Following the first woe, the sixth trumpet initiates the second woe.

Then the sixth angel sounded, and I heard a voice from the four horns of the golden altar which is before God, one saying to the sixth angel who had the trumpet, "Release the four angels who are bound at the great river Euphrates." (Rev 9:13-14)

When the sixth trumpet sounds, an angel next to the golden altar in the heavenly temple of God commands the angel of the sixth trumpet to release of four bound angels at the Euphrates River.

In contrast to the fifth trumpet which initiates an angel to go unlock the abyss (Rev 9:1), there is no mention of how the angel with the sixth trumpet releases the four bound angels at the Euphrates River.

While the location of the four bound angels is uncertain, it is likely not the abyss.

When the Bible speaks of confined or bound angels, it is always in the context of fallen evil angels (Luke 8:26-31; Matt 8:28-32; 2 Peter 2:4; Jude 1:6-7).

Just as with the fifth trumpet, God uses evil demonic instruments to carry out His judgment. Here the four bound fallen evil angels serve a divine purpose.

God's scheduled the release of the four bound angels to commence at the exact hour of a precise day of a certain month and of a specific year (Rev 9:15).

The four bound angels were limited to kill 1/3 of the population at that time. In consideration of the fourth seal, in which 25% of the population was killed, the death of 1/3 of the surviving 75% leaves at most 50% of the original population before the first seal was broken.

After the release of the four bound angels, the apostle John sees an army in his vision (Rev 9:16-17).

The armies of horsemen numbered two hundred million.

The riders had breastplates the color of fire, hyacinth (i.e. dark blue / purple), and brimstone (i.e. sulfurous yellow).

The heads of the horses were like the heads of lions.

Out of the horses proceeded fire, smoke and brimstone.

The tails of the horses were like serpents with heads from which they can do harm.

How human beings are killed is worth noting:

A third of mankind was killed by these three plagues, by the fire and the smoke and the brimstone which proceeded out of their mouths. For the power of the horses is in their mouths and in their tails; for their tails are like serpents and have heads, and with them they do harm. (Rev 9:18-19)

The riders did not kill anyone.

The Greek noun "plēgē," translated as "plague" does not place an emphasis on "disease" or "affliction;" instead the Greek meaning is "wound" as in a physical injury is in view. Here "plēgē" is a manifestation of God's wrath - the fire, smoke, and brimstone that proceeded from the mouths of the horses caused injuries that led to a quick death.

In the Septuagint (Greek Old Testament), "plēgē" is used as a means to rebuke and reprimand both Believers and non-Believers to turn to God.

Now the Lord said to Moses, "One more plague I will bring on Pharaoh and on Egypt; after that he will let you go from here. When he lets you go, he will surely drive you out from here completely." (Ex 11:1)

If also after these things you do not obey Me, then I will punish you seven times more for your sins. I will also break down your pride of power; I will also make your sky like iron and your earth like bronze. Your strength will be spent uselessly, for your land will not yield its produce and the trees of the land will not yield their fruit. If then, you act with hostility against Me and are unwilling to obey Me, I will increase the plague on you seven times according to your sins. (Lev 26:18-21)

As in the case of Pharaoh, when a heart is so hard, including those who profess a faith but in reality lack one, then God uses "plēgē" to exact His judgment of death.

If you are not careful to observe all the words of this law which are written in this book, to fear this honored and awesome name, the Lord your God, then the Lord will bring extraordinary plagues on you and your descendants, even severe and lasting plagues, and miserable and chronic sicknesses. He will bring back on you all the diseases of Egypt of which you were afraid, and they will cling to you. Also every sickness and every plague which, not written in the book of this law, the Lord will bring on you until you are destroyed. (Deut 28:58-61)

The apostle John observes that after the four bound angels complete their task of carrying out God's judgment, the surviving human beings, at most 50% of the original population before the first seal, do not recognize that the plagues confirm the existence of God and are unrepentant.

The rest of mankind, who were not killed by these plagues, did not repent of the works of their hands, so as not to worship demons, and the idols of gold and of silver and of brass and of stone and of wood, which can neither see nor hear nor walk; and they did not repent of their murders nor of their sorceries nor of their immorality nor of their thefts. (Rev 9:20-21).

This likely indicates that, at the end of the period defined by the sixth trumpet, there are no longer any Believers on earth; thus, there is no one left to evangelize and testify of God. And the seventh trumpet will sound soon…