The phrase “was given” is in the passive voice. On the surface, then, we do not know who gave the key. Yet repeatedly in Scripture the passive is used to indicate that the Lord God is the one doing the action. The section in the book of Revelation dealing with the seven trumpets (Revelation 8:1 – 11:19) begins with a reference to the Lamb (Revelation 8:1), the one who took the scroll from God’s hand and so is exalted as King of kings (Revelation 5:1–14). Thus, behind all the action of Revelation 9:1–12 is the exalted Christ.
1 And the fifth angel blew his trumpet, and I saw a star fallen from heaven to earth, and he was given the key to the shaft of the bottomless pit.