Masters of Forbidden Knowledge
The Thousand Sons Space Marine Legion
The Rubric of Ahriman
The Thousand Sons had nearly been destroyed by the threat of uncontrolled mutation in their earliest days before their reunion with their Primarch. Even the salvation presented by Magnus's instruction was imperfect, requiring constant and vigilant oversight. The terror of it never left some of the Legion's most veteran members, and the rampant corruption they beheld amongst other Traitor Legions as the Heresy ran its course appalled them. They dedicated themselves exclusively to their new master, and for a time Tzeentch seemed to shield them from a similar fate. Even when the Heresy ultimately failed, and the Thousand Sons were forced to fall back to the Eye of Terror with their comrades in rebellion, Tzeentch's favor seemed unilateral. Their patron god provided a new planet, rich in magical power for them, a haven from the madness within the Eye for them to continue their research. But the way of the Master of Sorcery is capricious, and no sooner were the Thousand Sons ensconced upon their new home world than the Changer of Ways began to alter them. Grotesque mutations in images favored by Tzeentch appeared spontaneously throughout the Legion. Many embraced these manifestations as tokens of their new destiny, but to the senior members of the Legion it was as if all they had been through counted for nothing. All their sacrifices; the loss of Prospero, the bloodshed of the Heresy, all was rendered meaningless. Their valiant pursuit of knowledge had resulted in the very madness and abomination they had always feared.
An inner cabal of the mighiest sorcerers, led by their Chief Librarian and Magnus's most trusted advisor, Ahriman, determined to counter the warping corruption. They laid te foundations of the mighty spell, and protected their workings with wards of secrecy, for they doubted Magnus would bless so risky an enterprise. They would dispel the violent mutations washing over their battle brothers and render the Thousand Sons immune to the warping effects of Chaos. The Grimoire Hereticus describes a spell of such unimaginable power that even daemonic horrors fled before the singular roaring maelstrom of magic unleashed by Ahriman and his cabal. The Planet of Sorcerers was enveloped in impenetrable storms of blue and yellow lightning, forks of the titanic energy arcing across the planet to strike down corrupted Thousand Sons one after another until it is said Magnus himself was forced to intervene.
The aftermath was nothing like what the cabal had hoped for. Across the breadth of the world, the fighting strength of the Thousand Sons Legion had in a single stroke been destroyed utterly - and preserved for eternity. The reviled mutations were no more, because the flesh of the affeced Space Marines had been reduced to dust, sealed forever inside armor mystically welded shut. Every clasp, joint and seam had been sealed through by infernal fire, trapping the animate spirit of the Thousand Sons irrecoverably within. Virtually the entirety of Magnus's Legion had been transformed into little more than implacable automata for all time. Magnus was enraged. The Legion he sacrificed everything for was no more. The pursuit of knowledge that had always been foremost to each of his brethren was now denied them for all time. By their own hands, the majority of this Legion of scholars could no longer even think.
Everything he had done, all he had sacrificed, every critical decision he had made in his life had been founded upon two sacred beliefs: that knowledge was pure, and that he was its master. With his home world destroyed, his father his sworn enemy, and his Legion in ruin, Magnus of the Thousand Sons ascended his tower in despair. Casting his bitter gaze out upon the war-weary and fragile Imperium of Man, Magnus vowed, as Horus had at the height of the Heresy, that he would see the galazy burn.
Home World
Prospero was chosen by its original settlers for one reason: its remoteness. Isolated from the most common Imperial travel lanes and boasting virtually no independent resources of any value, Prospero had only one redeeming quality: it was a good place to hide. In the end, it was not even that. Today it is a blasted ruin, declared Purgatus by the Inquisition.
