The Thousand Sons were the XV Legion of the original twenty Space Marine Legions before turning traitor after their Primarch Magnus the Red. After the Horus Heresy they relocated to Sortiarius, the Planet of Sorcerers in the Eye of Terror, and dedicated themselves to the Chaos God of change, Tzeentch. For a time following the Heresy Tzeentch protected the Thousand Sons from the corruption and mutation that most of the other Traitor Legions fell slave to. However Tzeentch, known to be a fickle god, eventually lost interest and allowed the Legion to suffer the same fate as their traitorous brethren simply for his amusement.

The Thousand sons started to suffer from mutation and de-evolution, seemingly at the whim of their patron-God. Horrified at this development, their chief sorcerer, Ahriman, cast a powerful spell designed to render the Thousand Sons immune to the warping effects of Chaos. The spell succeeded in this goal, but had a devastating unforeseen consequence, his fellow marines were transformed into dust sealed within their armour. Now most of the sons of Magnus are little more than mystically animated suits of armour, barely being capable of sentient thought without the guidance of a nearby Sorcerer.