"They have as king over them, the angel of the abyss; his name in Hebrew is Abaddon, and in the Greek he has the name Apollyon" (Revelation 9:11).
Both Abaddon and Apollyon mean "destroyer." It is the angel (or messenger) of the bottomless pit.
"Then the fifth angel sounded, and I saw a star from heaven which had fallen to the earth; and the key of the bottomless pit was given to him. He opened the bottomless pit, and smoke went up out of the pit, like the smoke of a great furnace; and the sun and the air were darkened by the smoke of the pit" (Revelation 9:1-2).
The fallen star is allowed to open the bottomless pit, from which a vast horde is released to torment those who did not follow God (Revelation 9:4-5).
One question is whether the star with the key to the pit is the same entity that leads the horde. It is a possibility. Stars represented God’s messengers as seen in Revelation 1:20, but there it was seven stars to represent the seven messengers of the seven churches. Continuing the idea of a star representing a messenger or an important figure, we know that Satan is referred to as the ruler of this world (Ephesians 2:2; John 12:31). Satan was cast out of heaven (Revelation 12:9; Luke 10:18). He is also a destroyer and a liar (John 8:44).
"Then I saw an angel coming down from heaven, holding the key of the abyss and a great chain in his hand. And he laid hold of the dragon, the serpent of old, who is the devil and Satan, and bound him for a thousand years; and he threw him into the abyss, and shut it and sealed it over him, so that he would not deceive the nations any longer, until the thousand years were completed; after these things he must be released for a short time" (Revelation 20:1-3).
An angel who has the key to the abyss binds Satan and sends him to the abyss (bottomless pit). You assume that the angel is the same person as Apollyon, the king leading the horde. However, it would make more sense to say the angel is the same as the fallen star because both temporarily hold the key to the abyss. Then the king of the horde could be the symbolic equivalent of Satan.
Notice that either view links Apollyon with Satan.