Remorseless. Cursed. Tragedy and fate conspired to drive the most psychically gifted of the Space Marine Legions into the clutches of Chaos. Thus they became an enemy of the Imperium like no other.
The path of damnation for the Thousand Sons was longer than most. One of the original Legiones Astartes, the Thousand Sons had many psykers within their ranks – hardly surprising as their Primarch, Magnus the Red, was a psychic giant. During the Great Crusade, the Thousand Sons continued to study arcane lore despite the Emperor’s warnings.
They remained loyal, using their occult powers to warn the Emperor of Horus’ impending betrayal. The Emperor declared the Thousand Sons’ arcane arts to be heresy, however, and unleashed the Space Wolves upon them.
Forced into a war they did not want, the Thousand Sons turned to Tzeentch in order to save themselves. Escaping certain destruction by fleeing into the Eye of Terror, the Legion continued their magical studies, although they were soon wracked by mutations. Disturbed by their decline, a cabal, led by the sorcerer Ahriman, risked the wrath of their Primarch, now risen to daemonhood, to cast a spell known as the Rubric.
The spell placed his kin beyond mutation, but reduced their mortal forms to dust sealed inside suits of armour – no more than magical automatons. The cabal was banished by the enraged Primarch, forced to scatter and fight for different Traitor Legions. After an age-long exile, however, the Legion has reconciled and reformed.