Thousand Sons
The Thousand Sons is a Chaos Spehss mahreen legion, they currently devote themselves fully to Tzeentch. Their Primarch is Magnus, an extremely powerful Psyker who's red skinned(literally) and has a cyclopean eye. While we don't exactly know what he does with this 3rd eye of his, he did use it to peer into the warp which turned him insane and used it to stabilize his mutating legion during the first founding. Their army is also 2/3s dust, no rly it is.
Pre-heresy
During the pre-heresy era where the Great Crusade was underway, the Thousand Sons was the first legion to exhibit psychic powers and they extensively used Psyker and Sorcerer powers at the same time, though because of the Imperium's never ending fear of psykers, they were usually distrusted by most of the Imperium. Nevertheless, they were tolerated because they were totally awesome and exceptionally bad ass.(Super human magicians for the win!!)
Heresy
During the Horus Heresy, Magnus used his B& powers of sorcery to warn the Empra about Horus' treachery, although instead of listening to Magnus, the Emperor instead questioned how he got the information. As Magnus got busted, he originally complied to accept any punishment the Empra could give him and then the Emperor sent the Space Wolves to arrest him. Horus, then sent a fake message to Leman Russ to destroy the Thousand Sons instead. Because of this, Magnus fell to the powers of Chaos on the counts of revenge and TTTHHHEEE AAABBBSSSOOOLLLUUUTTTEEE PPPOOOWWWEEERRR!!!!!! After these events Tzeentch walked up and sat on his throne in the Hidden Library, at which point he conjured a martini glass with his alcohol of choice and sipped from it. He then set it down on the arm rest, held his hands together, chuckled and muttered "Just as Planned".
Rubic Marines
As mentioned previously, the Thousand Sons' army is 2/3s dust, this was primarily because of Ahzek Ahriman's Rubric of Ahriman which damned everyone without Psychic powers to be sealed permanently into their armor unless the armor itself is broken. Originally this was suppose to stop the mutations they were experiencing as they defected to Chaos. Rubic Marines are pants-on-head retarded as without a sorcerer to guide them, they won't do anything else but stand however when controlled while in combat, they're like their former selves except much more deadly. Because of this: Most of the marines in the sons are now soulless automatons who fully obey whatever their master tells them to do, they are also totally fucking fearless. In short, they're a hybrid of the Necrons and 'nids