Exodus 10:6
And they shall fill thy houses, and the houses of all thy servants, and the houses of all the Egyptians; which neither thy fathers, nor thy fathers’ fathers have seen, since the day that they were upon the earth unto this day. And he turned himself, and went out from Pharaoh.
The verse is part of the warning delivered to Pharaoh by divine command that if he refused to let Israel go, a grievous plague of locusts would be brought into the coasts of Egypt. Moses and Aaron gave that warning in obedience to God, announcing what was about to come upon the land if Pharaoh would not release His people. The locusts would fill the houses of Pharaoh himself, the houses of his servants, and the houses of all the Egyptians, even entering the palace itself. They would cover the face of the earth and eat the residue of that which had escaped the hail, consuming every green thing that remained in the land. In scale the scourge would be unprecedented, beyond anything seen by their fathers or their fathers' fathers since they were upon the earth, with no generation in Egypt's remembered history having known such a plague.
The warning left Pharaoh with a plain choice, permitted to him by God, either to humble himself before God and let Israel go or to refuse and suffer the effects of the plague. At the same time it manifested God's power to confirm the faith of Israel in Him as the only true and living God. It gave unmistakable evidence of the difference He placed between the Egyptians and His people, showing that although Israel had been bound down by hard labor and despised, He had chosen them as His peculiar people and would work wonderfully for their deliverance so that all nations would know. It was given so that Israel could tell in the ears of their sons and their grandsons what He wrought in Egypt and the signs He did among them, that they might know that He is the LORD.
The threatened plague came at Moses' command and withdrew at his bidding. Its coming and going exposed the impotence of Serapis, in whom the Egyptians trusted to protect the country from locusts, showing that protection comes from Jehovah and not from idols.