Blood vengeance is a primitive form of the law of retribution according to which a kinsman must vindicate the rights of a relative whose blood has been shed.
According to the ancient Greek concept every act of bloodshed, even when committed in self-defence, created a certain defilement that required purification. Not only the criminal but his family also was defiled until the slain man's life was appeased by exacting vengeance. The initial crime could easily lead to a series of mutual crimes, a blood feud or vendetta. In primitive societies a whole family or even a whole clan might be annihilated for a murder committed by one of its members.