Comes AFTER or at The END of the Tribulation:

Revelation 16

King James Version (KJV)

16 And I heard a great voice out of the temple saying to the seven angels, Go your ways, and pour out the vials of the wrath of God upon the earth.

And the first went, and poured out his vial upon the earth; and there fell a noisome and grievous sore upon the men which had the mark of the beast, and upon them which worshipped his image.

The Tribulation has to have been and this is the end because they already have the Mark.

mark someone for life
Fig. to affect someone for life. The tragedy marked her for life and she was never the same. She was marked for life by her brother's untimely death.

 

And the second angel poured out his vial upon the sea; and it became as the blood of a dead man: and every living soul died in the sea.

  The blood of a dead man does not flow and therefore does not circulate, thus no oxygen is carried to the cells. This does not mean the sea will not be the blood, turning to blood , but rather "As", so we are looking at sea life, and with that, our life.      

And the third angel poured out his vial upon the rivers and fountains of waters; and they became blood.\

Now Waters we are told are people, Revelation 17:15 . By that I will assume that the fountains and rivers are towns and cities. In the same Revelation we also see that people have an Angel assigned.

And I heard the angel of the waters say, Thou art righteous, O Lord, which art, and wast, and shalt be, because thou hast judged thus.

For they have shed the blood of saints and prophets, and thou hast given them blood to drink; for they are worthy.

This is Israel, as mentioned in other scriptures, kills it;s prophets and saints

 

And I heard another out of the altar say, Even so, Lord God Almighty, true and righteous are thy judgments.

And the fourth angel poured out his vial upon the sun; and power was given unto him to scorch men with fire. 

And men were scorched with great heat, and blasphemed the name of God, which hath power over these plagues: and they repented not to give him glory.

  We all know by now that the sun has solar storms which spews out radiation. We are guarded from the most by out atmosphere, but it is thin and already has a growing hole in it. Anybody who has had to have radiation treatments can attest to not only melying of the skin but also destruction of the body in a most painful way. It is like being set on fire, a fire that does not end.

  

10 And the fifth angel poured out his vial upon the seat of the beast; and his kingdom was full of darkness; and they gnawed their tongues for pain,

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

While darkness can be many things, if it is literal then it would state that they were in physical pain from lack of sunshine, plants would die and they would be devout Vit C, but I don't think that is it. Darkness can also be evil deeds, thoughts, and actions. We know for instance that these will be violent times and with the first Vials poured out, things will be to the level of pain. A third possibility would be that genetic manipulation had went astray and completely backfired. In 2013, the started manipulation looks scary enough, and that is the part they tell us about. What lies in secret? What lies in the future? MOvies often give us a heads up as to events that will come. Notice the movies lately? Another possibility is nuclear war. Apparently the beast has a kingdom while ruling over other kingdoms. Given all the other clues we find of this kingdom, it is Jerusalem. (Luke 13:34). By this we see they will not change their ways, keyword there, very much like the we're saved so we do what we want group. 

12 And the sixth angel poured out his vial upon the great river Euphrates; and the water thereof was dried up, that the way of the kings of the east might be prepared.

 Note that this just uses the word, "water", so water is what it refers to. By this river drying up a new avenue of attack is opened up. But one soon finds out this isn't modern day Israel, yet it is in the plans to expand Israel to a thing called "Greater Israel". Zionism's intent is to rule from the Nile to the Euphrates. No way that was known when this was written.   

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.

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.

  Now we see 3 unclean spirits coming from the mouth of the dragon, 3 as in the 3 princes of Hell maybe?  Beelzebub, Lucifer, and Satan (Christian Bible) or I have heard Baal, Satan, Moloch, one of these, Baphomet. While Jewish and other beliefs name 7. These 3 deceive the leaders of whole world and pull them all to the last battle against God. 

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.

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

17 And the seventh angel poured out his vial into the air; and there came a great voice out of the temple of heaven, from the throne, saying, It is done.

18 And there were voices, and thunders, and lightnings; and there was a great earthquake, such as was not since men were upon the earth, so mighty an earthquake, and so great.

19 And the great city was divided into three parts, and the cities of the nations fell: and great Babylon came in remembrance before God, to give unto her the cup of the wine of the fierceness of his wrath.

20 And every island fled away, and the mountains were not found.

21 And there fell upon men a great hail out of heaven, every stone about the weight of a talent: and men blasphemed God because of the plague of the hail; for the plague thereof was exceeding great.

 

  Charter 17 continues this saga