Albert Barnes on Revelation 16:10

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 centre of the power of the beasts had not yet been reached. Here, however, there was a direct blow aimed at that power, still not such yet as to secure its final overthrow, for that is reserved for the pouring out of the last vial, Rev 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"-- τον θρονοντουθηριου--means the seat or throne which the representative of that power occupied; the central point of the Antichristian dominion. Compare Barnes on "Re 13:2" See also Rev 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, Isa 59:9-10, Jer 13:16 Eze 30:18, 32:7-8, 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.

(e) "seat of the beast" Rev 13:2-4 (f) "darkness" Rev 9:2