A chain was used to restrain the demoniac of Mark 5:3–4 and used also to confine the movements of prisoners (Acts 12:6–7; Acts 21:33). In the present case a “great” chain is needed lest the angel’s intended captive do with the chain what the demoniac of Mark 5:4 did.
1 Then I saw an angel coming down from heaven, holding in his hand the key to the bottomless pit and a great chain.