Plagues 8 & 9 were devastating for this reeling nation. They had already been through the ringer, but at this point God turned things up a notch. The locust plague was so terrible that, when the critters were gone, there were no green leaves left in Egypt. And, the plague of darkness was so severe that nobody dared to move from where they were for three solid days (Exodus 10:21-23).
For the first time Pharaoh actually considered acquiescing prior to a plague. When Moses just threatened to bring locusts, Pharaoh finally started discussing terms for surrender. Admittedly, he only did so under pressure from his court (Exodus 10:7), but clearly he was nearer to complying with God's man than he had been before. But, near is not far enough. When Moses refused to compromise God's plan for Pharaoh, the monarch would hear nothing of it and actually had the brothers driven away from him (Exodus 10:11). So, an east wind brought a swarm of locusts that was so large that the entire nation was literally coated with locusts. And then, when Pharaoh asked for mercy, a west wind blew the swarm into the Red sea. Incredibly (just like with the flies) not even one of those critters remained in Egypt after the plague.
But Pharaoh reneged again, so, a thick darkness descended upon the land. Surely as the people stayed put for three days they had plenty of time to meditate on the 9 afflictions that Moses had brought upon them. Yet their king remained immovably grounded in his cycle of foolishness ... even threatening to kill Moses if he ever came back into his court. Of course Moses agreed to stay away, but it wasn't because the death of Moses was on the horizon. God was about to reach out and afflict every household in Egypt with death. Finally, the nation was about to be coerced successfully.