And after these things I saw another angel come down from heaven; after [omitting "and"]… coming down, etc. The usual form of introduction to a new vision (cf. Revelation 4:1; Revelation 7:1, etc.). The "mystery" of the beast and the harlot having been declared, the angel now describes the doom in store for them.
The angel is from heaven, as carrying the news of the judgment which is sent from heaven (cf. Revelation 10:1; Revelation 19:6, Revelation 19:15, Revelation 19:17; Revelation 15:1, etc.). Having great power; and the earth was lightened with his glory.
The great "authority" refers to the judgment which follows, which, however, is not acted out before the seer, but only described. The last clause records the visible manifestation of the great power (cf.
the description in Ezekiel 43:2).