Revelation 16:10 (ESV)

10 The fifth angel poured out his bowl on the throne of the beast, and its kingdom was plunged into darkness. People gnawed their tongues in anguish

We were first introduced to this beast in Revelation 13:1, that ugly creature from the sea having ten horns and seven heads. To this brute “the dragon gave his power and his throne and great authority” (Revelation 13:2). The dragon was the devil, Satan, defeated in heaven and hence cast to the earth (Revelation 12:7–9) where he raged against the children of the woman (that is, God’s church; Revelation 12:17). The beast is his collaborator to accomplish his sinister purpose. This beast used the authority and throne it received from the dragon to “make war on the saints and to conquer them” (Revelation 13:7).