Crimson King, Primarch of Thousand Sons
The Thousand Sons were born of Magnus, the changeling Primarch. A physical giant possessed of coppery skin, fiery red hair and a single, baleful eye. Some say his massive size reflected his enormous intellect; his cyclopean eye his single-minded strength of purpose. At the height of the Great Crusade, Magnus was amongst the most imposing of the Primarchs and was always the one most suspected of taint. He would endure the whispers of Chaos throughout his life...
When the Primarchs were mysteriously scattered from their incubation on Terra, the infant Magnus fell upon the remote colony world of Prospero. He could hardly have been more fortunate: a grotesque cyclopean mutant who might have been feared and shunned on any other world came instead upon a hidden planet of kindred spirits: a commune of outcast human psykers. It would not be the last time Magnus's destiny would be so conveniently manipulated.
The original settlers of Prospero had chosen the world for its remoteness from Terra, and had gone to great lengths to sever contact with Humanity. Their single citadel was situated deep in the planet's ventral mountain range. Nourished by vast underground hydroponics for sustenance and techno-psychic collector arrays for sustainable energy, it was a construct of extraordinary beauty. The so-called 'City of Light' glittered amidst the desolation of Prospero, all gleaming silver towers, soaring obelisks and majestic pyramids. Within this carefully-artificed reclusium, far from the sight of Man, its commune of human refugees devoted themselves completely to the pursuit of knowledge and the mastery of the nascent mutation which had set them apart: their developing psychic powers.
Magnus became a ward to the community. He quickly exhibited the psychic traits common to the commune, and by the time he reached physical maturity, his power had grown so much that he surpassed every single person on the planet, possibly the greatest that ever existed other than the Emperor. He believed his intellect could master any discipline and his maxim, above all others, was 'Knowledge is power'. One day, Magnus used his single eye to look into the Immaterium, and changed the course of his life forever.
The Emperor of Man was attracted to the world of Prospero by the psychic beacon that was one of his lost sons. The actual meeting of the two was said to be a mere formality; the bond between father and son was believed to have been established via psychic communication.
Magnus was granted command of the 15th Space Marine Legion, the naming of the Legion as the Thousand Sons was a recognition that only a thousand original Marines were created from Magnus' genetic material. It was a reunion that could not have come at a better time for the Legion. Formed from Magnus' gene-seed, the Legion contained a high number of Marines who had undergone psychic mutation. Very few survived the mutation, of the few who did, many became the greatest psykers of the era. At this time, factions within the Imperium were calling for the extermination of all mutants and psykers.
Taking on the challenge of saving his Legion from destruction, Magnus bent his intellect towards training the Thousand Sons Legion in the way of the psyker. It is during this time that many believe that the Legion may have began to fall to Chaos, it is speculated by some that Magnus may have taken shortcuts in order to have his Legion battle-ready as soon as possible. It is not precisely known when the Legion crossed the threshold, but at some stage during the Great Crusade, the Thousand Sons passed the point of scholarly pursuit and began to dabble in sorcery.
Eventually, it came to the point where the Emperor himself held a council on the planet Nikaea, bidding that both sides present their view. Both sides argued each other to a standstill, and it was only the intervention of a contingent of Space Marine Librarians from other Legions that presented a compromise.
While meditating on Prospero, Magnus foresaw Horus being corrupted by Chaos and the future events of the Horus Heresy, the betrayal of half the Space Marine Legions, and the sundering of the Imperium. Burdened with the information imparted by this precognitive vision, he first tried to dissuade Horus through the warp but failing this he used the power of his Legion's greatest sorcerers to convey the news of the impending civil war to the Emperor himself via a sorcerous spell (instead of the slower method of Astropathic psychic communication).
Upon receiving the sorcerous message, the Emperor was furious to see that Magnus had broken his oath and was using sorcerous means once more. In light of this, the Emperor considered Magnus more suspect than his favoured Horus. Leman Russ, Primarch of the Space Wolves Legion, who had always been averse to witchcraft, was ordered by the Emperor to bring Magnus to Terra to account for his actions. During his voyage to Prospero Leman Russ was convinced by Horus to destroy Magnus's legion rather than simply bring them to account before the Emperor. Accompanying the Space Wolves were a full contingent of his own personal bodyguard and his elite witch-hunting units, the anti-psyker (pariah gene-bearing) Sisters of Silence (originally known as Null Maidens).
Ironically, given his precognitive abilities, this betrayal of the Thousand Sons came as a surprise to Magnus. Landing in a surprise assault on Prospero, the Space Wolves destroyed its capital city and with it much of the knowledge carefully collected by the Thousand Sons. In the ensuing, bloody battle on Prospero and in the capital of Tizca, the City of Light, Magnus and Russ met. Having moved from his ebony throne, and in command of great eldritch powers, Magnus first moved through the legion of the Space Wolves, slaying all who came near and killing even the oldest and staunchest of Long Fangs with his gaze alone. Soon Russ charged in to meet his foe on the battlefield, rushing from the melee he was in to stop Magnus. In mere moments of joining battle Russ and Magnus had traded blows that would shatter tanks. In one blow Magnus splintered Russ' breastplate. During the titanic clash, Magnus picked Leman up and as he did so, Leman kicked Magnus in the eye. Russ lifted the wounded Magnus with a grunt and broke Magnus' back upon his knee. Magnus appeared defeated, but fate had something else in mind for the cyclopean Primarch. Turning to the power of sorcery once more in order to save himself, his Legion, his world and all the knowledge he had accumulated, Magnus, in his anger at the Emperor and at his brother primarch crossed over to the side of Chaos in vengeance, he and his Legion becoming the pawns of the Chaos God Tzeentch in exchange for a mighty work of magic that transported the City of Light into the Eye of Terror. Though Prospero was destroyed that day, Magnus and the remainder of his Legion were not. The next time the betrayed Thousand Sons were seen in combat, they were on the side of Horus, and Magnus was Tzeentch's most powerful daemonic servant: the Prince of Change.
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 vapour. Towers jut from craggy fists of rock thrust up from 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 so 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 obelisk 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 Sorcerers to traverse the cosmos, launching vengeful assaults upon the Imperium of Man.