The sixth angel

The sixth angel sounded. I heard a voice from the horns of the golden altar which is before God, saying to the sixth angel who had the trumpet, “Free the four angels who are bound at the great river Euphrates!” The four angels were freed who had been prepared for that hour and day and month and year, so that they might kill one third of mankind.

Revelation 9:13-15

The sixth angel sounds his trumpet and a voice tells him to release the four angels bound at the great river Euphrates.