You guys know those 2 unnamed Space Marine Legions that were created before the Horus Heresy that make up the 20 Legions.

GW left them open so players could make up there own fluff about them and give themselves a bit of breathing space if they ever wanted to add anything else fluff wise.

So what are your theories concerning them.

Mine is that one of them must have sided with Horus and maybe it surrendered or something.

The other loyal one might have been completely destroyed or something during the Heresy.

P.S. Let me remind you that they are not the Grey Knights nor are the Legion of the Damned (they are the survivors of the Fire Hawks Chapter)