Opened (ηνοιξεν). First aorist active indicative of ανοιγνυμ. With the "key" (κλεις).

As the smoke of a great furnace (ως καπνος καμινου μεγαλης). The plague of demonic locusts is here turned loose. Καμινος is old word for a smelting-furnace, already in Revelation 1:15.

Were darkened (εσκοτωθη). First aorist passive indicative of σκοτοω, old causative verb from σκοτος, in N.T. only here, Revelation 16:10; Ephesians 4:18.

By reason of (εκ). "Out of," as a result of (Revelation 8:13).