The path of damnation for the Thousand Sons Legion of Space Marines was longer than most. Even before the Heresy, Magnus led his sons in the study of arcane lore and the practice of sorcery despite the Emperor's warnings to stay clear of such matters. However, they remained loyal to both the Emperor and humanity despite their growing preoccupation with magic.
Magnus the Red was an imposing figure; a red cyclopean giant. He was a very powerful psyker, probably the most powerful psyker of all the primarchs. Furthermore he had been raised on the Planet of Sorcerors � Prospero. This unusual human world was a paradise planet that embraced the psychic mutations that had engulfed its population. Instead of becoming outcasts the pyskers of Prospero became its leaders. Magnus had become the planet's pre-eminent sorcerer and leader of the ruling Commune of Prospero.
Upon meeting Magnus the Emperor had cautioned him about the dangers of sorcery and the nature of the Warp. The Emperor realised that Magnus was very powerful. Possibly feeling it was more dangerous for Magnus to remain ignorant of the secrets of the Warp than it was for him to know them the Emperor showed him the truth.
Magnus feigned shock and horror at what the Emperor revealed to him. He immediately agreed to renounce sorcery and to re-educate the peoples of Prospero. But Magnus secretly dismissed the Emperor�s warnings. He had already peered into the Warp with his one great eye and was obsessed with the power and beauty it promised.
Magnus had been united with his Thousand Sons, the Space Marine Legion who bore his genes. The legion adopted Prospero as its new Homeworld and many of Magnus' followers on the planet were recruited into their ranks.
Despite the Emperor's warnings the Primarch was obsessed with the Warp and the study of sorcery. The Legion of the Thousand Sons had been plagued with psychic mutancy in the years leading to their unification with Magnus. When they did meet Magnus he was able to offer a solution to them. He secretly taught them the ways of the Sorcerors of Prospero, to embrace their psychic talents as a gift rather than a curse. The Thousand Sons had become a secret cabal of warlocks.
The Thousand Sons fought bravely during the Great Crusade and few could doubt their achievements. However some of the other Primarchs could never bring themselves to fully trust Magnus and his Thousand Sons. Leman Russ regarded Magnus as sly and devious, Mortarion openly accused the Cyclops of dabbling in sorcery and Corax of the Raven Guard refused to field his Legion alongside the Thousand Sons.
When Horus gathered his forces against the Imperium, the Thousand Sons at first tried to use their occult powers to warn the Emperor. Mistrustful of anything tinged by Chaos, the Emperor declared the Thousand Sons heretics and sent Leman Russ and the Space Wolves to devastate the Thousand Sons' home world of Prospero. The Thousand Sons were driven into a war against their Emperor and had to fight alongside Chaos to survive and protect their accumulated wisdom. The Thousand Sons sought the patronage of the Chaos Power Tzeentch, Changer of the Ways, greatest wielder of magic among the Chaos gods. Tzeentch favoured them well and the Thousand Sons escaped from the aftermath of the Heresy into the Eye of Tenor to find the daemon world which was promised them. This world became known as the Planet of the Sorcerers, where the Thousand Sons established themselves under the rule of Magnus the Red.
The former Primarch of the Thousand Sons was elevated to the rank of daemon prince of Tzeentch, and installed in a volcanic fortress-pinnacle called the Tower of the Cyclops. Like its master, the topmost tower of this fortress has a single living eye which peers over the surrounding landscape, watching over the minions of its lord: the eternal guardian and watchdog of the Sorcerer King.
Beneath the gaze of the Cyclops the Thousand Sons continued their study of magic and many became mighty sorcerer-lords of Tzeentch. But as time passed many of the Thousand Sons declined into mutation and madness under the warping influence of the Lord of Change. The sorcerers became increasingly disturbed by the degeneration of their brethren until a cabal lead by Ahriman, the greatest among them, risked the wrath of their daemon primarch by undertaking the drastic spell, the Rubric of Ahriman, and purging the Thousand Sons. This brought about a great schism within the Legion as the cabal was banished by the enraged Primarch and scattered to fight in different Traitor Legions across the Eye of Terror.