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!!). After much debate however, the use of sorcery was banned throughout the Imperium but Psykers could still be trained.
Heresy
Before the actual start of the Horus Heresy, Magnus used his B& powers of sorcery to warn the Empra about Horus' impending treachery. This caused the Webway portal and the Golden Throne which the Emperor was building to become damaged beyond repair. Magnus got all emmo when he realized that he totally screwed up the Emperor's master plan and then the Emperor sent the Space Wolves to arrest him. Horus, who was digging Chaos already, then sent a fake message to Leman Russ to destroy the Thousand Sons instead. The Space Wolves cause massive damage to the Sons' legion. 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".
Rubric Marines
As mentioned previously, the Thousand Sons' army is 2/3s dust, this was primarily because of Ahzek Ahriman's spell: The Rubric of Ahriman which damned everyone in the legion without Psychic powers to be turned into ash and their soul to be sealed for all eternity into their permanently sealed armor unless the armor itself is broken. Originally this was suppose to stop the mutations they were experiencing as they defected to Chaos. Rubric 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 kind of a hybrid of the Necrons and 'nids. Rubric Marines are soulless automatons with bolters that fire rounds that have been enchanted by their Sorcerer masters to burn through anything but fucking Terminator armor. Also, they are all protected by the powers of the Warp, so on top of that, they're a bitch to kill. It is worth noting that they are nothing like 'nids. It is also worth noting that they're being compared fluff-wise. OFC they aren't nids, they're not even organic anymore. It doesn't seem like the Thousand Sons can make anymore Rubric Marines though, so they're going to slowly whittle away as the Imperium employs it's ever successful tactic of drowning them in corpses.
