Merem Solomon

You've met them, haven't you? Some of them are known as Servants. Oh, they serve various purposes. But when humanity's collective unconsciousness becomes threatened, legendary beings who have formed a contract with that same unconsciousness are summoned to protect humans from the threat. Your Emperor and Primarchs that you speak of, after their death, I guarantee they will become such beings. Except they will be nigh unkillable, backed by the formerly dormant power of humanity.