The Crimson Sons are a warband of Heretic Astartes dedicated to the Chaos God Tzeentch that was originally formed by members of the Thousand Sons Traitor Legion. The Crimson Sons warband was exiled by its Primarch Magnus the Red for aiding in the casting of the disastrous sorcerous ritual known as the Rubric of Ahriman.
In the wake of this ritual, the Crimson Sons were exiled from the Planet of the Sorcerers, and have wrought mayhem in isolation from the main body of the Thousand Sons ever since. Like so many sects in the Thousand Sons Cult of Duplicity, how their actions ultimately serve the will of Magnus the Red is unknown to all but the Crimson King.
The Crimson Sons warband's colours are a deep red with golden trim, not unlike the original, pre-Horus Heresy colours of their parent Space Marine Legion. Despite their exile from the main body of the Thousand Sons, the warband paid tribute to the Crimson King, their Primarch Magnus the Red, with their name and also by colouring their Power Armour a deep red.
The Crimson Sons warband's sigil is a yellow coloured sigil of the Cult of Duplicity centred upon a field of crimson and trimmed in gold.
- Codex Heretic Astartes - Thousand Sons (8th Edition), pg. 22
- War Zone Fenris: Wrath of Magnus (7th Edition), pg. 134