Hopefully the title wasn't misleading. I'm not interested in exploring what the two missing space marine legions and their primarchs did wrong, but rather in something that has been bothering me for a long time. Who ordered the records of their existence to be purged?
In the 3rd edition SM codex it said: RECORDS PURGED, ORDER ORIGIN UNKNOWN.
Now, because the viewer of the legion table was an inquisitor (=][= approval codes were visible in the corners), we have to assume that the great secret of chaos was known to the viewer, but not to the more average imperial citizen. At least, that was the theory of the day.
In later SM codeci the ORDER UNKNOWN became blurred entries that suggested complete deletion, or corroded databanks that would suggest physical destruction of the information.

Now, everything we know about the Heresy has been 'updated' (to be polite) through the Horus Heresy artbooks and the BL series. In one of the little books, the Lightning Tower, Primarch Rogal Dorn ponders the questions as to why Horus defected. As he ponders this, he walks into a stadium where there used to be twenty statues of the primarchs. Now, here's where my question and argument revolves around:

Quote Originally Posted by The Lightning Tower, page 9
Dorn's aimless wanderings had taken him to the Investiary. In that broad space, an amphitheatre open to the night sky, statues of the twenty stood on ouslite plinths in a silent ring. (...). Under the glittering stars, it felt like an arena, where twenty warriors had gathered to make their combat.
The second and eleventh plinth had been vacant for a long time. No one ever spoke of those absent brothers. Their separate tragedies had seemed like aberrations. Had they, in fact, been warnings that no one heeded?
Sigismund had urged that the effigies of the traitors also be removed from the Investiary. He had offered to do the work himself. This, Dorn recalled, had made the Emperor laugh. For the time being, the traitors had been shrouded. Their towering, draped forms seemed like phantoms in the blue darkness.
At first glance, nothing really new here. The missing legions are not discussed and the physical memory of those primarchs have been removed. So, the order for the purging of the legion archives seem to point to the Emperor. After all, he either removed their statues, ordered it, or allowed it.
The traitor legions however, seem to get a preferential status regarding their two siblings. They are ordered shrouded by the Emperor, rather then removed as requested by Sigismund. And knowing Sigismund, he is probably the most vocal of a larger group of people believing this is the right thing to do. But, as can be read, the Emperor laughed this away.

Now, he will probably remove the statues of the traitors after the war is over. But, the deletion of the records doesn't sit well with me. After all, deleting all records means that their 'seperate tragedies' aren't allowed to be known by anyone, right? Destroying their records and their statues would eventually delete the public knowledge of these two primarchs, right?

If this was the goal of the Emperor, why didn't he remove the plinths?
This may sound strange, but think about it. If the Emperor truly wanted to destroy any and all memory of the legions, why would he allow the memory to remain that there were twenty primarchs? Nobody would understand those two empty slabs if the records were deleted. And the people, and with that I mean the normal and mortal citizens, that were around at the time would eventually die out, leaving these points of reference void. So, removing those plinths and remodelling the theatre would be completely possible since it happened 'a long time' ago. None would be the wiser. The Emperor was rather busy with his palace in the years leading up to Horus Rebellion.

Here's my theory: the Emperor never wanted to destroy the memory of his two missing children, but couldn't bear to look on their faces because of what happened. All records were most likely sealed. The order to purge them was of a later date. I do not know who did it, but I believe the following options are possible:

a). The surviving loyalist primarchs decided to purge all memory and knowledge of these two. This would have been done in the interest of preserving a positive image of the Emperor. With the data on those two gone, no-one could point a finger to the Emperor that he didn't see the Heresy coming because of what happened previously, or to protect the remaining primarchs against accusations that there's something wrong with their design itself and defection/insanity/whatever is bound to happen to all of the primarchs.

b). The data was deleted by the newly appointed High Lords of Terra to ensure a positive image of the Emperor, thus ensuring their own rule.

c). The newly founded church of the emperor ordered the deletion, either when they came to power amongst the high lords, or one or more of their adherents amongst the high lords persuaded the others.

What do you think?