Informally known as 'Shadow Masters', these were a small cadre of infiltration squads within the Legion already well known for their skill in such tactics. Each is a veteran of the Lycaen Uprising who fought at the side of Corax himself. When the Emperor arrived on Deliverance he judged many of the rebels who had fought alongside his son worthy of transformation into a Space Marine and helped remind Corax of his roots as a revolutionary and freedom fighter.
At the beginning of the Horus Heresy, the Mor Deythan were few in number after eight decades of constant warfare and rarely admit new recruits into their ranks. Most who remained were grizzled veterans who were masters of moving silently through the shadows and attacking at the best opportunity. They utilized a range of specialized equipment. However rumor had it that they possessed the ability of their Primarch's ability of nigh-invisibility.