The Grey Death are a Renegade Chapter of Chaos Space Marines that are sworn to the service of Nurgle. They were once a Loyalist Chapter of Space Marines known as the Iron Drakes. They were corrupted after entering the Eye of Terror in 321.M37 as part of the Imperial Abyssal Crusade.
At some point during their sojourn in the Eye, the Iron Drakes were corrupted by Nurgle after being marooned on the Daemon World of Anathrax by the Death Guard and became the Grey Death, ready and willing to spread their plague across the stars.
The Grey Death were a formerly Loyalist Chapter known as the Iron Drakes. Following the Ecclesiarchal Purges of 321.M37, a dozen star systems were engulfed by Warp Storm Dionys, its echoes rippling along the spiral arms of the galaxy as it raged through the Empyrean. Records of mutation and Chaos Cultist activity quadrupled overnight. Worse yet, it was not only the citizens who were affected by the sudden influx of Chaos. Many of the Space Marine Chapters with homeworlds affected by the Warp Storm found that the secret imperfections in their gene-seed were writ large upon their new recruits, giving rise to a wave of disturbing manifestations both physical and psychological. The Iron Drakes were one such Loyalist Chapter of Astartes that had been affected by these Warp Storms.
When the Ecclesiarchy heard of this sinister tum of events, Saint Basillius the Elder demanded that all those Chapters of the Adeptus Astartes whose homeworlds had been touched by the Warp Storm be rendered unto his judgement. Such was the elder's influence with the High Lords of Terra that within a standard year this had come to pass. After a series of stringent tests and prognostications, hundreds of Chapters were deemed unaffected by the Warp Storm. No less than thirty were found wanting. The Judged, as these fallen Chapters came to be known, volunteered for a redemptive Penitent Crusade. The most militant of their number demanded the right to purify their tainted flesh in the fires of battle, to make a noble end from tragic misfortune. To the surprise of his closest advisors, Saint Basillius agreed to this proposal. He saw it fitting to send the accused into the Eye of Terror, taking the fight for the Imperium's future to the Daemon Worlds inhabited by the Traitor Marines.
A representative from each of the doomed Chapters held an emergency Council of Dismay to discuss the proposed Crusade. After scant hours of debate, they acquiesced to Basillius' demands, for they believed that martyrdom was preferable to an existence of suspicion and doubt. The last few days of 321.M37 saw a solemn procession of Strike Cruisers and Battle Barges pass through the Cadian Gate into the Eye of Terror, relay systems dormant and heraldic colours obscured by black mag-plates. One by one, the Chapters of The Judged disappeared into the iridescent dust nebulae that surrounded the Eye. As the massive Space Marine flotilla entered the Eye of Terror, they were set upon by a massive Chaos warfleet. The resultant battle was so fierce that the ships of The Judged were forced to retreat and were scattered to the furthest corners of the Eye.
After the Death Guard crippled their fleet, the Iron Drakes Chapter was marooned on the pestilent Daemon World of Anathrax. The Iron Drakes had made planetfall in force, smashing through bulbous fungus forests of giant, puffy-eyed humanoid heads that coughed and sneezed grey mucous onto the Space Marines deploying from their Drop Pods. For over a standard century, the Plague Marines present on Anathrax harried the stranded Loyalists, always aiming to debilitate their targets and damage their wargear rather than slay them. The Iron Drakes fought bravely throughout this time, but one by one they were brought low. Though their respirators filtered the air of disease, many of the Space Marines found their Power Armour often cracked and laid open by the torrent of Bolter fire levelled upon them by hidden Plague Marines. By the time a full half of the Chapter had bulged screaming out of their armour to become fungus-trees themselves, the rest were finally ready to surrender and end the century of torment, pledging their souls to the service of Nurgle. Thus did the Iron Drakes become the Grey Death, ready to serve the will of the Plague Lord and spread his pestilence across the galaxy. By the time the Iron Drakes were sighted once more in the Imperium, they fought under the name of the Grey Death, and their new loyalties were plain to all.
The Grey Death's Chapter Colours is pale sage battle-plate with black trim. The usual Chaos fetishes and accouterments dedicated to their patron Chaos God are also prominently displayed on their armour.
The Grey Death's warband badge is a white skull displayed on a three-pointed Chaos octed, centred on a field of pale sage.
- Codex Heretic Astartes - Chaos Space Marines (8th Edition), pg. 53
- Codex: Chaos Space Marines (6th Edition), pg. 18