At the sounding of the fifth angel, a star falls from heaven, to whom is given the key of the bottomless pit, 1. He opens the pit, and there come forth locusts like scorpions, 2-11. The first woe past, 12. The sixth trumpet sounds, 13. Four angels who were bound are let loose. 14-21.
1 And the fifth angel sounded, and I saw a star fall from heaven to the earth. And the key of the bottomless pit was given to him. 2 And he opened the bottomless pit, and there arose a smoke out of the pit, as the smoke of a great furnace. And the sun and the air were darkened by reason of the smoke of the pit. 3 And there came out of the smoke locusts upon the earth, and power was given to them, as the scorpions of the earth have power. 4 And it was said to them that they should not hurt the grass of the earth, neither any green thing, neither any tree, but only those men who do not have the seal of God in their foreheads. 5 And to them it was given that they should not kill them, but that they should be tormented five months. And their torment was as the torment of a scorpion when he strikes a man. 6 And in those days men shall seek death and shall not find it. And they shall desire to die, but death shall flee from them.
7 And the shapes of the locusts were like horses prepared for battle. And on their heads were something like crowns like gold, and their faces were as the faces of men. 8 And they had hair as the hair of women, and their teeth were as the teeth of lions. 9 And they had breast-plates, like breast-plates of iron. And the sound of their wings was as the sound of chariots of many horses running to battle. 10 And they had tails like scorpions, and there were stings in their tails. And their power was to hurt men five months. 11 And they had a king over them, who is the angel of the bottomless pit, whose name in the Hebrew language is Abaddon, but in the Greek language he has his name: Apollyon.
12 One woe is past, and behold, there are still two more woes coming hereafter.
13 And the sixth angel sounded, and I heard a voice from the four horns of the golden altar which is before God, 14 Saying to the sixth angel who had the trumpet, "Release the four angels who are bound in the great Euphrates River." 15 And the four angels were released who were prepared for that hour, day, month, and year to slay the third part of men. 16 And the number of the army of the horsemen was two hundred million. And I heard their number. 17 And thus I saw the horses in the vision and those who sat on them, having breast-plates of fire, of jacinth, and brimstone. And the heads of the horses were as the heads of lions, and out of their mouths issued fire, smoke, and brimstone. 18 By these three the third part of men was killed: by the fire, by the smoke, and by the brimstone which issued out of their mouths. 19 For their power is in their mouth and in their tails. For their tails are like serpents and have heads, and with them they do harm.
20 And the rest of the men who were not killed by these plagues still did not repent of the works of their hands, that they should not worship demons, idols of gold, silver, brass, stone, and of wood, which neither can see, hear, nor walk, 21 Neither did they repent of their murders, of their sorceries, of their lewdness, nor of their thefts.
Matthew Henry Commentary - Revelation, Chapter 9[➚]
[v.4] - "the seal of God in their foreheads" - Compare with Deuteronomy 6:8 which instructs that the words and commandments of God should be placed on the hands and as "frontlets between your eyes"—between the eyes being in the region of the forehead. Then, think of the words and commandments of God as a seal.
[v.20] - Reference, Deuteronomy 4:28; Psalm 115:4-8, 135:15-18; Daniel 5:23.