Board Thread:Warhammer 40k General Discussion/@comment-24383134-20140303213741/@comment-24944355-20140716111608

One of the missing primarchs was 3rd found according to the lists I've seen and the other was 19th found. Now hints in the Horus Heresy books have led to the inference that one of the missing primarchs was & his legion was destroyed by the Space Wolves, this was probably the one found 3rd. The hints also infer that the 2nd of the missing primarchs was brought low by some other reason, possibly for violently disagreeing with the Emperor about some part of his goals and refusing to join the crusade.

Now why one primarch was killed by Russ we don't know, but it must have been for something pretty bad considering what the Emperor let Curze & Angron get away with. My theory is that the slain primarch either attempted a coup or tried to set up his own empire, either offense would have been enough to for the Emperor to set Russ upon him.

As for the 2nd missing primarch it's unlikely that the Emperor had him killed for refusing to join him so he was most likely imprisoned in a stasis chamber. Whether he is still imprisoned or some how escaped or was set loose is anybody's guess, but after 10,000 years an unmaintained stasis device could quite easily fail. As to what he disagreed so violently with the Emperor about probably had to do with either the violently aggressive nature of the crusade or with the empires policy of exterminating even benign alien races.