Black Crusade (2011), p27 — The Death Guard

The Death Guard followed Horus into heresy, their sense of loyalty to their Warmaster and their Primarch, Mortarion triumphing over their duty to the Emperor on distant Terra. The rebel Death Guard Legion was marooned in the warp during the long journey to Earth to join the attack on the Imperial Palace. A mysterious, unstoppable contagion spread through the trapped fleet, putrefying all it touched. Mortarion himself became infected and in his delirium he called upon the Powers of Chaos to aid his Space Marines. Mortarion's fevered ravings were answered by Nurgle, and Mortarion became Nurgle's Champion.

The Death Guard survived but they continue to bear the marks of Nurgle's first blessings upon them. Their once-white armour became stained and cracked where the bubbling foulness of their mortal bodies has erupted to the surface. They bear the three-lobed mark of Nurgle rendered as flies or rotting heads upon banners and shoulder guards. Their bolt guns and chainswords are caked with filth and rust but are no less deadly. Plague and contagion have become the Death Guard's primary weapons and they can be found anywhere in the galaxy spreading Nurgle's blessings.