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. Roughly 66% of their army, at one point, was dust.
No, seriously. Incredibly pissed-off dust.
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, used their abilities to wondrous effect in service of the Imperium, and in general were exceptionally badass (Superhuman warrior-monk 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, and that his favored son, if not stopped, was about to fuck everything up and destroy everything the Emperor had sought to build. Unfortunately Magnus' contacting the Emperor in this fashion could not have occurred at a worse time: It caused the Webway portal and the Golden Throne which the Emperor was building to become damaged beyond repair.
Only slightly bummed that he had screwed-up the Emperor's master-plan, Magnus headed back to Prospero in order to prepare to strike against Horus before Horus had a chance to destroy the Imperium of Man. The Emperor, meanwhile, sent the Space Wolves to arrest him. Horus, who was digging Chaos already, pulled a move worthy of Eldrad Ulthran in terms of dickery, and sent a fake message to Leman Russ - with orders to destroy the Thousand Sons instead.
With no warning, the loyalist Space Wolves opened fire on Prospero, Magnus' homeworld, and caused immeasurable damage to some of the largest caches of knowledge in the fucking Imperium. Scribes and the Thousand Sons struggled to recover what they could, but suffered enormous casualties in the assault and though they forced the Space Wolves back, the Legion had suffered immensely for it. Magnus swore vengeance against Russ and the other Marines who had stupidly ignored his warnings, his hubris eventually leading him to fall to Chaos himself. 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 was at one point 66% dust.
This was primarily because of Ahzek Ahriman's FML spell: The Rubric of Ahriman.
This rite 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; Ongoing mutation paired with the earlier slaughter of the Thousand Sons by the Space Wolves meant that they were slowly dying out, and Magnus and the Thousand Sons actively sought a way to stop it. Figuring out his own method from Magnus' spellbooks, Ahriman crafted a ritual that caused any Marines with mutations to either survive with their mutations cured and psychic powers augmented - or turned to dust if they weren't a Sorcerer. Suffice to say Magnus was fucking pissed and kicked Ahriman's ass out of the eye of terror. The spirit-armor Marines became known as Rubric Marines.
Rubric Marines are automatons - without a strong guiding presence, they lapse into a passive state - they are immortal and thus make the perfect guardians for the libraries and bases of the Thousand Sons. When guided by a Sorcerer or placed in the front lines against a foe, their old battle-hunger returns and they move with more of a sense of purpose. They obey orders without hesitation, and know no fear. Making them far more fearsome, their Sorcerers equip them with modified bolter rounds that are enchanted to explode into bursts of white-hot warp-fire on impact, burning through armor with incredible speed. Additionally, since they're functionally animate suits of armor, they possess phenomenally strong supernatural protection due to their construction and are a bitch to kill as a result. They are, however, incredibly slow.
Whilst the current generation of Rubric Marines date back to the Thousand Sons' expulsion of Ahriman, it's believed they have the ability to make more by using altered and notably scaled-down versions of the rite that resulted in the Rubric. Even with this, however, the creation of them is lengthy and time-consuming, ergo guaranteeing that their enemies will slowly whittle down their numbers by drowning them in corpses.
Tabletop-wise Rubric marines are of debatable use. Yes they're fearless, are fairly good at being shooty especially if their bolters are enchanted, has a +4 invul save, so basically, they rape any kind of infantry that doesn't have the toughness of a Terminator. But on the downside, they're slow like termies and aren't necessarily as tough as them, they're nearly useless if the sorcerer leading them dies, they don't wow anyone in CQC, and overall they're hard to use for beginners. Overall, sort of a bridge between the shootiness Slaanesh Noise Marines, toughness of Nurgle Plague Marines, and the fearlessness of Khornate Berzerkers. Throw them in a Rhino and let the games begin.
The Thousand Sons have a distinctive battle cry "All is Dust."
Connections with the Blood Ravens?
It's never proven to be absolutely canon, but in the "A Thousand Sons" novel from the Horus Heresy book series, there are some hints that the Blood Ravens stemmed from a Thousand Sons cult who remained loyal to the Emperor which was sorta planned by Magnus. Specifically, the most painfully obvious of the hints is this particular line from the novel:
And note that the Blood Ravens have no idea which legion or chapter they originally came from and the fact that they produce an unusually high number of Psykers than your average marine chapter and the Thousand Sons were known to have a large number of Psykers in their legion, as well as having a fetish for various flavors of knowledge.