Given how heavily the Blood Ravens Dawn of War novels, a certain passage in the Thousand Sons novel, and the short story with the Thousand Sons who missed Prospero beat you over the head with the connections, the Blood Ravens being a successor of anyone but the Thousand Sons would be a baffling reversal. It would make especially little sense to have them be a White Scars successor, there's absolutely nothing tying them back. Though, maybe it's just a typo on the part of the OP? Arvida was the character in that Thousand Sons short story no?
This all still sounds like nonsense, to some extent, especially with stuff like Omegon being a founding member(Primarch then?) of the Grey Knights.
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And I missed the "Magnus being a C'tan shard" bit. What? No, seriously, what? The greatest Psyker humanity has seen bar the Emperor is now somehow an embodiment of the antithesis of the warp? That is basically what the C'tan are, no?