A son f the God Empeor of Man and Perpetual Erda , these beings are nearly 12 thousand years old and are basically Gods , half have fallen to the dark powers of chaos , the others remain loyal. Currently Roboute Guilliman, greatest of them is the only remaining loyalist ,the rest dead or missing , the traitors are now all accended to be daemon princes of chaos , apart from Konrad Curze and Horus Lupercal who are dead.
Primarch ,ie. Holy shit the primarch Angron just threw a titan that tried to stomp on him