Board Thread:Warhammer 40k General Discussion/@comment-69.143.134.163-20150419161351/@comment-2.104.154.21-20150519125140

The lost legions obviously fills/has filled the design purposes of a) allowing players to fill on the blank spots with their own traitor/loyalist legions and b) leaves GW with a cliffhanger to develop once the 40k franchise begins to see diminishing sales like WHFB. And obviously 40k lore has been inconsistent since the early days of the game. As such, I doubt anyone - even at GW HQ - have any parrticular detailed idea of the fate. BUT I do like the following theory:

One legion is wiped out by Russ and the Spacewolves - most likely because of Chaos corruption within the entire legion. The fate of the primarch is anybody's guess, though most likely he would have been tainted along with his legion.

If the second legion is absorbed into The Ultramarines I doubt that there would have been corruptiion involved among either either Primarch or Astartes. The risk of tainted gene-seends would have been too great. One possibility is that the Primarch died before reacong maturity to take control of his legion. Another is that the Primarch (along with some of the legion) is destroyed in battle. It is possible that the initial eradication from Imperial records is essentially a PR stunt to maintain the picture, that Primarchs are invincible. More likely is the involvement of Chaos and/or the Emperors "secret project", as this would be the only real reason for a massive coverup.

Of course it is entirely possible that the two legions are wiped out fighting each other (Traitors fight loyalists, Traitors kill loyalist Primarch, Russ kills traitor Primarch and Space Wolves clean up the mess. Everything is covered up to hide the fact that even the superhuman Space Marines and their Primarchs are fallable + the risk of revealing the existence of Chaos to the general public), but again, it is all just speculation.