The Shadow Wardens were an elite cadre of veteran Astartes who served as the Honour Guard for the Primarch Corvus Corax of the Raven Guard Legion during the Great Crusade and Horus Heresy eras in the late 30th and early 31st Millennia.
They had served their lord since he was the leader of the slave uprising on Lycaeus. Following their Primarch's disappearance, it is not known if this elite cadre was dissolved as an active formation within the newly formed Raven Guard Chapter or if it still exists.
The origins of the Shadow Wardens can be traced back to the time of the slave uprising on the prison moon of Lycaeus. Before he was reunited with the XIXth Legion, Corax served as the rebels' leader during the slave uprising. Corax's peers insisted that he be watched over by a bodyguard lest the overlords or some other rogue element attempt to assassinate him.
While he resented this concern, Corax allowed such a force to be employed, as much for his followers' peace of mind as for his own safety. After he joined his Space Marine Legion, the bodyguard followed, formed into an honour guard known informally as the "Shadow Wardens." It was said that they were ever present, though often unseen when their Primarch went to war, and that none could avoid their gaze. Indeed, only one individual was ever able to do so, and that was Corax himself, giving rise to the wry saying that Corax was able to slip his own shadow should he choose to do so.
- The Horus Heresy - Book Three: Extermination (Forge World Series) by Alan Bligh, pg. 138