Well it is a good idea however, fluff indicates that all twenty Legions and their Primarchs fought at least during the Great Crusade. Also, when Dorn talks with the Sigillite on Terra by the plithes of the Primarchs, he mentions something along the lines of "They are lost to us forever" and in one of the Audio-Books, if you look at the second sentance in my signature that is aimed at the pair. Not trying to ruin your idea or anything, but rather trying to strengthen it.
Nyctophobia- Fear of the Dark Angel.
"No one ever spoke about of those two absent brothers. Their separate tragedies had seemed like aberrations. Had they, in fact, been warnings that no one had heeded?"
'Killing a man is like fucking, boy, only instead of giving life you take it. You experience the ecstasy of penetration as your warhead enters the enemy's belly and the shaft follows. You see the whites of his eyes roll inside the sockets of his helmet. You feel his knees give way beneath him and the weight of his faltering flesh draw down the point of your spear. Are you picturing this?'
'Is your dick hard yet?'
''What? You've got your spear in a man's guts and your dog isn't stiff? What are you, a woman?'