Arcanis161 wrote:I'll have to dig out my Codex again, but I think he's referring to a group listed in there as the "Sons of Magnus", which have a red and gold color scheme.
The "Sons of Magnus" are a generic name for all Thousand Sons marines. You're thinking of the Crimson Sons:
In the wake of the Rubric of Ahriman, 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 Cult of Duplicity, how their actions ultimately serve the will of Magnus is unknown to all but the Crimson King.