Saving a post by Saby on reddit:
It should be noted that there are MANY instances of chapters going against their general culture or otherwise using iconography of different parents. But they are Sons of the Phoenix.
- Fulgrim was the Phoenician, and his personal retinue was made up of the Phoenix Guard.
- Their chapter symbol is basically the Emperor’s Children logo: one wing fanned out
- Emperor’s Children, Sons of the Phoenix. Admittedly there are plenty of Sons chapters out there unrelated to Fulgrim.
- Their colors are very Fulgrim coded too – and they’re a lot more ornate than most Dorn successors.
- Basically any description of their fighting contains phrases like spectacular and precise, which matches Crusade Era ECs more than the stalwart and unyielding nature of most sons of Dorn.
Additionally, there is a whole argument between Cawl and Gulliman in the Great Work (I think) that debates the merit of using the preserved geneseed of the traitor legions. Cawls insists on using it, citing that the genetics are pure even if the character of their primarchs was corrupted. Gulliman forbids him from using it, but Cawl is a maverick of sorts. He clearly uses an illegal AI copy of his brain as an assistant and lies about its true extent. He built a secret legion of space marines and hid it for 10k years. He casually reverse engineers lost technology like Vulkite and Grav tech.
Futhermore, most of the Primaris chapters started out as ‘grey shields’ that fought without chapter banners and were basically just any of the marines taken out of stasis. He easily could have released some Emperor’s Children Primaris marines in this period, and then just told them that they were the sons of Dorn once things settled down.
For some reason, the Thousand Sons specifically targeted them for capture and experimentation. Could be nothing, could be that they smell the scent of traitors in their blood.
It wouldn’t be the first time that lost legions were incorporated into surviving ones. It is heavily implied that Gulliman and Dorn assimilated portions of the II and XI legions after their primarchs were [redacted]. Gulliman knows that he and Dorn would have advocated for that, and that their memories would have been wiped over it had it happened. Meanwhile, the White Templars (Fist Successor) were discovered to not be of Dornian ancestry recently despite having been a Successor chapter for like 10k years. Possibly meaning they were among these assimilated chapters.
So in Summary:
- Basically every superficial feature of the Sons of the Phoenix exhibits features of Fulgrim and the Emperor’s Children
- Cawl is basically all but confirmed to have utilized traitor geneseed in making Primaris Chapters.
- Imperial Fists have canonically (almost certainly) adopted non-Dorn forces in the past and this would mirror that
They also pose a very interesting possibility with the existence of the perfect, uncorrupted Fulgrim clone. And the possibility of him leading his own sons (of the phoenix) one day.
