing where a normal child would have suffocated and died long before. The Overlord of Barbarus took the child in with the intention of creating a son and heir, naming him Mortarion - child of death.
Mortarion was kept in a fortress positioned at the limit of even his superhuman tolerance to the toxins in the air, while the Overlord moved his own fortress to the highest peak of the world, beyond where even Mortarion could go. He trained the child, who had a highly keen intellect and voracious appetite for knowledge; Mortarion learned everything from battle doctrine, to arcane secrets, from artifice to stratagem. However, the young Primarch's questions began to turn towards subjects the Overlord did not want to talk about, namely the pitiful creatures in the valleys that the many warlords preyed upon for corpses to reanimate and bodies to warp.
The Deliverance of Barbarus
Finally, knowing he would be unable to find the answers he desired from his adoptive father, Mortarion broke out of the fortress that had been his home and prison after killing several guards stationed at the gates of the fortress, and headed for the valleys of Barbarus. Breaking through the poisonous mists, Mortarion discovered that the prey of the warlords were in fact the same species as he, and swore to deliver them from their oppression. The people of Barbarus were slow to accept this pale, gaunt stranger from the mountains, but Mortarion was given a chance to prove his worth when creatures enthralled to another warlord attacked the village. Seeing that the peasants were unable to effectively fight back, Mortarion joined the fray, wielding a massive harvesting scythe that made short work of the beasts. The warlord smiled when Mortarion advanced upon him and withdrew to the apparent safety of the deadly fog, only to be pursued and butchered by this inhumanly resilient Primarch.
Accepted into the village without further reservation, Mortarion began to train the villagers in the art of warfare. Soon, representatives from other villages journeyed to learn from Mortarion, while the villages scattered across the valleys of the world were transformed into strongpoints. Mortarion travelled from settlement to settlement, teaching, building and defending his people.
He recruited the toughest, most resilient men he could find, forming them into small units that trained under his supervision. He enlisted the aid of blacksmiths, craftsmen and artificers to create suits of armour that would allow men to travel through the poisonous fog. As each battle in the mists was fought, Mortarion and his Death Guard would learn how to better adapt the armour, and themselves, to reach the more poisonous heights. Eventually, only one peak denied them access, the one on which Mortarion's adoptive father had made his home.
The Coming of the Emperor
Despite his adoptive father being a ruthless necromancer, Mortarion felt reluctant to attack the man who took him in and called off the planned attack. Returning to the village, Mortarion's mood darkened when he found his people talking not of his victory but of the arrival of a benevolent stranger who promised salvation to the people of Barbarus. Finding this stranger in conference with the village elders, Mortarion claimed that his people needed no outside help. The stranger commented that even Mortarion and his Death Guard were having trouble pacifying the final warlord, and offered a challenge. If Mortarion could defeat the Overlord, the stranger would leave. If not, Mortarion had to swear fealty to the stranger and the Imperium of Man he represented.
Ignoring the protests of his Death Guard, Mortarion left alone to confront his adoptive father, motivated by a compulsion to prove himself to the stranger below. The confrontation was brief. The air surrounding the Overlord's fortress was so poisonous, that parts of Mortarion's armour began to rot. He collapsed at the gates of the Overlord's citadel, bellowing challenges. The final thing Mortarion saw before he passed out was the Overlord of Barbarus coming to kill him, then the stranger leaping between the two and slaying the Overlord with a single sword thrust.
When he recovered, Mortarion swore fealty to the stranger, who revealed himself to be Mortarion's father, the Emperor of Mankind. The Emperor granted Mortarion command of the XIV Space Marine Legion, then known as the Dusk Raiders, who quickly adopted the name and dogma of Mortarion's Death Guard. However, The Emperor's slaying of his adoptive father proved to become a grudge Mortarion long held against him.
The Great Crusade
Mortarion believed that victory in battle came through sheer resilience, and Horus, who used the strengths and weaknesses of the different legions to create the most efficient fighting force possible, used his legion in co-ordination with Mortarion's frequently. Mortarion and the Death Guard would draw out the enemy and tire them down, and then the Luna Wolves would strike. This combat tactic worked brilliantly, and Mortarion grew close to Horus.
Mortarion was a grim and driven Primarch, his breathing apparatus and scythe an inseparable component of his aspect. The pallid, hairless Primarch was viewed by others as a freak, and was distant from all his brother Primarchs save Horus the Warmaster and Konrad Curze the Night Haunter, the leader of the Night Lords Legion. Some Primarchs, such as Roboute Guilliman, feared that Mortarion was more loyal to Horus than he was to the Emperor; however the Emperor claimed that loyalty to Horus was de facto loyalty to the Emperor. Events would prove the Emperor sorely mistaken.
The Horus Heresy
When Warmaster Horus turned to Chaos, he did not require much effort to drag Mortarion and his Legion down with him. Horus had been one of the few Primarchs with whom Mortarion had felt comfortable, and as such he showed more loyalty to the Warmaster during the Great Crusade than to the Emperor himself. In addition to this, First-Captain Calas Typhon, Mortarion's right-hand man, had long been a secret follower of the Ruinous Powers and eagerly manipulated the rest of the Death Guard into treading the path of damnation.
Mortarion revealed his true colours during the scouring of Istvaan III, when he willingly sent potentially Loyalist elements of the Death Guard into Horus' trap. Once the Astartes who remained loyal to the Emperor were purged, the Death Guard then fought alongside their Traitor brethren during the Drop Site Massacre on Istvaan V.
During the subsequent assault on Terra itself, the Death Guard were part of Horus's invasion force. However, en route, the entire Death Guard Fleet became trapped in the Immaterium due to the actions of Typhon. Typhon slew the fleet's Navigators (claiming their loyalty was still to the Emperor), then promised to lead the fleet himself with his psychic powers. The Destroyer Plague infected the fleet while they drifted aimlessly through the Warp, making a mockery of the Death Guard's legendary resistance to poison and contagions. The plague that came could not be resisted, something that terrified Mortarion and the Death Guard. It transformed them into bloated mutants, yet none could die, their own body being their undoing. None suffered more than Mortarion, for it was like being on the mountain top again, surrendering to the toxins, but this time without the Emperor to save him. Eventually, Mortarion could suffer no more and gave himself to Chaos. Desperate, Mortarion offered his Legion and his own soul up to the Ruinous Powes in exchange for deliverance. Father Nurgle responded and took the Legion and Mortarion as his own.
What emerged from the Warp bore little resemblance to what had gone in. The Death Guard Space Marines' once gleaming grey armour was corroded and shattered, barely containing their bloated, pustule-riddled bodies. Their weapons and armour were powered by the energies of Chaos and they became known as the Plague Marines, although they would still use the name of the Death Guard for themselves. Mortarion himself left his humanity far behind and was transformed into Nurgle's greatest mortal Champion: the Prince of Decay.
In the end, Horus was defeated by the Emperor, but unlike the other Legions, who splintered and fled into the Eye of Terror, Mortarion's Legion, now calling themselves the Plague Marines, made an orderly withdrawal, wave after wave of Loyalists breaking themselves on the Legion. Mortarion led his forces, in an ordered formation, back to the Eye of Terror. Mortarion claimed the Daemon World known as the Plague Planet as his new homeworld and it proved to be an ideal base for launching attacks from out of the Eye of Terror into the physical universe. He shaped it so well that Nurgle promoted him to become the greatest of his Daemon Princes. Mortarion had finaly gotten what he wanted, a world of his own. He ruled over a toxic death world of poison, horror and misery. For better or worse, the Death Lord had come home.