The Thousand Sons were the XV Legion of the original twenty Space Marine Legions. Their Primarch is Magnus, often called Magnus the Red. The Legion turned traitor during the Horus Heresy, after which it relocated to the Planet of Sorcerers in the Eye of Terror and dedicated itself to the Chaos God of change, sorcery and magic, Tzeentch. For a time it appeared that Tzeentch protected the Thousand Sons from the corruption and fragmentation that most of the other Traitor Legions underwent, but eventually the Legion went through a crisis of mutation and de-evolution, seemingly at the wish of their patron-God. Aghast at this development, their chief sorcerer, Ahriman, cast a powerful spell designed to render the Thousand Sons immune to the warping effects of Chaos. While the spell succeeded in this goal, it had an unforseen consequence of transforming the mutated Thousand Sons into little more than mystically animated suits of armour barely capable of being termed as anything more sentient than automata.
With the only surviving cognisant members being those incredibly strong in heretical psychic powers, the Thousand Sons are now a legion of ghosts led by the damned.
Kairos Fateweaver. Greater Daemon of Tzeentch
“Do not ask which creature screams in the night. Do not question who waits for you in the shadow. It is my cry that wakes you in the night, and my body that crouches in the shadow. I am Tzeentch and you are the puppet that dances to my tune...”
- The Changer of Ways.
Tzeentch, the Changer of Ways, is the Chaos God of Sorcery, Change, and Manipulation. Within the Realm of Chaos his domain covers the chaotic magic, evolution and change, scheming and manipulation of all kinds, mundane as well as arcane. He is known under many other names, among them the Lord of Change, the Grand Schemer and the Lord of Sorcery, while in the Chaos Wastes he is known as the Great Eagle. In the Age of Reckoning, the Northmen tribes that make up the Army of Chaos led by the elite Raven Host worship him as the Raven God. Tzeentch's number is nine; as such, his followers frequently take to the battlefield in squads made up of multiples of nine. The colors used to represent Tzeentch are generally blue, purple, yellow, and gold.