Explanation and summary
The ninth plague of the 10 plagues that the God of the Israelites sent as judgment upon Egypt, through Moses, was a covering of darkness, as recorded in the book of Exodus. In this plague, complete darkness covers Egypt for three days. The Egyptian gods this plague insulted include Re (or Amun, the sun god and chief deity of Thebes) and perhaps others as well.
The progress of the seasons has been quite marked from the first plague, just before the rising of the waters, on through the year until now the khamsin period has come. When this dreadful scourge comes with its hot sand-laden breath, more impenetrable than a London fog, it is in very truth a "darkness which may be felt."
The dreadful horror of this monster from the desert can hardly be exaggerated. Once again Pharaoh said "Go," but this time he wished to retain the flocks and herds, a hostage for the return of the people (Exodus 10:24). Upon Moses' refusal to accept this condition, he threatened his life. Why had he not done so ere this? Why, indeed, did he let this man Moses come and go with such freedom, defying him and his people in the very palace?
Biblical Text (KJV)
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.
22 And Moses stretched forth his hand toward heaven; and there was a thick darkness in all the land of Egypt three days:
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.
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.
25 And Moses said, Thou must give us also sacrifices and burnt offerings, that we may sacrifice unto the Lord our God.
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.
27 But the Lord hardened Pharaoh's heart, and he would not let them go.
28 And Pharaoh said unto him, Get thee from me, take heed to thyself, see my face no more; for in that day thou seest my face thou shalt die.
29 And Moses said, Thou hast spoken well, I will see thy face again no more.
Probably Moses' former career in Egypt explains this. If, as is most probable, he had grown up at court with this Merenptah, and had been known as "the son of Pharaoh's daughter," heir to the throne and successor to Rameses II, instead of Merenptah, then this refugee had undoubtedly many friends still in Egypt who would make his death a danger to the reigning Pharaoh.
International Standard Bible Encyclopedia, which is in the public domain (with minor edits).