A.T. Robertson 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)

Upon the throne of the beast (επ τον θρονον του θηριου). That is Rome (13:2). The dragon gave the beast his throne (2:13).

Was darkened (εγενετο εσκοτωμενη). Periphrastic past perfect passive with γινομα and σκοτοω (9:2). Like the darkness of the Egyptian plague (Exodus 10:22) and worse, for the effects of the previous plagues continue.

They gnawed their tongues (εμασωντο τας γλωσσας αυτων). Imperfect middle of μασαομα, old verb (to chew), from μαω (to knead), only here in N.T.

For pain (εκ του πονου). "Out of distress" (cf. εκ in 8:13), rare sense of old word (from πενομα to work for one's living), in N.T. only here, 21:4; Colossians 4:13. See Mt 8:12.