John’s eye is diverted from the sun to “the beast” (and the kings of the earth accompanying it). John’s use of the definite article “the” indicates that John expects his readers to be familiar with this beast. Given the wording of Revelation 16:13 we need to understand that this beast is the monster of Revelation 13:1, the first assistant to the dragon in his pursuit of the woman. This beast represents evil forces that lean against the depraved human heart to conspire against God and his holy purposes (see further on Revelation 13:1). That “the kings of the earth” and “their armies”—depraved men all—are in league with this beast is in step with Revelation 13:3, Revelation 13:7, and Revelation 16:12–13.
19 And I saw the beast and the kings of the earth with their armies gathered to make war against him who was sitting on the horse and against his army.