A.T. Robertson Commentary Revelation 9:1

"And the fifth angel sounded, and I saw a star from heaven fallen unto the earth: and there was given to him the key of the pit of the abyss." — Revelation 9:1 (ASV)

Fallen (πεπτωκοτα). Perfect active participle of πιπτω, already down. In Lu 10:18 note πεσοντα (constative aorist active, like a flash of lightning) after εθεωρουν and in Re 7:2 note αναβαινοντα (present active and linear, coming up, picturing the process) after ειδον.

Of the pit of the abyss (του φρεατος της αβυσσου). Αβυσσος is an old adjective (alpha privative and βυθος, depth, without depth), but η αβυσσος (supply χωρα place), the bottomless place. It occurs in Ro 10:7 for the common receptacle of the dead for Hades (Sheol), but in Lu 8:31 a lower depth is sounded (Swete), for the abode of demons, and in this sense it occurs in Re 9:1,2,11; 11:7; 17:8; 20:1,3. Φρεαρ is an old word for well or cistern (Luke 14:5; John 4:11f.) and it occurs in Re 9:1f. for the mouth of the abyss which is pictured as a cistern with a narrow orifice at the entrance and this fifth angel holds the key to it.