Board Thread:Warhammer 40k Novels/@comment-27376167-20151210113237/@comment-95.241.59.5-20160121015144

My guess would be from stuff in the Horus Heresy that one legion mutated too much and the other defied the Emperor.

Ironically I'd say the 'defied Emperor' legion would be the 'loyal' legion. Imagine something like what happened to the Word Bearers being ordered to wipe out an entire planet but the Primarch of the IInd or XIth defies the Emperor and is put down for it.