Important Note: The critical text seems to have preserved the text better here. The star is already "fallen"; this positions it as likely an angel that fell prior to this moment, or as the Wormwood star. Let's explore those two options, but note that stars in Revelation can be literal stars, humans (women, 12 stars in Revelation 12), and heavenly bodies such as Satan and his angels.

Option 1: Wormwood — Literal stars are very unlikely to be given keys to the bottomless pit — the bottomless pit is connected to spiritual beings. Unless the wormwood star is an angel, it then seems unlikely the star here is that star.

Option 2: Fallen Angel — This seems more likely given the spiritual nature of the bottomless pit.

Is this fallen star, Wormwood, or a fallen angel? Or, maybe it is both? I'll let you consider it. But who the angel is will not be the point. The main focus of the 5th trumpet is the locusts.

When the locusts come out of the bottomless pit, it is also fitting for them to be demonic in nature, as they seem to come from a supernatural place – the bottomless pit. Yet it should not be ignored that the locusts are linked to a material world by their ability to sting men. So, the spiritual and material worlds meet at the 5th trumpet and first woe.

These locusts plague the earth for five months and only touch "those men which have not the seal of God in their foreheads". This implies the 144,000 are present at this time.

9And the fifth messenger blew, and I saw a star fallen from heaven unto the earth: and to him was given the key [r.0] of the shaft of the bottomless pit [r.1] [r.2].
[lex]

“Shaft of the abyss” (τὸ φρέαρ τῆς ἀβύσσου) — φρέαρ (phrear) denotes a “well,” “pit,” or more precisely a vertical shaft or opening, not the abyss itself. The phrase describes an access point leading to the abyss (ἄβυσσος). The term ἄβυσσος (abyssos) means “deep,” “unfathomable,” or “bottomless,” referring to a depth without measure rather than a “pit” as such. The traditional English rendering “bottomless pit” (e.g., KJV) reflects this idea interpretively and has been retained due to longstanding usage, though “abyss” more directly represents the Greek term.

[r.0]

Re 1:18, Re 3:7, Re 9:1, Re 20:1 — This key is given to the star not owned by it.

[r.1]

Re 9:1, Re 9:2, Re 9:11, Re 11:7, Re 17:8, Re 20:1, Re 20:3

[r.2]

Re 9:1, Re 20:1 — Key of the bottomless pit.

[time]
Trumpet 5
2 And he opened the the shaft of the bottomless pit [r.3]; and there ascended a smoke out of the pit, as the smoke of a great furnace; and the sun and the air were darkened [r.4] from the smoke of the pit.
[r.3]

Re 9:1, Re 9:2, Re 9:11, Re 11:7, Re 17:8, Re 20:1, Re 20:3

[r.4]

Re 9:2, Re 16:10

3 And there came out of the smoke locusts upon the earth: and unto them was given power, as the scorpions of the earth have power.
4 And it was commanded them that they should not hurt the grass of the earth, neither any green thing, nor any tree; but only those men who have not the seal of God in their foreheads [r.5] [r.6] [r.7].
[r.5]

Re 9:4, Re 16:2 — Seal of God to mark of beast pivot. In Rev 9:4, the 144,000 appear to be present, and in Rev 16:2, they are not because the mark of the beast is the identifiable mark.

[r.6]

Re 7:4, Re 9:4, Re 11:14, Re 14:1, Re 22:4

[r.7]

Re 7:3, Re 9:4, Re 13:16, Re 14:1, Re 14:9, Re 17:5, Re 20:4, Re 22:4

5 And to them it was given that they should not kill them, but that they should be tormented five months [r.8]: and their torment was as the torment of a scorpion, when he strikes a man.
[r.8]

Re 9:5, Re 9:10

6 And in those days shall men seek death, and shall not find it; and shall desire to die, and death shall flee from them.
7 And the appearance of the locusts was like unto horses prepared unto war; and on their heads were as it were 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 breastplates, as it were breastplates of iron; and the sound of their wings was as the sound of chariots of many horses running to war.
10 And they had tails like unto scorpions, and there were stings in their tails: and their power was to hurt men five months [r.9].
[r.9]

Re 9:5, Re 9:10

11 And they had a king over them, who is the messenger of the bottomless pit [r.10], whose name in the Hebrew tongue is Abaddon, but in the Greek tongue his name is Apollyon.
[r.10]

Re 9:1, Re 9:2, Re 9:11, Re 11:7, Re 17:8, Re 20:1, Re 20:3

12 One woe is past; and, behold, there come two woes more hereafter.

Important Note: Again, we seem to be in the theme of supernatural plagues and yet still literal ones too. The spiritual and material world meet here again. These are a judgment on the whole earth as it says "the third part of men". If this is not the whole world, the scope points to those men prior "which have not the seal of God in their foreheads".

13 And the sixth messenger sounded, and I heard a voice from the four horns of the golden altar which is before God [r.11] [r.12],
[r.11]

Re 8:3-5, Re 9:13, Re 14:18, Re 16:7

[r.12]

Re 9:13, Re 16:7

[time]
Trumpet 6
14 Saying to the sixth messenger, who had the trumpet, Loose the four messengers who are bound in the great river Euphrates [r.13].
[r.13]

Re 9:14, Re 16:12

15 And the four messengers were loosed, who were prepared for an hour, and a day, and a month, and a year, to slay the third part of men [r.14].
[r.14]

Re 9:15, Re 9:18

16 And the number of the army of the horsemen was two hundred thousand thousand: and I heard the number of them. [r.15]
[tr]

ESV, "The number of mounted troops was twice ten thousand times ten thousand; I heard their number." The ESV renders the phrase idiomatically ("twice ten thousand times ten thousand"), preserving the apocalyptic numeric form of δισ μυριαδες μυριαδων. However, the added clause "I heard their number" (ηκουσα τον αριθμον αυτων) indicates an explicitly enumerated total, not a vague expression of magnitude. This supports the KJV's inference of a precise count, even while the Greek presents that count in a stylized, idiomatic form.

[r.15]

Re 9:16, Joel 2:11 — Not the army of Joel 2:11; it is the army of the LORD, not four angels: "The LORD utters his voice before his army". Joel 2:11 better fits 19:14.

17 And this I saw the horses in the vision, and those who sat on them, having breastplates of fire, and of jacinth, and brimstone: and the heads of the horses were as the heads of lions; and out of their mouths issued fire and smoke and brimstone.
18 By these three [plagues] was the third part of men killed [r.16], by the fire, and by the smoke, and by the brimstone, which issued out of their mouths.
[fn]

In the TR/KJV the word "plagues" (πληγή) does not appear here, but it is present in the TR at 9:20.

[r.16]

Re 9:15, Re 9:18

19 For their power is in their mouth, and in their tails: for their tails were like unto serpents, and had heads, and with them they do hurt.
20 And the rest of the men who were not killed by these plagues yet repented not of the works of their hands [r.17], that they should not worship demons, and idols of gold, and silver, and brass, and stone, and of wood: which neither can see, nor hear, nor walk:
[r.17]

Re 9:20, Re 14:13, Re 18:6, Re 20:12-13, Re 22:12

21 Neither repented they of their murders, nor of their sorceries, nor of their fornication, nor of their thefts [r.18].
[r.18]

Re 9:21, Re 21:8, Re 22:15