“And 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. And 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. And out of the smoke came forth locusts upon the earth; and power was given them, as the scorpions of the earth have power. And they were told that they should not hurt the grass of the earth, nor any green thing, nor any tree, but only the men who do not have the seal of God on their foreheads. And they were not permitted to kill anyone, but to torment for five months; and their torment was like the torment of a scorpion when it stings a man.” ~Revelation 9:1-5
This fallen star from heaven is Satan.
“How you have fallen from heaven, O star of the morning, son of the dawn! You have been cut down to the earth, you who have weakened the nations!” ~Isaiah 14:12
“And He said to them, ‘I was watching Satan fall from heaven like lightning.'” ~Luke 10:18
The key, which represents power and authority, to the bottomless pit was given to him by God. God has full control of all the events happening upon the earth. When Satan opens the pit, the locusts are given special limits on what they can and cannot do. They are only allowed to do what God allows them to do. And they are instructed to leave the believers- those with “the seal of God on their foreheads”-alone. The Tribulation saints who are still alive will be protected from this woe.
These locusts are demons.
“For if God did not spare the angels when they sinned, but cast them into hell and committed them to pits of darkness, reserved for judgment;” ~2 Peter 2:4
When Jesus cast the demons out of the Gerasene man, He spoke with those demons- “And Jesus asked him, ‘What is your name?’ And he said, ‘Legion’; for many demons had entered him. And they were entreating Him not to command them to depart into the abyss.” ~Luke 8:30-31
We are told “the appearance of the locusts was like horses prepared for battle; and on their heads, as it were, crowns like gold, and their faces were like the faces of men. And they had hair like the hair of women, and their teeth were like the teeth of lions. And they had breastplates like breastplates of iron; and the sound of their wings was like the sound of chariots, of many horses rushing to battle. And they have tails like scorpions, and stings; and in their tails is their power to hurt men for five months.” ~Revelation 9:7-10. These are definitely not ordinary locusts.
In verse 11, John tells us they have a king over them. The insect locusts have no king, according to Proverbs 30:27~ “The locusts have no king, yet all of them go out in ranks;”
This king is the angel of the abyss, named Apollyon in Greek, Abaddon in Hebrew. Apollyon means “the angel of the bottomless pit, the Destroyer”. Abaddon means “the name of the angel-prince of the infernal regions, the minister of death and the author of havoc on the earth”. We are told that Satan is seeking to destroy us, and he is the prince of this world. It sounds to me like this king is indeed Satan.
“The first woe is past; behold, two woes are still coming after these things.” ~Revelation 9:12