Thus saith the LORD, About midnight will I go out into the 
midst of Egypt: 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 there shall be a great cry throughout all the land of Egypt, such as there was none like it, nor shall be like it any more.
But against any of the children of Israel shall not a dog move his tongue, against man or beast: that ye may know how that the LORD doth put a difference between the Egyptians and Israel.
Exodus 11:4-7
"Finally, the 10th plague, of death of the firstborn, was an
attack on the divinity of Pharaoh, whom the Egyptians believed was an
incarnation of the sun-god and of Osiris, the giver of life. 
---It was the
Pharaoh’s task to retain the favor of the gods and to uphold the laws
of Ma‘at, goddess of order.
---However, he was powerless to prevent the
death of his own son, the next-in-line ‘divine ruler’, or that of anyone
else’s son in the land of Egypt.
---Thus, Yahweh alone had absolute control of life and death."
CMI