User blog comment:Cryptomania/Primarch Rubinek??/@comment-10514705-20120429140619/@comment-10514705-20120430150506

Well, I'm pretty sure quite a few guys here would love to make their own Primarch and Legion, but we have purposefully taken this into account in the rules in order to avoid ONLY Lost Legions appearing which cannot have a canon friendly link with other Fanon articles, or even Canon sources.

Second, it would change a lot as then there would only be one lost unarmed Primarch. This is a problem as it is implied that one lost legion fell to chaos while the other did not. This is still speculation.

Secondly, being named as Primarch directly contradicts the whole, you know, Lost Legions and their Primarchs being wiped from history. ALL records were removed, so it would cointer to what is regarded as Canon.