The Thousand Sons were the 15th Legion of the First Founding. Their armour was red with black trim before the Heresy but changed to blue with gold trim to represent their devotion to Tzeentch after the Heresy. Their icon was a large black M on a red background before the Heresy. Magnus the Red was their Primarch. Their homeworld was Prospero which was later destroyed by the Imperium.
The Pre-Heresy Emblem of the Thousand Sons
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-haired 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 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 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 Terror 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.
Taken from Codex: Chaos v2
The path of damnation for the Thousand Sons legion was longer than most. Even before the Heresy they studied arcane lore and sorcery despite the Emperor's warnings to stay clear of such matters. 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 them heretics and sent the Space Wolves to devastate the Thousand Sons' homeworld of Prospero.
Fleeing this onslaught the Thousand Sons sought the patronage of the Chaos power Tzeentch, greatest wielder of magic among the Chaos gos. Tzeentch favoured them well and the Thousand Sons escaped to the Eye of Terror, where they settled on the world which became known as the Planet of the Sorcerers. As ever with events that take place within the Eye of Terror, it is difficult to find confirmatory evidence, but it appears that as time passed many 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 bretheren until they eventually risked undertaking the drastic Rubric of Ahriman which purged the Thousand Sons of those members that were considered 'unworthy', and which created the strangely robotic Thousand sons Chaos Space Marines that are encountered today. Taken from Codex: Chaos Space Marines v3
The Primarch of the Thousand Sons
Magnus the Red
Even before the Horus Heresy the Thousands Sons became involved with arcane lore and the practice of sorcery. Despite warnings from the Emperor their Primarch, known as Magnus the Red or Cyclopean Magnus for his flaming red hair and single eye, continued to delve deeper into the mysteries of the warp. Magnus remained loyal, however, even attempting to warn the Emperor about Horus through his arcane powers. But the Emperor, mistrustful of anything tinged by the warp and Chaos, sent Leman Russ and the Space Wolves to destroy the Thousand Sons homeworld. Once driven into war Magnus had little choice but to ally himself with Tzeentch, greatest magician of the Chaos Gods, to avoid total destruction. Magnus escaped the aftermath of the Horus Heresy by using his sorcerous powers to open a warp interface through which the ships of the Thousand Sons could flee to the Eye of Terror. There Tzeentch granted Magus the Planet of Sorcerers to rule as his own.
Over the centuries Cyclopean Magnus has become a sorceror of the most consummate power. His single eye blazes with mystic energy and his limbs constantly burn with blue-white witchfire. Taken from Renegades