Fire, Smoke, and Brimstone
“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.
19 For their power is in their mouth, and in their tails: for their tails were like unto serpents, and had heads, and with them they do hurt.”
(Rev 9:17-19 KJV)
The horsemen have an interesting description. Many have taken these descriptions to be symbolic of modern warfare. John would not have known of tanks, guns, planes, ships, missiles, helicopters, etc., and he could have been using descriptions that the people of his time would understand. The problem I have with this explanation is that no modern vehicles of war fit these descriptions. It is more likely that they are demons, such as the locusts (9:2-12). Either way, their mission is to destroy a third of the remaining population by fire and brimstone. This is similar to the judgment on Sodom and Gomorrah.
“24 Then the LORD rained upon Sodom and upon Gomorrah brimstone and fire from the LORD out of heaven;
25 And he overthrew those cities, and all the plain, and all the inhabitants of the cities, and that which grew upon the ground.”
(Gen 19:24-25 KJV)