What emerged from the Warp when the Death Guard fleet broke out bore little resemblance to what had entered. 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. Mortarion's fevered ravings were answered by Nurgle, and he became Nurgle's champion. Eventually even Mortarion became infected, and in his delirium he called upon the powers of Chaos to aid the Space Marines. 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. It roiled in their guts, warping their once superhuman bodies, transforming them into horrible, pustulent grotesques. 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. 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. For the Space Marines of the Death Guard there was nothing so terrifying as the Destroyer plague which made their legendary resilience meaningless. The stinking pestilence bloated the gut, distended the flesh, and turned its victims rotten from the inside. Meanwhile a mysterious contagion began to spread from ship to ship. Days passed while the fleet's Navigators searched for a Warp-tide that would bring them back to the material universe. 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. 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. What Mortarion and his adepts could not devise means to compensate for, the Death Guard overcame through sheer resilience.

Thanks to their superhuman ability to resist poison and disease, no toxic smog or corrupted atmosphere deterred their course. There was no environment which Mortarion, their Primarch, or the Death Guard feared. 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.

Traitor legions codex