The sixth angel poured his bowl on the great river Euphrates, and its water was dried up in order to prepare the way for the kings from the east.
And I saw three foul spirits like frogs coming from the mouth of the dragon, from the mouth of the beast, and from the mouth of the false prophet.
These are demonic spirits, performing signs, who go abroad to the kings of the whole world, to assemble them for battle on the great day of God the Almighty.
("See, I am coming like a thief! Blessed is the one who stays awake and is clothed, not going about naked and exposed to shame.")
References for Revelation 16:15
And they assembled them at the place that in Hebrew is called Harmagedon.