Exodus 11:5
And all the firstborn in the land of Egypt shall die, from the firstborn of Pharaoh that sitteth upon his throne, even unto the firstborn of the maidservant that is behind the mill; and all the firstborn of beasts. And all the firstborn in the land of Egypt shall die, from the firstborn of Pharaoh that sitteth upon his throne, even unto the firstborn of the maidservant that is behind the mill; and all the firstborn of beasts.
This was the fearless final warning spoken directly to Pharaoh face to face just before Moses left his presence in great anger. It announced that every firstborn in the land of Egypt would die, from the heir of Pharaoh sitting upon his throne to the child of the maidservant behind the mill, together with all the firstborn of beasts, so that the stroke would reach every level of Egyptian society. About midnight God Himself would go out into the midst of Egypt to execute the stroke. His angel would slay Egypt's firstborn if Pharaoh continued to refuse to let Israel go.
The threat followed stubborn refusal to obey God, and specifically refusal to let Israel go and take their cattle with them. Repentance after the earlier losses could have spared the children, yet Pharaoh humbled not himself. It would be the last plague, coming closer than those before it and more dreadful than any that had yet visited Egypt, bringing a great cry throughout the land. The plague would make a distinction between Egypt and Israel, whose children would be spared in distinguished mercy. The threatened loss of Pharaoh's son answers God's claim that Israel is His firstborn son, honored with special privileges among the nations.
Faithfully portraying its nature and effects enraged Pharaoh, because he could not intimidate Moses or make him tremble before kingly authority. Moses stood fearless because he leaned for support on a mightier arm than that of any earthly monarch. It foretold that Egypt's great counselors would bow down before Moses and entreat Israel to leave. This was fulfilled when the firstborn lay dead in every house, which compelled the rebellious monarch to relinquish his grasp on his bondmen. As the final judgment it shows that Jehovah is the God who executes judgment on the earth, executing His judgments upon Pharaoh and upon the gods of Egypt. It proved His power superior to all other power, and declared His name and divine power and justice to all nations. It was designed to strengthen the faith of His people so that their posterity would worship Him alone.