...the seat of the beast... i.e., the nations that support his rule.

...his kingdom was full of darkness... This language is Jewish and it takes us back to the Exodus from Egypt (See. Ex. 10:21-22) when Israel was delivered from bondage in Egypt only to go into the wilderness, and Egypt came under judgment for mistreating Abraham's seed through Isaac.

...and they gnawed their tongues for pain...

From the Merrian-Webster Collegiate Dictionary:
"gnaw" ... 2 a : to be a source of vexation to : PLAGUE <anxiety always gnawing him> b : to affect like gnawing <hunger gnawing her vitals>...

The terms "full of darkness" and "gnawed their tongues" both suggest spiritual destitution / torment. Surely those who have sold their souls to Lucifer by accepting his mark will experience both. Like Judas who betrayed Christ, they will experience the anguish that comes from realizing the enormity of what they have done, and that there is no remedy for what they've set in motion.

11 And blasphemed the God of heaven because of their pains and their sores, and repented not of their deeds.

...blasphemed the God of heaven... I've heard it preached that those cast into the lake of fire will spend eternity alternately praying, and then blaspheming God. If that will be so, then these will be getting a head start on eternity.

...because of their pains and their sores... Cf. the plaques of Egypt at the time of the Exodus (Ex. 10:21-27) (Is this a continuation of the sores of Revelation 16:2 / the first vial? or is it referring to a new plague?).

...and repented not of their deeds. Will this be because "the door to the ark will be shut"?, or because they will have hardened their hearts to the point of no return? or both?

 

And the sixth angel poured out his vial upon the great river Euphrates... The river Euphrates was one of the four rivers which bound the Garden of Eden in the beginning — i.e., as it was before the earth was divided in the days of Peleg (Gen. 2:10-14; 10:25).

...and the water thereof was dried up... Because of its place in history / Scripture, the river Euphrates is symbolically the mother of all nations (click here for "prophetic rivers") — for from this area, all life began. Consequently, is it not fitting that all life should end there?

...that the way of the kings of the east might be prepared. God's blessings and cursings for Israel were often spoken of as coming from the east — the direction from which the sun rises. Assuming that not all Jews at the time will have chosen to give Christ His rightful place, this will definitely not be a time of blessing for those left.

13 And I saw three unclean spirits like frogs come out of the mouth of the dragon, and out of the mouth of the beast, and out of the mouth of the false prophet.

And I saw three unclean spirits like frogs... This brings to mind the devil's frustrated attempts to supernaturally imitate God at the time of the Exodus.

Exodus 8:
1 And the LORD spake unto Moses, Go unto Pharaoh, and say unto him, Thus saith the LORD, Let my people go, that they may serve me.
2 And if thou refuse to let them go, behold, I will smite all thy borders with frogs:
3 And the river shall bring forth frogs abundantly, which shall go up and come into thine house, and into thy bedchamber, and upon thy bed, and into the house of thy servants, and upon thy people, and into thine ovens, and into thy kneadingtroughs:
4 And the frogs shall come up both on thee, and upon thy people, and upon all thy servants.

5 And the LORD spake unto Moses, Say unto Aaron, Stretch forth thine hand with thy rod over the streams, over the rivers, and over the ponds, and cause frogs to come up upon the land of Egypt.
6 And Aaron stretched out his hand over the waters of Egypt; and the frogs came up, and covered the land of Egypt.
7 And the magicians did so with their enchantments, and brought up frogs upon the land of Egypt.

Note that although the Pharaoh's magicians were able to copy a few of the miracles (i.e., curses) wrought at the hands of Moses and Aaron, they were never able to remove any of the curses!

...come out of the mouth of the dragon, and out of the mouth of the beast, and out of the mouth of the false prophet. All three of these will be able to "speak" supernatural curses (but never blessings) into existence. Is this the devil's feeble attempt at having a "trinity"?

14 For they are the spirits of devils, working miracles, which go forth unto the kings of the earth and of the whole world, to gather them to the battle of that great day of God Almighty.

For they are the spirits of devils, working miracles... The words of Jesus were "spirit" and they were life. The words of the beast will be "devilish" and they will be death.

...which go forth unto the kings of the earth and of the whole world, to gather them to the battle of that great day of God Almighty. This will be in preparation for the final, great battle between those on the earth (Satan and his fallen angels and the ungodly men on the earth) and the powers of heaven.

15 Behold, I come as a thief. Blessed is he that watcheth, and keepeth his garments, lest he walk naked, and they see his shame.

Behold, I come as a thief. Again, we have an expression that is definitely Israeli in nature. Again, we have the seed of Abraham at odds with the God of Abraham. And Once again--as it was during their Visitation and as it will have been at the end of our age — Christ will come into their midst like a thief in the night.

Blessed is he that watcheth, and keepeth his garments, lest he walk naked, and they see his shame. Once more, we have Jewish language: "that watcheth" — a part of their heritage and tradition — and "keepeth his garments" which has to do with their "conditional salvation under the law." Apparently, this is referring back to the "true Jews" that were harvested in set 13.

16 And he gathered them together into a place called in the Hebrew tongue Armageddon.

...Armageddon...(Greek for to gather or to pluck) God is gathering the fowl (i.e., Lucifer, his fallen angels, and all who are under their control) for the great and final battle on the earth where He will pluck their feathers and throw them in the fire.