And I saw an ANGEL coming down from HEAVEN
Who this ANGEL is, we aren't told, but given the task he is to perform (next verse), he must be very strong like the archangel, Michael (Revelation 12:7, Daniel 10:10-17, Daniel 12:1-4, and Jude 5-9-16).
The phrase 'coming down from HEAVEN' implies that this takes place on the earth. Recall that since Revelation 12:9, where archangel Michael threw 'the great dragon' down to the earth, the devil (dragon) has been relegated to doing his dirty work down here (Revelation 12:10).
having
When you have the KEY, you control who goes in or who comes out. More than that, the KEY carries with it the AUTHORITY to do the binding of the devil (next verse).
The ABYSS is a temporary place of incarceration for demons. As with the rest of the Revelation, it is both physical and symbolic. Physically, past scholars have indicated that the ABYSS was in the depths of the ocean. That thought has some merit, as we have previously made the case that water is the container of spirits. It was not a coincidence that "THE FLOOD" was used to purge the earth of evil mankind during Noah's time (Genesis 7:6-24).
From a symbolic or spiritual viewpoint, the ABYSS was an awful place even for demons. The demons who were operating on the earth in Jesus' time begged Jesus to not send them to the ABYSS (Luke 8:26-31-39). During the fifth trumpet (Revelation 9:1-12), some of the demons from the bottomless pit were released back onto the earth. Whether their leader, Abaddon Revelation 9:11) came back with them is not known. We do know that the devil will be bound and spend 1000 years in the ABYSS.
That this is a GREAT CHAIN implies that it is stronger than any evil prank or evil accusation the devil can concoct. When one reads the scriptures, we become aware that evil demons comes in varying strengths. The disciples were surprised that they were unable to take authority (due to lack of faith, Jesus said) over some demons (Matthew 17:14-21). Here we have the leader of the demonic world, and obviously he has much evil within him. As Isaiah 14:1-14-23 puts it, this devil (king of Babylon) thought himself as powerful as God. Thus it will take a GREAT CHAIN to hold him.
From a duality viewpoint, it is difficult to know what the GREAT CHAIN is physically. Many scholars say this is symbolic only; yet, strong binding forces at a molecular level could act like a GREAT CHAIN. Physicists are well aware how "sticky" water molecules are......just try to create a perfectly dry container. It is near impossible on earth.
Symbolically, it is much easier to accept that the GREAT CHAIN is that which stops evil from existing/controlling the earth. Believers should be well aware of their STAND against evil (Ephesians 6:10-11-20). Individually, we know that we can bind and lose things on earth (and in heaven - Matthew 16:13-19-20). But to bind the devil will take an ANGEL with a GREAT CHAIN.