Hidden Ones
The Hidden Ones were the secret spies and agents of the Thousand Sons Space Marine Legion, during the Great Crusade and the Horus Heresy. Since they were Scouts, it is likely they weren't that psychic to begin with, so they were most likely Rubric'd during Ahriman's shenanigans.
Overview[edit | edit source]
When it comes to what they actually do, the Hidden Ones actually stays true to their name, as we barely know jackshit about them. So you could say they are really, really, memetically well hidden. The only thing we know about them was that their commander was Amon, 9th Fellowship Captain of the legion and Equerry to Magnus the Red. Amon was noted to have trained the Hidden Ones personally. But other than Amon, we don't know what weapons they use, what equipment they carry nor what their methodology was really like.
It is possible that the Hidden Ones were not Astartes Scouts in the traditional sense; that is, not new recruits employed as reconnaissance troops, but a type of infiltrator akin to the Officio Assassinorum. They may even be Daemonic in origin. Evidence for this comes not only from the Hidden Ones being referred to as the 'Scout Auxilia', but also from the revelation that Kasper Ansbach Hawser was a Hidden One of the Thousand Sons despite being completely unaware of this possibility for most of his known life. So yeah, we are in some deep CIA/KGB psy-ops/Men-in-Black shit here.
Hawser operated as a deep-cover spy planted inside the Space Wolves legion apparently by the Thousand Sons, who had psychically (and perhaps sorcerously) reprogrammed his mind so that he would go to Fenris and become attached to the Space Wolves. From this position his handlers in the Thousand Sons - believed to be Amon and even Magnus himself - could perceive his experiences and glean secret intelligence on the inner operations of the Wolves of Fenris.
HOWEVER, it should be noted that most of the information concerning Hawser's status as a Hidden One came from a power of the warp masquerading as Amon and that Hawser may have in fact just have been a puppet designed to appear as a Hidden One by the machinations of Chaos. Therefore, while the information given about the Hidden Ones appears reliable, the fact that the only one known of currently was possibly not one at all makes most of what we know about them even more uncertain.
As it was stated that Hidden Ones were more useful to the Thousand Sons when they were unaware of their status as spies, it is implied that unconscious Hidden Ones similar to Kasper Hawser existed in other institutions the Thousand Sons wanted to establish intelligence on.
Tl;dr, they give the Alpha Legion a run for their money.