Board Thread:Warhammer 40k General Discussion/@comment-91.224.27.226-20131031140001/@comment-2.218.141.117-20131122200632

Games Workshop has stated that the missing legions were deliberately left blank to allow people to create their own heresy era legions.

However there are a few clues:

In Lightning Tower, Rogal Dorn wishes his two brothers were still with them, Malcador says they are lost forever.

In First Heretic Lorgar reveals that they were lost fifty years before the heresy, Magnus reveals the Emperor swore his sons to silence on the matter. Also it is revealed that it was at this point the Ultramarines swelled to outnumber all other legions, perhaps absorbing survivors of the lost legions.

In Prospero Burns Leman Russ comments that there is precedent in Astartes fighting Astartes and that he gets 'all the dirty jobs'.