The third [angel] poured out its vessel on the rivers and the fountains of waters, and there was made blood. And I heard the fourth angel saying: "You are just, who are, who was, the Holy one, because You have judged these things...And I heard another from the altar saying: Lord God almighty, true and just are your judgments.(Apocalypse 16:4-7)
Two of the angels mentioned in the text appear in the image, one pouring the vessel out into the rivers and the other standing before an altar and gesturing toward Saint John, who stands at the far left.