‘They have over them as king the angel of the abyss. His name in Hebrew is Abaddon, and in Greek he has the name Apollyon (‘Destroyer').'
‘Abaddon, meaning ‘destruction', occurs six times in the Old Testament (Job 26:6; Job 28:22; Job 31:12; Psalms 88:11; Proverbs 15:11; Proverbs 27:20) as a synonym for Sheol (the grave, the place of the dead), or of Death. It is the ultimate destruction. Here it is used of the prince of Death. Whether it is Satan or one of his leading princes is not too important. But as Satan was bound there (see on Revelation 20:3) Satan may be seen as a prime contender.
The Greek means ‘the one who destroys'. (This drawing attention to the Hebrew or Greek is typical of John (John 5:2; John 19:13; John 19:17; John 19:20; John 20:16)). Those who follow him go to destruction. But the fact that Jesus descended to ‘the abyss', to be raised from there (Roman Revelation 10:7), warns against making it specific to evil spirits and fallen angels. It is the land of the dead, the land of emptiness. It includes the place in which the fallen angels are ‘bound'. The fact that Jesus ‘descended' there does not mean there is a world below peopled by some kind of ghosts. It was ‘below' because graves are below the surface. Apart from that it is ‘below' only in spiritual terms, i.e. a retrograde place.