Was and is not (ην κα ουκ εστιν). Imperfect and present of ειμ, an apparent antithesis to ο ην κα ο ων of Revelation 1:4. This is a picture of the beast of Revelation 13:1 which the woman is riding, but no longer just the empire, but one of the emperors who died (ουκ εστιν, is not).

And is about to come up out of the abyss (κα μελλε αναβαινειν εκ της αβυσσου). That is, he is going to come to life again.

And to go into perdition (κα εις απωλειαν υπαγε). So (and he goes into perdition) the best MSS. read rather than the infinitive υπαγειν. Most interpreters see here an allusion to the "Nero redivivus" expectancy realized in Domitian, who was ruling when John wrote and who was called Nero redivivus.

Shall wonder (θαυμασθησοντα). First future passive (deponent) of θαυμαζω, with which compare εθαυμασθη in Revelation 13:3. John had wondered (εθαυμασα) in verse Revelation 17:6 "with the amazement of a horrible surprise; the world will wonder and admire" (Swete).

Whose name (ων ονομα). Singular ονομα, like πτωμα in Revelation 11:8. See Revelation 13:8 for the same description of those who worship the beast and for discussion of details.

When they behold (βλεποντων). Genitive plural of the present active participle of βλεπω, agreeing with ων (genitive relative) rather than with ο κατοικουντες (nominative just before ων).

How that (οτ). "Namely that."

He was, and is not, and shall come (ην κα ουκ εστιν κα παρεστα). Repetition of what is in verse Revelation 17:7 with παρεστα (future of παρειμ, from which παρουσια comes) in place of μελλε, "parody of the divine name" (Charles) in Revelation 1:4; Revelation 1:8; Revelation 4:8, "as the hellish antitype of Christ." The Neronic Antichrist has also a παρουσια.