Albert Barnes Commentary Revelation 16:10

"And the fifth poured out his bowl upon the throne of the beast; and his kingdom was darkened; and they gnawed their tongues for pain," — Revelation 16:10 (ASV)

And the fifth angel poured out his vial upon the seat of the beast. The previous judgments had been preparatory to this. They all had a bearing on this and were all preliminary to it; but the "seat"—the home, the center of the power of the beast—had not yet been reached. Here, however, a direct blow was aimed at that power, still not yet such as to secure its final overthrow, for that is reserved for the pouring out of the last vial, Revelation 16:17–21. All that is represented here is a heavy judgment which was merely preliminary to that final overthrow, but which affected the very seat of the beast.

The phrase "the seat of the beast"—ton yronon tou yhriou—means the seat or throne which the representative of that power occupied: the central point of the Antichristian dominion. Compare Barnes on Revelation 13:2.

See also Revelation 2:13. I understand this as referring to the very seat of the Papal powers, Rome—the Vatican.

And his kingdom was full of darkness. Confusion—disorder—distress; for darkness is often the emblem of calamity (Isaiah 59:9–10; Jeremiah 13:16; Ezekiel 30:18; Ezekiel 32:7–8; Ezekiel 34:12; Joel 2:2).

And they gnawed their tongues for pain. This is a "most significant expression of the writhings of anguish." The word rendered gnawed does not occur elsewhere in the New Testament, nor is the expression elsewhere used in the Bible; but its meaning is plain—it indicates deep anguish.