Please explain "Loose the four angels which are bound in the great river Euphrates. Revelation 9:14Saying to the sixth angel which had the trumpet, Loose the four angels which are bound in the great river Euphrates.
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Here is the passage in question.

Revelation 9:13-15 [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 which had the trumpet, Loose the four angels which are bound in the great river Euphrates. [15] And the four angels were loosed, which were prepared for an hour, and a day, and a month, and a year, for to slay the third part of men.
See All... And the sixth angel sounded, and I heard a voice from the four horns of the golden altar which is before God, Saying to the sixth angel which had the trumpet, Loose the four angels which are bound in the great river Euphrates. And the four angels were loosed, which were prepared for an hour, and a day, and a month, and a year, for to slay the third part of men.

This passage deals with the sixth trumpet and what is called the second woe. The three woes refer to Revelation 8:13And I beheld, and heard an angel flying through the midst of heaven, saying with a loud voice, Woe, woe, woe, to the inhabiters of the earth by reason of the other voices of the trumpet of the three angels, which are yet to sound!
See All... where an angel cries, "Woe, woe, woe, to the inhabiters of the earth by reason of the other voices of the trumpet of the three angels, which are yet to sound!" The last three trumpets bring special woes to the inhabitants of the earth. The second of these woes deals with the loosing of four very special angels.

Notice several things about these four angels. First, their purpose is to slay the third part of mankind. Their loosing sends forth an army of two hundred million horsemen (Revelation 9:16-18 [16] And the number of the army of the horsemen were two hundred thousand thousand: and I heard the number of them. [17] And thus I saw the horses in the vision, and them that 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 was the third part of men killed, by the fire, and by the smoke, and by the brimstone, which issued out of their mouths.
See All...). It is difficult to determine whether these horses and riders are supernatural or marvels of modern technology. But it is clear that they are deadly and that they fulfill their purpose. Second, these angels are prepared for a very special time. They are to be released at the very year, month, day, and hour for which they are prepared. Third, they are bound until the time of the purpose for which they are ordained. Fourth, they are bound in the great river Euphrates. Let's see if we can make sense out of this.

These angels are certainly fallen angels. Their sole purpose is to kill. This "power of death" (Hebrews 2:14Forasmuch then as the children are partakers of flesh and blood, he also himself likewise took part of the same; that through death he might destroy him that had the power of death, that is, the devil;
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See All...). Other fallen angels have been delivered "into chains of darkness, to be reserved unto judgment" (2Peter 2:4For if God spared not the angels that sinned, but cast them down to hell, and delivered them into chains of darkness, to be reserved unto judgment;
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Why they are bound in the Euphrates is a bit of a mystery. A couple of details about the Euphrates are significant. First, the Euphrates was the farthest eastern boundary of the land promised to Israel (Genesis 15:18In the same day the LORD made a covenant with Abram, saying, Unto thy seed have I given this land, from the river of Egypt unto the great river, the river Euphrates:
See All...; Deuteronomy 1:7Turn you, and take your journey, and go to the mount of the Amorites, and unto all the places nigh thereunto, in the plain, in the hills, and in the vale, and in the south, and by the sea side, to the land of the Canaanites, and unto Lebanon, unto the great river, the river Euphrates.
See All...). Those crossing the Euphrates going east were definitely leaving the promised land. Second, the Euphrates River is especially connected to Babylon (2Kings 24:7And the king of Egypt came not again any more out of his land: for the king of Babylon had taken from the river of Egypt unto the river Euphrates all that pertained to the king of Egypt.
See All...; Jeremiah 46:2Against Egypt, against the army of Pharaohnecho king of Egypt, which was by the river Euphrates in Carchemish, which Nebuchadrezzar king of Babylon smote in the fourth year of Jehoiakim the son of Josiah king of Judah.
See All...) and Babylon is prominent in the book of Revelation (Revelation 14:8And there followed another angel, saying, Babylon is fallen, is fallen, that great city, because she made all nations drink of the wine of the wrath of her fornication.
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