User blog comment:Aryeh.wolfblade/The missing legions aka fergotten ones./@comment-3514402-20120221020750

I've been re-reading some of the Heresy books and i believe there are two particular lines in The First Heretic that have not yet been posted here. From page 163 of that book.

QUOTE (Magnus the Red) "Russ knows that, though he lacks the intelligence to give it voice. Instead, he swore that he already lost two brothers and had no desire to lose a third."

QUOTE (Magnus the Red) "The Word Bearers will not walk the same paths as the forgotten and the purged. I trust you Lorgar."

This occurred around forty-three years before the outbreak of the Heresy. This suggests that whatever happened to the Lost Legions, it was either during the early or middle parts of the Great Crusade, not at the start of the Heresy. Also, since it was suggested in Prospero Burns and The Outcast Dead that the Wolves of Fenris have been unleashed against a brother Legion before, and since the Wolves themselves make no secret of their role as the Emperor's executioners, it would appear that the Burning of Prospero may have been the second time the Rout was used in this capacity. Plus, that should lay to rest any ideas that one of the Primarchs was a girl.