Mortarion - Doom of the Death Guard: During the subsequent assault on Terra itself at the end of the Heresy by the Traitor Legions, the Death Guard were part of Horus' invasion force. However, en route, the entire Death Guard Legion fleet became trapped in the Immaterium due to the actions of First Captain Calas Typhon, trapping them in a perpetual nightmare.It was whilst on campaign with the Word Bearers Legion earlier in the Great Crusade that Typhon had learned of a different path for the Legiones Astartes to follow, a future where his hard-won but hidden psychic abilities would be a source of greatness instead of a taboo to be hidden from sight.The foremost Chaplain of the Word Bearers, Erebus, inducted Typhon into the secrets of the Seven Pillared Lodge, one of the Warrior Lodges that had begun to spread throughout the Space Marine Legions in the later days of the Great Crusade. It was during this time that Typhon caught a glimpse of what the Astartes could truly become if they shrugged off the yoke of the Emperor's ambitions.Perhaps Typhon's revelation was instrumental in Mortarion's own fall to the Ruinous Powers; perhaps Mortarion would have walked a dark path on his own. Either way, the troubled Death Guard Primarch saw a worthy leader in Horus, whereas in the Emperor he saw only a self-serving and pompous pretender who had stolen Mortarion's hard-won kingship in a single day while engaging in hypocrisy concerning his own use of the Warp. As the Heresy reached its conclusion, Mortarion ordered his fleet to head for Terra with all haste, intending for the Death Guard to join the other Traitor Legions in the destruction of the False Emperor.By this point, Calas Typhon served one master alone, and it was not his Primarch. Typhon had seen to it that the fleet's Navigators were killed to a man (claiming their loyalty was still to the Emperor), but reassured Mortarion that the Warp-gift he possessed would see them through their journey in the Empyrean safe enough. Though he hated the concept of relying on what he considered witchery, even in one of his own Astartes, Mortarion was left with little choice. The Death Guard fleet made transition into the Warp, and in the process damned themselves to an eternity of war as the puppets of a foul and ancient god.In leading the Death Guard into the Warp, Typhon had delivered them into the clutches of his new master, Nurgle, the Lord of Decay. The strange tides of the Empyrean are notoriously fickle, and during their voyage the entire Death Guard fleet was becalmed within its shifting dimensional tides. As their warships lingered, directionless and without hope, the cloying influence of Grandfather Nurgle began to take hold. The Death Guard were subjected to the terrible infection of the Destroyer Plague and Nurgle's Rot, as Nurgle's power managed to infiltrate the vessels of the XIV Legion.It polluted the vessels themselves as easily as it did the transhuman warriors within. The virulent plagues infected the fleet while they drifted aimlessly through the Warp, making a mockery of the Death Guard's legendary resistance to toxins 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 on Barbarus, surrendering to the toxins, but this time without the Emperor to save him. Eventually, Mortarion could suffer no more and gave himself over to Chaos to stop the pain. Desperate, Mortarion offered his Legion and his own soul up to the Ruinous Powers in exchange for deliverance.Nurgle responded gleefully and took the XIV Legion and Mortarion for his own champions. As Mortarion at last swore his soul to the eternal service of the Plague God in return for an end to the ceaseless agony, Nurgle responded with his usual generosity. Mortarion was transformed into a Daemon Prince of the Plaguefather, the living embodiment of life in death, and death in life. The Death Lord at last was worthy of his title.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, the very image of death. Condemned to a deathless state of decay, the Death Guard would spread their pestilent diseases the length and breadth of the galaxy for the greater glory of Chaos.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 later 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. Mortarion had finally 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.