Here is the passage in question.
Revelation 9:13-15 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 8:13 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). 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 2:14) is the lot of the devil. They are bound until they are released for their special purpose. The only other creature bound in the book of Revelation is the devil who is bound in the bottomless pit for a thousand years (Revelation 20:2). Other fallen angels have been delivered "into chains of darkness, to be reserved unto judgment" (2Peter 2:4). The four angels bound in the Euphrates are certainly fallen angels.
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 15:18; Deuteronomy 1:7). Those crossing the Euphrates going east were definitely leaving the promised land. Second, the Euphrates River is especially connected to Babylon (2Kings 24:7; Jeremiah 46:2) and Babylon is prominent in the book of Revelation (Revelation 14:8; 16:19: 17:5; 18:2, 10, 21). Beyond this, much is speculation. Are these fallen angels connected to Mystery Babylon? Did they commit earlier sins for which they were bound? I cannot tell. We are told what part God wants us to know at this time. The rest will be revealed when God sees fit.