The Thousand Sons are one of the Traitor Legions of Chaos Space Marines who are sworn solely to the service of Tzeentch, the Chaos God of Change, Intrigue and Sorcery, though they were once the Imperium of Man’s XV Legion of Space Marines.
The main feature that distinguishes the Thousand Sons from the other Traitor Legions is the sheer number of psyker mutations that have always existed amongst the Space Marines in this Legion. Every current member of the Legion possesses psychic abilities and they prefer to use Chaos Sorcerers and guile in combat, relying on their psychic powers and sorcerous knowledge to earn them victory.
As a result of an ancient spell known as the Rubric of Ahriman that was cast soon after the Thousand Sons fell from their homeworld of Prospero during the early days of the Horus Heresy, every Thousand Sons Astartes that lacked psychic powers had his soul fused directly into his Power Armour. The Thousand Sons have become living embodiments of sorcery animated by the power of Chaos and the will of the Lord of Change.