Darkness!

Exo 10:21 And the LORD said unto Moses, Stretch out thine hand toward heaven, that there may be darkness over the land of Egypt, even darkness which may be felt.
Exo 10:22 And Moses stretched forth his hand toward heaven; and there was a thick darkness in all the land of Egypt three days:
Exo 10:23 They saw not one another, neither rose any from his place for three days: but all the children of Israel had light in their dwellings.
Exo 10:24 And Pharaoh called unto Moses, and said, Go ye, serve the LORD; only let your flocks and your herds be stayed: let your little ones also go with you.
Exo 10:25 And Moses said, Thou must give us also sacrifices and burnt offerings, that we may sacrifice unto the LORD our God.
Exo 10:26 Our cattle also shall go with us; there shall not an hoof be left behind; for thereof must we take to serve the LORD our God; and we know not with what we must serve the LORD, until we come thither.
Exo 10:27 But the LORD hardened Pharaoh's heart, and he would not let them go.
to Exo 11:8