The Brotherhood of Lethe are a warband of Chaos Space Marines that were once a Loyalist Space Marine Chapter known as the Crusaders of Dorn. Though their origin and Founding is unknown, their Chapter name suggests lineage from the Imperial Fists. The Crusaders of Dorn were corrupted after entering the Eye of Terror in 321.M37 as part of the Imperial Abyssal Crusade. It is unknown what they endured during their sojourn into the Eye, but by the time they made their way back into realspace, they had become the warband of Chaos Space Marines known as the Brotherhood of Lethe.
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 Crusaders of Dorn were one such Loyalist Chapter of Astartes that had been affected by these Warp Storms.
When the Ecclesiarchy heard of this sinister turn 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.
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 the Crusaders of Dorn is unknown, but most of the tales of the Chapters of The Judged ended in tragedy and sorrow. By the time they reemerged from the Eye many standard centuries later, the Crusaders of Dorn were no more, for they had become the Chaos Space Marine warband known as the Brotherhood of Lethe, wholly dedicated to the service of the Ruinous Powers.
The Brotherhood of Lethe warband's colours are not listed in current Imperial records.
The Brotherhood of Lethe warband's badge is not listed in current Imperial records.
- Codex: Chaos Space Marines (6th Edition), pg. 19