Board Thread:Warhammer 40k General Discussion/@comment-25618639-20150112024430/@comment-26859246-20171006104045

Well apart for the Word bearers there were loyalists within every traitor legion in the first place and though the bulk of the Thousand sons seems to have sided with their primarch not all of them would have. So its certainly possible that the BR were descended from loyalist elements of them. Remember the Grey knights founding members were drawn from the loyalist marines whose legion had sided with Horus.