Revelation 9:13-21

Revelation 9:13-21

13 The sixth angel sounded his trumpet, and I heard a voice coming from the four horns of the golden altar that is before God. 14 It said to the sixth angel who had the trumpet, “Release the four angels who are bound at the great river Euphrates.” 15 And the four angels who had been kept ready for this very hour and day and month and year were released to kill a third of mankind. 16 The number of the mounted troops was twice ten thousand times ten thousand. I heard their number.

17 The horses and riders I saw in my vision looked like this: Their breastplates were fiery red, dark blue, and yellow as sulfur. The heads of the horses resembled the heads of lions, and out of their mouths came fire, smoke and sulfur. 

(Testing of the Peacekeeper reentry vehicles: all eight (of a possible ten) were fired from only one missile. Each line shows the path of an individual warhead captured on reentry via long-exposure photography.)

18 A third of mankind was killed by the three plagues of fire, smoke and sulfur that came out of their mouths. 19 The power of the horses was in their mouths and in their tails; 

(In the cone, the mouth)

(In the tail.)

for their tails were like snakes, having heads with which they inflict injury.

20 The rest of mankind who were not killed by these plagues still did not repent of the work of their hands; they did not stop worshiping demons, and idols of gold, silver, bronze, stone and wood—idols that cannot see or hear or walk. 21 Nor did they repent of their murders, their magic arts, their sexual immorality or their thefts.

The passage is not about demons as they would be unaccounted for, no location is given, they are not, "in the air" nor "in the abyss", nor in need of a habitation, no commander is given them, or where they came from or how they got there etc.

So unaccounted for demons summoned from somewhere?

Or they are something else.

I think its pretty obvious 

what the passage refers to.

"Sulfur" mentioned twice in vs 17 and once in vs 18 kinda gives it away.

 Multiple independently targetable reentry vehicle