Through the millennia of endless raiding since, it has been discerned the City of Light survived its transit through the Immaterium intact. It came to rest within the Eye of Terror, upon a world that has come to be known as the Planet of Sorcerers. That daemonic place is a seething cauldron of magical power, reflected across its breadth in the form of infernal volcanism and tempestuous skies saturated with magical vapor. Towers jut from craggy fists of rock thrust up from the plains of lava, twisted and obscene mockeries of the spires and pyramids of learning which were the hallmarks of the City of Light before its fall. Mightiest of them all is the obsidian monolith that is the Tower of the Cyclops, said to be some massive it can be discerned from space with the naked eye. More obscenely, the Tower of the Cyclops looks back, as the pinnacle of the abelisk is a glowing warp eye, through which Magnus watches the paths of the future. The Silver Towers of the City of Light have been transfigured by the sorcerous might of the Thousand Sons into space-faring fortresses in which psyker lords set out from the Planet of the Sorcerers to traverse the cosmos, launching vengeful assaults upon the Imperium of Man.
Combat Doctrine
The Thousand Sons were well known for preferring to avoid close combat, instead relying upon their mastery of psychic power and sorcery to carry the day. Guile, fient, confusion, and misdirection were their hallmarks; all stratagems better used at range. Many were the occassions a Thousand Son detachment would accomplish through illusion of trickery what a brother Legion would pay for dearly in blood.
Whatever else it may have changed, the Rubric of Ahriman affected that doctrine very little. The sorcerer lords of the Thousand Sons still use their ghost-brethren as implacable bulwarks of gunfire, around which they construct elaborate plans of guile and misdirection to achieve victory, all driven home with a timely application of potent magic.
Organization
Magnus placed great faith in his subordinates, believing he had taught them well, that their powerful sorceries gave them the necessary tools to function independently of him. Before the Heresy, individual Thousand Sons squads were not led by Veteran sergeants but by those who showed the most psychic promise. These 'thrall-wizards' were apprenticed to more experienced sorcerers for their cabalistic training, but at the same time gained experience leading men in small units. While this practice meant it was rare for a Thousand Son who did not possess some measure og psychic talent to become a ranking officer, it also meant those sorcerers who did gain prominence had considerable combat experience. As a result, the Thousand Sons, a numerically small Legion to begin with, rarely took to the field en masse. Instead, they campaigned in smaller detachments under the command of sorcerers who often acted with much more authority independent of their Primarch than the officers of other Legions. This command experience had stood them in good stead as independent leaders of warbands since the Heresy, to the Imperium's considerable and continuing misfortune.
Beliefs
For the Primarch Magnus, knowledge was power. He believed there was no discipline his intellect could not master, no secret he could not unlock and make serve his purpose. For the Thousand Sons, knowledge was salvation, the means to controlling the psychic legacy of their Primarch's gene. Every book was sacred, every writing worthy of study, every document a resource to be drained. The ultimate knowledge was sorcery, the way to final enlightenment, the key to the universe. Before the Heresy, the Thousand Sons were publicly dogmatic, swearing oaths of loyalty and singing the Imperial hymns. They fought for the expansion of the Emperor's realm with diligence, but as their oath-breaking illustrated, their final loyalty rested not with the Emperor, but with their Primarch. When Magnus's reach for intellectual mastery exceeded his grasp, Tzeentch was waiting for him, and the Space Marines who believed as he did could do nothing but fall with him.
Gene-seed
Magnus was unquestionably the most profoundly mutated of the Emperor's Primarchs, both physically and psychically, and the Legion imprinted with his gene-seed reflected that with a high percentage of Thousand Sons manifesting some level of psychic ability. Early in the Legion's history a small, but significant percentage were prone to physical mutation, but in the wake of falling thrall to Tzeentch that percentage escalated wildly. The Rubric ended that forever for the battle brothers of the Thousand Sons, but the sorcerers who command those armored shells still carry the gene-seed of their Daemon Prince and wear their grotesque mutations proudly as tokens of their mercurial patron's favor.
Battlecry
A ghostly whisper of: "All is Dust!". Taken from White Dwarf 266US