A third of mankind was killed by the three plagues of fire, smoke, and sulfur that proceeded from their mouths.

Revelation 8:7
Then the first angel sounded his trumpet, and hail and fire mixed with blood were hurled down upon the earth. A third of the earth was burned up, along with a third of the trees and all the green grass.

Revelation 8:8
Then the second angel sounded his trumpet, and something like a great mountain burning with fire was thrown into the sea. A third of the sea turned to blood,

Revelation 8:9
a third of the living creatures in the sea died, and a third of the ships were destroyed.

Revelation 8:10
Then the third angel sounded his trumpet, and a great star burning like a torch fell from heaven and landed on a third of the rivers and on the springs of water.

Revelation 8:11
The name of the star is Wormwood. A third of the waters turned bitter like wormwood oil, and many people died from the bitter waters.

Revelation 8:12
Then the fourth angel sounded his trumpet, and a third of the sun and moon and stars were struck. A third of the stars were darkened, a third of the day was without light, and a third of the night as well.

Revelation 9:15
So the four angels who had been prepared for this hour and day and month and year were released to kill a third of mankind.

Revelation 9:17
Now the horses and riders in my vision looked like this: The riders had breastplates the colors of fire, sapphire, and sulfur. The heads of the horses were like the heads of lions, and out of their mouths proceeded fire, smoke, and sulfur.

Revelation 9:19
For the power of the horses was in their mouths and in their tails; indeed, their tails were like snakes, having heads with which to inflict harm.

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.