Exploring John's Visionary Experience in Revelation

Part 2 of our in-depth look into the role played by the Old Testament in Revelation 3 focuses the otherworldly “keys” in Rev 3:7 (“who has the key of David, who opens and no one will shut, who shuts and no one opens” and its earlier counterpart, Rev 1:18 (“[I am] the living one. I died, and behold I am alive forevermore, and I have the keys of Death and Hades). What are these keys? What does the metaphor represent? Are Death and Hades places or entities, or both? How does earlier Old Testament material help us understand what John is saying?