And I saw an angel come down from heaven, having the key of the bottomless pit and a great chain in his hand. An angel came down from heaven - One of the executors of the Divine justice, who receives criminals, and keeps them in prison, and delivers them up only to be tried and executed.
The key of the prison and the chain show who he is; and as the chain was great, it shows that the culprit was impeached of no ordinary crimes.