Board Thread:Warhammer 40k General Discussion/@comment-25618639-20150112024430/@comment-26859246-20150901152804

It may simply be that the BR are loyalist splinters from one of the traitor legions. We know that Horus and the other traitor primarchs did not manage to kill all of the loyalist marines in their legions as some were likely on detachement elsewhere.

I never completely believed that there were loyalist marines within the thousand sons and that the whole legion willingly followed Magnus to hell.

But between Space wolves and the Silent Sisters I doubt that there were many marines that surrended