Exodus 11:5

11:5 and all the firstborn in the land of Egypt will die, from the firstborn son of Pharaoh ¹  who sits on his throne, to the firstborn son of the slave girl who is at her hand mill, and all the firstborn of the cattle.

Isaiah 47:2

47:2 Pick up millstones and grind flour!

Remove your veil,

strip off your skirt,

expose your legs,

cross the streams!

[11:5]  ¹ sn The firstborn in Egyptian and Israelite cultures was significant, but the firstborn of Pharaoh was most important. Pharaoh was considered a god, the son of Re, the sun god, for the specific purpose of ruling over Re’s chief concern, the land of Egypt. For the purpose of re-creation, the supreme god assumed the form of the living king and gave seed which was to become the next king and the next “son of Re.” Moreover, the Pharaoh was the incarnation of the god Horus, a falcon god whose province was the heavens. Horus represented the living king who succeeded the dead king Osiris. Every living king was Horus, every dead king Osiris (see J. A. Wilson, “Egypt,” Before Philosophy, 83-84). To strike any firstborn was to destroy the heir, who embodied the hopes and aspirations of the Egyptians, but to strike the firstborn son of Pharaoh was to destroy this cardinal doctrine of the divine kingship of Egypt. Such a blow would be enough for Pharaoh, for then he would drive the Israelites out.