"At the sounding of the sixth trumpet, 4 angels are released from their bondage at the Euphrates river by the 6th angel"
https://himitsustudy.com/2020/02/23/study-of-revelation-the-sixth-trumpet/
"The four "angels" which are "bound" in the great River Euphrates are the four countries through which the Euphrates runs. The Euphrates begins in southeast Turkey, continuing through Syria and Iraq on its way to the Shatt al-Arab waterway, which forms the border between Iraq and Iran."
https://vigilantcitizenforums.com/threads/are-the-4-angels-bound-in-the-great-river-euphrates-about-to-be-loosed.6912/
"The Euphrates was considered a boundary between the netherworld and the land of the living, and Tell Halaf was on the Khabur River—Mesopotamia’s equivalent of the River Styx."
https://www.skywatchtv.com/videos/unraveling-revelation-the-angels-in-the-river-euphrates
"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."
https://www.learnthebible.org/four-angels-in-the-euphrates.html