In the Jewish story of the Exodus, commemorated on the holiday of Pesach (Passover), The Children of Israel were enslaved in Egypt and Moses, as God's prophet, was petitioning the pharoah for their release. The pharoah refused, and God visited on him a plague for each refusal (see the ten plagues). The final plague, which broke him, was the slaying of the firstborn. God sent an angel of death over Egypt, and in every house the firstborn son was killed (I am uncertain as to whether families where the firstborn was a daughter lost their oldest son or not). The Children of Israel were told to sacrifice a lamb and paint its blood on their doorposts. If a family did this, the angel Passed Over their home and their children were spared.