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

My personal, unsourced, and totally in-my-head theory is that the two lost primarchs were either

1) lost in the warp permanently upon being scattered or 2) died on the planets they landed on some time later, probably pretty quickly afterwards. Either way they died before the Emperor found them

After all they were just infants. I mean, you drop 20 superhuman babies across the galaxy at least one of them is going to wander into traffic and get hit by a bus. Primarchs are not invincible, even against mortal armies. After all the Emperor refused to allow Angron to fight with his rebel army, fearing that he would die. And he was full grown...'and also freaking Angron. '

There are incredibly powerful forces in the galaxy, some which full grown and geared-out primarchs with their entire Legions. Drop an infant Primarch on an active Necron Tomb world, see what happens.

Their Legions were then absorbed into the Ultramarines (this is actually hinted at) when it became clear that their Primarchs were dead, and the perceived 'weakness' that they were not able to survive and had died so ignobly (hilariously?) is the source of the shame associated with talking about them - and a sore spot about the Legions whose gene seeds they share. I'm sure the Emperor was probably a bit dissapointed when he found out two of his kids hadn't been able to survive.

Another reason why the Emperor might have expunged them from history:  Note that several times in canon the Emperor arrived seemingly 'in the nick of time' to save one of his sons from death (Mortarion, Angron). Maybe the Emperor just arrived a few days late, or had to choose between sons who were both in immediate peril. ''He probably wasn't thrilled about that. ''So he had everything associated with them expunged. And since nobody was stupid enough to bring up the Emperor's dead kids, even talking about them was taboo. 