Two Primarchs and their legions as missing from Imperial records. What mention of them is little some records state they are invisible meaning the Primarchs as in fact missing or were psychic blanks as they could not be sensed by those of the warp or with psychic abilities. Though it could be an actual power that all Primarchs possess as it is said that Corax of the Raven Guard can walk among people without being seen and only the Emperor knows of this. However, this could also be more attributed to his specialty of stealth tactics and guerrilla warfare. Other records briefly mention they fought for the Imperium while others state they fought for chaos. In either case, legions 2 and 11 are a mystery.
With Twenty Primarchs in all this Warhammer Wednesday will take a bit so bear with me sorry if I overload your dash. Also I take no credit for these pictures I will try to credit these pictures if I can but if not it is not my work.