Codex Supplement: Traitor Legions (2016), p20 — The Death Guard

Even before the Horus Heresy, the Death Guard did not manoeuvre fancifully, or concern themselves with confusing their opponents; they picked the best ground upon which to fight, then smashed their foes after they had broken themselves against the Death Guard line. There was no environment which Mortarion, their Primarch, or the Death Guard feared. Thanks to their superhuman ability to resist poison and disease, no toxic smog or corrupted atmosphere deterred their course. What Mortarion and his adepts could not devise means to compensate for, the Death Guard overcame through sheer resilience. The Warmaster Horus above all others recognised the value of the Death Guard; he would often place Mortarion and his Legion in the centre of his battle line, counting on the enemy's inability to oust them so that he could either lever his advance from the rock of Mortarion's immovable position, or use it as the anvil upon which his hammerblow assault would break the foe.

When Horus led his attack on the Emperor, the rebel Death Guard Legion became marooned in the Warp while attempting the long journey to Terra. Days passed while the fleet's Navigators searched for a Warp-tide that would bring them back to the material universe. Meanwhile a mysterious contagion began to spread from ship to ship.

The stinking pestilence bloated the gut, distended the flesh, and turned its victims rotten from the inside. For the Space Marines of the Death Guard there was nothing so terrifying as the Destroyer plague which made their legendary resilience meaningless. These were the warriors who the Imperium had sent to conquer worlds no other man could set foot upon, much less fight on and win. There had been no pestilence, contagion, toxin or pollution potent enough to overcome Mortarion and the Death Guard, until the plague raced through their fleet. It roiled in their guts, warping their once superhuman bodies, transforming them into horrible, pustulent grotesques. They were made corrupt within and sickening to behold without and they grew sicker and sicker, yet could not die, their own constitution becoming their worst enemy. Eventually even Mortarion became infected, and in his delirium he called upon the powers of Chaos to aid the Space Marines. Mortarion's fevered ravings were answered by Nurgle, and he became Nurgle's champion.

The Death Guard, newly dedicated to Nurgle, arrived in time to take part in Horus' attack on Terra and fought in all of the major battles of the campaign. What emerged from the Warp when the Death Guard fleet broke out bore little resemblance to what had entered. The gleaming white and grey armour of Imperial champions was no more, burst and shattered from the horrific bloating of infected bodies, scabbed with boils, putrescence and the filth of corruption. Their weapons and machinery of war were now powered by the sickly sorcery of Chaos, glowing with lambent green luminescence and oozing gangrenous pus. The name Death Guard itself would pass into secondary use, as the walking pestilence-carriers became a terrifying sight across the Imperium. To their victims, to their erstwhile allies, even to themselves, they had become the Plague Marines.

The beliefs of the Death Guard echoed those of their Primarch, and a resolute determination that individuals should be free of oppression and terror became a conviction that individuals were not suited to decide what was just for them. A faith in inner strength, iron will and unshakeable resolution in the face of hardship led to pride, arrogance and an utter contempt for those they deemed inferior. When Nurgle's blessings came to the stranded Death Guard, their pride and arrogance were revealed, and their contempt for weakness turned upon themselves. Their surrender to Nurgle left them with only one seething, burning outlet, stoked white-hot by the depth of their self-loathing: to infect the strong, slay the weak and rot the foundations of everything in their paths until it collapses. Their debasement would no longer seem so shameful, if the pestilence of their Unclean Lord eventually brought everything to ruin.

After Horus' defeat, Mortarion led his Death Guard in a merry dance of destruction over a score of planets, until finally retreating with his Legion into the Eye of Terror - but not in disarray, as many of the other Legions did. Even in damnation, the resilience of the Death Guard remained, and under the direction of their Master they withdrew into the Eye intact, loyalist Space Marines and Imperial Army regiments breaking upon them time and again. There, Mortarion received Nurgle's ultimate reward, and became a full-fledged Daemon Prince, ruling over one of Nurgle's greatest Plague Worlds in the Eye of Terror.

With Mortarion elevated to daemonhood, his hand upon the Legion became more remote and the Death Guard gradually became broken up over space and time into smaller units. Warriors of the Death Guard are most often seen afoot, or at best accompanied by mad, plague-infested Helbrutes. Few of the tanks and transports of the Legion still function, their upkeep and maintenance being no priority to Space Marines dedicated to the Incarnation of Rot and Decay. The Daemon Prince Mortarion remains master of the Death Guard even after their fall, however dispersed they become, orchestrating their movements unseen from his bubonic throne.

Even after the end of the Horus Heresy, the primacy of the foot soldier remained ever the trademark of the Death Guard. Their grim Primarch Mortarion prefers to utilise huge waves of infantry, well equipped and highly trained on an individual level. He demanded that they be able to function and fight in almost any kind of atmosphere, and gave little emphasis on specialised units such as those using jump packs or bikes. The Death Guard were particularly renowned for their success at such high-risk missions as space hulk clearance and the Plague Marines continue that success, using hulks to spread disease, infection and the cult of Nurgle throughout the body of the Imperium. The combat doctrine which served the Death Guard so well in life now suits the damned character of the Plague Marines to perfection.