possible fluff for my chaos list

a chapter separated during the retreat after the horus heresy into the eye of terror into to different factions each lead by a brother they meet up and the younger brother the chao lord of khorne is trying to prove that he as as worthy of daemonhodd as his newly appointed demon prince brother
blood for the blood god

From what I was able to discern it was good, but you still have to explain how 20 suits of terminator armour got in your army. I would imagine it something like the deamon prince and chaos lord broke the minds of some first company chapter so they defected towards Chaos.

Fundamental flaw is that all of the 18 original chapters of the horus heresy are accounted for. The chapter you suggested would have to exist after the heresy, perhaps they were a warband that was retreating after the black crusade. One brother becoming a demon prince, while the other did not.