Anathrax is a Daemon World within the hellish realms of the Eye of Terror devoted to Nurgle, the Lord of Plague. A lush but pestilent planet, it is covered by giant bulbous fungi made up of puffy-eyed heads that cough and sneeze a grey mucous which corrupts those who touch it. The Iron Drakes Space Marine Chapter was corrupted on this world 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 on the Daemon World of Anathrax and became the Grey Death warband of Heretic Astartes, ready and willing to spread their plague across the stars.
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 in this way.
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 Imperial Crusade. After scant solar 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.
On the pestilent Daemon World of Anathrax, the Iron Drakes Space Marine Chapter made planetfall in force, smashing through bulbous fungi forests of giant puffy-eyed heads that coughed and sneezed grey mucous onto the Space Marines deploying from their Drop Pods. The Iron Drakes fought bravely, 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 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 ready to surrender, pledging their souls to 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.
Chapter after Chapter fell to the perils of the Eye, for the Abyssal Crusade had entered a hell from which very few emerge unchanged. The true account of what occurred within the Eye to another of The Judged Chapters, known as the Argent Hammers, is unknown, but most of the tales of the Chapters of The Judged ended in tragedy and sorrow. What is known is that at some point, the Argent Hammers also made planetfall upon the Daemon World of Anathrax, the same Plague Planet which had already corrupted one Chapter of The Judged, the Iron Drakes, transforming them into the warband known as the Grey Death.
By the time they reemerged from the Eye many standard centuries later, the Argent Hammers were no more, for they had become the Chaos Space Marine warband known as the Talons of Anathrax, wholly dedicated to the service of the Ruinous Powers.
- Codex: Chaos Space Marines (6th Edition), pg. 19