You know... I wouldn't have put it past Horus to set up some kind of secret team to infiltrate loyalist forces. But then, if they decide to stay loyal to the Emperor... that wouldn't have gone over so well.
Doesn't seem like Horus's MO. Sure Alpha Legion but Horus liked to have his forces with him. To be honest most Legions liked to have their forces consolidated. Even a few thousand would be a force that could change a battle. Unless it is one of the bigger ones where Titans are involved or most of a Legion's strength. Also all it'd take is one to check them out as usually those with a Gene-Seed of a Primarch carries some of their features like little Horus for example so if someone looked at the Blood Ravens and they were almost identical to the Sons of Horus they'd be scratching their heads questioning it. Even if they were loyal they'd be watched like hawks as the level of suspicion hanging over them would be incredible. Dorn kicked out all those who were of other legions from the Sol System when the Ivstaan Atrocity became known to them. So the Word Bearers 34th company was sent to join the other Word Bearers into defeating the Traitor legions at the Dropsite Massacre.
Horus was known to be sneaky on occasion, just not in the same league as Alpharius. There were others from his legion who broke with him after he started the Heresy, renamed themselves the Luna Wolves and went absolutely crazy trying to take him down. Most died. Not all.
All were locked but at this point it was before the Heresy was fully realised, only ones who weren't locked were those who weren't Legion Crusading forces so the Imperial Fists because they're not Crusaders but Praetorians of Terra. Also the only other exception was the Space Wolves under direct orders from the Malcador the Sigillite. I think all Legions that were sent to defeat Horus's rebellion had taken everyone in their Legions from Terra unless they for some reason were left imprisoned. Also explains how almost all of the Terran Born Word Bearers were "removed".
Deliverance Lost, Alpha Legion with Face Transplants and mental programming from Librarians infiltrate the Raven Guard.
As I said Alpha Legion do that. Other legions do not. No other Legion has been known to send Legionaries disguised as others into another Legion. Alpha Legion are the ones who seem to do it by being World Eaters, Raven Guard, Iron Hands, Ultramarines and no doubt have been others. They might dress up as Imperial Fists but I haven't read through Praetorian of Dorn yet.
http://www.gamegrin.com/articles/warhammer-40000-dawn-of-war-iii-interview/ GAMEGRIN: Dawn of War has been a driving force behind the background of the Blood Ravens - now a well-established part of 40k canon. Are we going to see a deeper exploration of their background? BOULLE: We will see some more - the story is mostly about a new situation they have to deal with and where they are now. The game is picking them up some time after Dawn of War 2 as Gabriel Angelos has been chapter master for a while. The game will show the direction that the chapter is going in. There will always be secrets about the Blood Ravens - we don't want to reveal everything - but old school Dawn of War fans will have some pleasant surprises.
