Then the sixth angel poured out his bowl on the great Euphrates River. The river dried up, to provide a way for the kings who come from the east. 1
References for Revelation 16:12
Then I saw three unclean spirits that looked like frogs. They were coming out of the mouth of the dragon, the mouth of the beast, and the mouth of the false prophet.
They are the spirits of demons that perform miracles. These three spirits go out to all the kings of the world, to bring them together for the battle on the great Day of Almighty God.
"Listen! I am coming like a thief! Happy is he who stays awake and guards his clothes, so that he will not walk around naked and be ashamed in public!" 2
References for Revelation 16:15
Then the spirits brought the kings together in the place that in Hebrew is called Armageddon. 3
References for Revelation 16:16