User blog comment:Supahbadmarine/By Tzeentch! Here We Go!/@comment-9465750-20130212130355/@comment-3258762-20130212134631

I do believe it has been heavily stated that no First Founding Legion (Those who also happen to have the most say in everything, as they are given more respect than normal chapters) would ever side with Chaos no matter what. They would rather die.

The Inquisition would, in such a case, be torn into a civil war where they are either completley wiped out or the forces remaining (Pro or Con Chaos) Inquisition would be far to little to do anything.

Truth be told, I think the 10 Primarchs have the largest chance of winning and, if any Loyalist Primarchs would return (The Daemon Primarchs would never be allowed to exist by their brothers) they would probably side with the new Primarchs when they see the stagnant Imperium.