The eighth plaque connects plaques 7-9 all together. It is finishing what the seventh plaque started by destroying all the plant life that the hail left. "Covering the face of the land" is a prelude to the darkness of the eighth plaque.
God reminds Moses that all that is happening isn't just to punish Egypt but to reveal who He was the one and only true Lord over all. God could have just told Moses to gather the people to leave and the Pharaoh couldn't have done anything to stop them. But He wanted to show what He was capable of and start to reveal His redemption plan for us thru Jesus.
The ninth plaque being a darkness so strong it could be felt for 3 full days. Yes, it was a warning to the death that was about to happen. I feel like it's more than that too. It's a warning to us now of what it would be like without the light of God in our lives. I know when it's really dark fear tries to take it's hold most. You hear things more, like every sound is amplified. Darkness feels less safe. We need the Light of Jesus shining in your lives to guide us from the darkness and save us. Three days of darkness had to be nearly unbearable, without Jesus as your Savior you face an eternity of darkness.