And the LORD hardened the heart of Pharaoh, and he hearkened not unto them; as the LORD had spoken unto Moses.

Wesley's Notes

9:12 Now the Lord hardened Pharaoh's heart - Before he had hardened his own heart, and resisted the grace of God, and now God justly gave him up to his own heart's lusts, to strong delusions, permitting Satan to blind and harden him. Wilful hardness is commonly punished with judicial hardness. Let us dread this as the sorest judgment a man can be under on this side hell.

Matthew Henry's Concise Commentary

9:8-12 When the Egyptians were not wrought upon by the death of their cattle, God sent a plague that seized their own bodies. If lesser judgments do not work, God will send greater. Sometimes God shows men their sin in their punishment. They had oppressed Israel in the furnaces, and now the ashes of the furnace are made a terror to them. The plague itself was very grievous. The magicians themselves were struck with these boils. Their power was restrained before; but they continued to withstand Moses, and to confirm Pharaoh in his unbelief, till they were forced to give way. Pharaoh continued obstinate. He had hardened his own heart, and now God justly gave him up to his own heart's lusts, permitting Satan to blind and harden him. If men shut their eyes against the light, it is just with God to close their eyes. This is the sorest judgment a man can be under out of hell.