Exodus 10:20
“But the Lord hardened Pharaoh’s heart, so that he would not let the children of Israel go.”
And he went out from Pharaoh, and intreated the Lord.
And the Lord turned a mighty strong west wind, which took away the locusts, and cast them into the Red sea; there remained not one locust in all the coasts of Egypt.
But the Lord hardened Pharaoh’s heart, so that he would not let the children of Israel go.
And the Lord said unto Moses, Stretch out thine hand toward heaven, that there may be darkness over the land of Egypt, even darkness which may be felt.
And Moses stretched forth his hand toward heaven; and there was a thick darkness in all the land of Egypt three days:
Counter-Arguments
The strongest case that this verse does not belong in this theme.

Cross-References
“And the Lord said unto Moses, When thou goest to return into Egypt, see that thou do all those wonders before Pharaoh, which I have put in thine hand: but I will harden his heart, that he shall not let the people go.”
“And he hardened Pharaoh’s heart, that he hearkened not unto them; as the Lord had said.”
“And the Lord said unto Moses, Pharaoh’s heart is hardened, he refuseth to let the people go.”
“Therefore hath he mercy on whom he will have mercy, and whom he will he hardeneth.”
“And he hardened Pharaoh’s heart, that he hearkened not unto them; as the Lord had said.”