The Hidden Ones were a specialist unit of the Thousand Sons Space Marine Legion that served as the Legion's Scout Auxilia. There is very little in the way of information in Imperial records of how this mysterious unit was deployed or what kind of operations it undertook. Imperial historical accounts show that there is the distinct possibility that these specialised reconnaissance troops served a far more sinister role for the XV Legion that was more akin to that of traditional espionage agents than a unit of typical Scout Marines composed of Astartes Neophytes engaged in reconaissance-in-force and combat infiltration missions, though the Hidden Ones also carried out those types of missions as well. An operative of the Hidden Ones would infiltrate a target organisation of interest to gather intelligence while operating under a cover so deep that they never even realised that they had been psychically-reprogrammed to act as a sleeper agent for the Thousand Sons. The Hidden Ones' status following the Thousand Sons Legion's transformation into a Traitor Legion devoted to Tzeentch and the enactment of the Rubric of Ahriman is unknown.
- A Thousand Sons (Novel) by Graham McNeill
- Prospero Burns (Novel) by Dan Abnett