'Key' in the Bible

"To the messenger of the church in Philadelphia, write: "The one who is holy, who is true, who has the key of David, who opens a door that no one can shut, and who shuts a door that no one can open, "says this:

When the fifth angel blew his trumpet, I saw a star that had fallen to earth from the sky. The star was given the key to the shaft of the bottomless pit.

Then I saw an angel coming down from heaven, holding the key to the bottomless pit, with a large chain in his hand.