Board Thread:Warhammer 40k General Discussion/@comment-98.81.158.208-20140211041148/@comment-82.27.120.214-20140213114658

point 2 is a valid arguement as it does mention one of the legions as "the purged". a purge is the removal of people who are considered undesirable by those in power from a government,  so to speculate that the space wolves purged them is highly likely as they also were sent to quel the world eaters and thousand sons.

another theory i have thought of as to what happened to the other legion is that they could of got lost in the warp and never seen again ie like the fire hawks chapter. now im not saying they are the legion of the damned but something similar happened to them. it sorrowed the emperor that much in grief he expunged the legion from memory so not to think about it anymore and so they came to be known as "the forgotten". this also could explain the rapid swelling of the ultramarine legion at the time who could of exumed the lost legions recruits from thier homeworld when the legion dissapeared

but all said and done this is just a theory lol