The ancient social brotherhoods were based on the rite of blood drinking; the early Jewish fraternity was a sacrificial blood affair.  Belief blood was seat of soul gave origin to the practice of blood drinking. 
The embryonic state was made possible by the decline of the blood bond in favor of the territorial, and such tribal federations were usually firmly cemented by conquest.  The first treaties of peace were the “blood bonds”. 
The adherents of the cult of Mithra worshiped in caves and other secret places, chanting hymns, mumbling magic, eating the flesh of the sacrificial animals, and drinking the blood.  Blood drinking became common, and it was customary to mix the “edible” parts of the body with medicines.  Blood drinking as a seal of personal friendship and as a token of group loyalty.