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.