Their Primarch is Magnus the Red, called the Red Cyclops or Cyclopean Magnus, due to his one large eye and his flaming-red hair. Magnus had already been touched by Chaos long before the Horus Heresy, from his long study of the Arcane arts. Even though the Thousand Sons tried to use their occult powers to warn the Emperor of Horus's heresy, the Emperor, mistrustful of anything tinged by Chaos, declared the Thousand Sons heretics and sent Leman Russ and the Space Wolves to devastate the Thousand Sons' homeworld of Prospero. The Thousand Sons were driven to war against their Emperor, and had to fight alongside the Traitor Legions for their survival. The Thousand Sons turned to Tzeentch and asked for his patronage, as she is the greatest wielder of Magic among the Chaos gods. Magnus the Red was elevated to the rank of Daemon Prince of Tzeentch and given a Daemon World to rule over. He rules from his great volcanic fortress called The Tower of The Cyclops. The topmost level of the fortress has a single living eye, which watches over the landscape and the minions of its Lord, The Eternal Watchdog of The Sorcerer King. Over time, the Thousand Sons started to degenerate and become mutated. The Sorcerers of the Thousand Sons joined together in a cabal led by Ahriman. Even though they risked the wrath of their Daemon Primarch, they cast the spell The Rubric of Ahriman, purging the Thousand Sons of mutation for all of eternity. This caused the great schism within the Legion, and caused the cabal to be banished by the enraged Primarch and scattered to fight in different Traitor Legions across The Eye of Terror.
Codex Chaos 2nd ed.
The name of the Thousand Sons was taken from the initial series of genetic imprints made from the tissue of their Primarch, Magnus the Red. Magnus was a ruddy-hared and extremely large Primarch with a single eye set deep in his broad forehead. Because of this he was sometimes known as Cyclopean Magnus or the Red Cyclops. The truth was that his soul had already been touched by Chaos, and Tzeentch insinuated into him a fascination with the occult forces of the warp and the secrets that lay deep within its fabric. From his imprint a thousand Space Marines were created: the Thousand Sons of Magnus. An entire Legion of many thousands of Space Marines was subsequently raised to take part in the Great Crusade, but the Legion always kept the title of the Thousand Sons. 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. 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 with 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 the 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 favored them well and the Thousand Sons escaped from the Eye of Terror to find the demon 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 demon 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 the 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 demon 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.
