In Exodus 11, after many plagues, the Lord said to Moses, "I will bring one more plague on Pharaoh and on Egypt. After that, he will let you go from here, and when he does, he will drive you out completely." This last plague would be the worst, which would cause every firstborn son in Egypt to die, even Pharaoh's son. Even the animals belonging to the Egyptians would lose their firstborn, but the Israelites would not lose any firstborn. Hearing about this, Pharaoh still refused to listen and let the Israelites go out of his country, since the Lord hardened his heart.