Upon the throne of the beast (epi ton thronon tou thēriou). That is Rome (Rev 13:2). The dragon gave the beast his throne (Rev 2:13).
Was darkened (egeneto eskotōmenē). Periphrastic past perfect passive with ginomai and skotoō (Rev 9:2). Like the darkness of the Egyptian plague (Exo 10:22) and worse, for the effects of the previous plagues continue.
They gnawed their tongues (emasōnto tas glōssas autōn). Imperfect middle of masaomai, old verb (to chew), from maō (to knead), only here in N.T.
For pain (ek tou ponou). “Out of distress” (cf. ek in Rev 8:13), rare sense of old word (from penomai to work for one’s living), in N.T. only here, Rev 21:4; Col 4:13. See Mat 8:12.