Exodus 10:15
For they covered the face of the whole earth, so that the land was darkened; and they did eat every herb of the land, and all the fruit of the trees which the hail had left: and there remained not any green thing in the trees, or in the herbs of the field, through all the land of Egypt.
The verse describes the dreadful plague of locusts that the Lord sent as a judgment on Egypt while Pharaoh chose not to submit to Him and let Israel go, choosing rather to receive the plagues and watching without remorse as his whole kingdom lay under the miracle of these dreadful judgments. Moses and Aaron warned Pharaoh beforehand that if he refused, locusts would be brought into the coasts of Egypt to cover the face of the earth and eat the residue that escaped the hail. Each plague that followed refusal came exactly as told, leaving Pharaoh free to humble himself and obey or refuse and suffer its effects.
The locusts were brought by an east wind blowing upon the land all day and all night, arriving in the morning to go up over all Egypt and rest in all its coasts. It was a visitation more grievous than any before or after, with none like it before or after. They covered the face of the whole earth so that the land was darkened and ate every herb of the land and all the fruit of the trees left by the hail until no green thing remained in the trees or herbs of the field through all Egypt. The plague nearly destroyed the land and left it ruined.
The completeness of the destruction made the Egyptians fear that after eating everything in the field the locusts would next attack and devour the people themselves. The devastation exposed the impotence of Serapis, whom the Egyptians believed protected the country from locusts, showing the folly of trusting any protection other than Jehovah. God bore with Pharaoh's continual rebellion and suffered his resistance in order to show His great signs and wonders before Pharaoh and before Israel. This was to confirm Israel's faith in Him as the only true and living God, to give unmistakable evidence of the difference He placed between the Egyptians and His people, and so that parents would tell their sons and grandsons what He wrought in Egypt that they might know He is the Lord.
While death threatened him, Pharaoh hastily summoned Moses and Aaron, confessed sin against God and against them, asked forgiveness only this once, and begged them to entreat God to take away this death. At Moses' intercession and entreaty God turned a mighty strong west wind that took away the locusts and cast them into the Red Sea, so that not one locust remained in all the coasts of Egypt. After relief Pharaoh broke his promise and refused to let the children of Israel go, persisting in rebellion without remorse and yielding only what he was compelled to yield by dreadful visitations. He first permitted sacrifice only in the land of Egypt, then permitted the men alone to go, then after the near-destruction by locusts permitted the children and wives also to go but withheld the cattle, after which Moses announced that the angel of God would slay the firstborn.
In one account, after relief from the plague Pharaoh's heart was hardened by the Lord. In another account, after relief Pharaoh himself hardened his heart again and refused to let Israel go despite his humility and promise while death threatened him.