Revelation 16:10,11

10  And the fifth angel poured out his vial upon the seat of the beast; and his kingdom was full of darkness; and they gnawed their tongues for pain,

11  And blasphemed the God of heaven because of their pains and their sores, and repented not of their deeds

When the fifth vial is poured out the kingdom of the Beast will be full of darkness. It will be a darkness that can be felt and they will gnaw their tongues for pain. There will be no light in the kingdom of the Beast because God will show the world that the Beast ruling the earth has no light in himself. An example of this type of darkness can be found in Exodus when God judged Egypt.

In Egypt, the darkness lasted three days; we are not told how long this darkness will last. The people in the Beast’s kingdom living with sores, heat, thirst and darkness will not repent of following Satan as God, or of their sins. They will instead blaspheme the God of heaven. 

With this plague of darkness God is graphically demonstrating to the people on the earth who have not yet worshiped the Beast or taken his mark what hell will be like for eternity if they do. They must by their own free will call on the God of heaven for mercy and salvation and accept Jesus to be saved from Satan. If they did, God would have his angels protect them from the plagues yet to come. Yet, most left on earth will continue to believe the lie that Satan is God.