The Holy Bible, Revelation 16:12
12 And the sixth angel poured out his vial upon the great river Euphrates; and the water thereof was dried up, that the way of the kings of the east might be prepared.
The Holy Bible, Revelation 9:13-18
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.
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.
Journal of Discourses, Orson Pratt 16:43
Read also concerning the sounding of the sixth trump, and you will find that there is a great work to be accomplished before the seventh angel shall sound, for in the time intervening between the sounding of the sixth and seventh trumps the four angels which are bound in the great river Euphrates are to be loosed, and they are to gather together a very great army. If I recollect aright, that army is to consist of two hundred millions of people, who are to ride on some kind of beasts or animals which the Lord, probably by some supernatural means, will prepare for the occasion. These personages who come forth riding upon these beasts are prepared for an hour, for a day, for a month, and for a year; and their work is to slay a third part of the hosts of men then existing upon the earth, and as they are prepared for an hour, a day, a month, and a year, it shows there will be quite a lapse of time between the sounding of the trump of the sixth and seventh angel.