User blog comment:Aryeh.wolfblade/The missing legions aka fergotten ones./@comment-5821760-20121211034539

What you guys are refering to is the part where the remembrancer kallista had a vision about ravens. I also believe another person had the same vision and admitted that he too had seen them. the only possible way that could have happened is when magnus sent part of his main fleet away from prospero before the attack. They would have to have found a way to reinsert themselves into the imperium. Its possibly because they have a lot of the same qualities and distinct pieces of the thousand sons such as the thirst for knowledge, the many psykers in the legion. The only part that doesnt make sense is if it was the remaining parts of the thousand son fleet they would also have suffered from the flesh change unless tzeentch decided to spare them. So a lot of speculation but its still possible.