'Key' in the Bible

“Write to the angel of the church in Philadelphia:“The Holy One, the True One, the One who has the key of David, who opens and no one will close, and closes and no one opens says:

The fifth angel blew his trumpet, and I saw a star that had fallen from heaven to earth. The key to the shaft of the abyss was given to him.

Then I saw an angel coming down from heaven with the key to the abyss and a great chain in his hand.