Jamieson, Fausset and Brown on Revelation 16:10

10. angel--omitted by A, B, C, Vulgate, and Syriac. But Coptic and Andreas support it.

seat--Greek, "throne of the beast": set up in arrogant mimicry of God's throne; the dragon gave his throne to the beast (Re 13:2).

darkness--parallel to the Egyptian plague of darkness, Pharaoh being the type of Antichrist (compare Notes, see on Re 15:2, 3; compare the fifth trumpet, Re 9:2).

gnawed their tongues for pain--Greek, "owing to the pain" occasioned by the previous plagues, rendered more appalling by the darkness. Or, as "gnashing of teeth" is one of the accompaniments of hell, so this "gnawing of their tongues" is through rage at the baffling of their hopes and the overthrow of their kingdom. They meditate revenge and are unable to effect it; hence their frenzy [Grotius]. Those in anguish, mental and bodily, bite their lips and tongues.