My hat in the ring on the Blood Ravens origins
I know the popular fanon theory behind the Blood Ravens is that they were Thousand Sons successors, but the simple fact is if that were the case the Flesh Change would have ruined them and made them nonviable thousands of years ago.
But that's not the only option.
They're Word Bearers descendants. They have many sayings and quotes that originate from the Word Bearers. Also Lorgar and the Word Bearers had great psychic potential, with Lorgar even being the second most potent psyker of the primarchs after Magnus. They also share similar color schemes with the later stage Word Bearers. Finally and probably the most importantly, it's very easy to see how the things that characterize the Blood Ravens make sense as purified remnants of Word Bearer culture. The Word Bearers were not fundamentally stupid fanatics. Lorgar even said it himself: "All I wanted was the Truth." They were scholars. They collected texts. They studied religions. They preserved histories. They sought universal truths. They wanted to understand the metaphysical structure of reality. Their fatal flaw was that they found answers in the Warp and then decided that the answers they found must therefore be worthy of worship. Strip away that conclusion, retain the intellectual temperament, and you get something remarkably Blood Raven-like.
The Word Bearers treasured scripture → the Blood Ravens treasure knowledge.
The Word Bearers were obsessed with understanding the divine → the Blood Ravens are obsessed with understanding the universe.
The Word Bearers preserved texts, relics and histories → the Blood Ravens hoard artifacts and lore.
The Word Bearers were extraordinarily doctrinal → the Blood Ravens have an almost obsessive relationship with Chapter traditions and teachings.
The Word Bearers possessed significant psychic potential → the Blood Ravens have an unusually high concentration of psykers.
The Word Bearers were intensely religious → the Blood Ravens are intensely devoted to the Emperor.
It's almost like someone took the cultural DNA of the Word Bearers and asked:
"…What if you removed Chaos?"
Also, like, their homeworld is called Aurelia and they have major beef with Eliphas the Inheritor, which makes the Blood Ravens' relationship with the Word Bearers feel less like "Oh, that's another Chaos Legion we fight." and more like "There is something about these people that we hate with a depth we don't entirely understand."
So, this is the fanon that I've found the most compelling and interesting and latched onto. All it would have taken was enough Word Bearers to go "The Emperor denies that He is a God, but He is mistaken. He may deny his nature but He cannot change it, and we worship Him because He is Worthy of that Worship, not because He wants it.", harden their resolve, and due to the shifting atmosphere of their brothers, suppress their expression of that belief for a few decades until a more opportune moment before going ahead with the Schism.
That is essentially how early Christianity survived in Rome: keeping quiet, keeping their heads down, and waiting.
Eventually that moment happens; maybe it's Calth, or the Siege of Terra, or a battle simply no longer remembered, and they declare for the Loyalists and join the fight. There's tons of historic precedent for just such a military rug pull of loyalties.
After the war, they get snapped up by Dorn or Guilliman when they go on their post-Heresy adoption sprees.
It's probably Dorn if I'm honest, because religious loyalty looks a lot like the Duty he loves so much and his code also produced the Black Templars anyway. But most importantly, he's not an idiot. He recognizes their livery and the telltale signs of their gene seed, but they've proven their valor and loyalty, and his Legion is desperate to fill in the holes in their ranks done by the Heresy and the Siege of Terra.
"You understand Loyalty. You understand Duty. The path you chose was hard and fraught with peril, but it preserved your honor, and when our need was dire, it brought you back to us. Though your sins are forgiven, you will understand they are not forgotten. Though I know what you were, and I know what you endured, I also know what you chose, and a man's choices show the world who he really is. By your actions, you have shown me all that I need to know. Welcome home, Brothers. Welcome to the Seventh Legion."
And either they or their descendants eventually become distinctive enough and their talents necessary enough that they get spun off into their own Chapter, be it during the Second Founding or later.
It's a DIFFICULT path, but hardly an impossible one considering real history is full of similar stories. And it's Rule of Cool as hell, which makes it very 40K friendly, so long as it's never officially CONFIRMED. Sadly, officially confirmed Loyalist Traitor descent is very much the main purview of fanon chapters. Which is a shame because the whole "we are the sons of those who held fast and stayed the course" that comes with Loyalist sons of Traitor stock is so very cool.
Anyway, Knowledge is Power, Guard it Well… and None Shall Find Us Wanting!
