Albert Barnes Commentary Revelation 9:18

"By these three plagues was the third part of men killed, by the fire and the smoke and the brimstone, which proceeded out of their mouths." — Revelation 9:18 (ASV)

By these three. Three things—explained immediately as referring to the fire, the smoke, and the brimstone.

Was the third part of men killed. See Barnes on Revelation 8:7-12, on each of which verses we have notices of calamities that came upon the third part of the human race, of the sea, of rivers, etc. We are not to suppose that this is to be taken literally, but the description is given as it appeared to John. Those immense numbers of horsemen would sweep over the world, and a full third part of mankind would seem to fall before them.