The opening of the seventh and last seal of the scroll in the hands of the Lamb marks the onset of all subsequent events until the establishment of the Christ's millennial kingdom. The silence in heaven for about half an hour draws the attention of all who are there to the terrible things that are about to take place as God's fury is released over sinful and unrepentant mankind.
Seven angels are given a trumpet each, to be blown in sequence, to proclaim the phases of the wrath of God. The sound of the trumpet is a sign of judgment for Israel, and will be heard on the day of God's wrath over Israel and mankind.
Before the trumpets are sounded, another angel offers much incense with the prayers of the saints on the altar: these are the tribulation saints on earth, persecuted by the false religion "mystery Babylon", and they pray for vengeance and deliverance from their enemies. This indicates that it is now time for them to be answered: the censer is now filled with fire from the altar (fire represents the consuming judgment of God) and thrown upon the earth followed by noises, thundering, lightning and earthquakes, indicating the presence of God in the proceedings.
What are generally known as the "trumpet judgments" then take place: the first four in the form of physical catastrophes, the last three are announced by an angel as the three woes, the result of each being:
Trumpets were used to call for the gathering of the people in assemblies, for war, for worship, and to proclaim religious festivals such as the year of jubilee and the feast of tabernacles (Num.10:2-10); seven trumpets were used in the conquest of Jericho (Josh.6:4); the sound of the trumpet was heard when the Lord descended on Sinai (Exo.19:9,16,19).
First four trumpets
It is very likely that special physical changes in substances take place in order to fulfil the prophecy of the first four judgments, as happened in the plagues of Egypt (Exo.7:10, Joel2:30,31); it may also be that God uses what already exists, like volcanic eruptions, landslides and tsunamis, collision with a large meteor and the resulting atmospheric pollution.
The angel or eagle, depending on the translation of verse 13, is understood to be a seraph (Isa.6:6) proclaiming in heaven the great suffering yet to come.
Fifth trumpet
The star fallen from heaven is an angel (Rev. 20:1). The bottomless pit, or the abyss is a place of unclean spirits, or demons (Luke 8:31); the entrance to it is kept under lock and key, holding prisoner the spirits inside it (there are also insubordinate angels held in hell, in darkness, bound in everlasting chains (2Peter2:4, Jude6), and there are many others still loose in the world and in the heavenly places (Eph.6:12), destined for judgment and the lake of fire with the devil (Mat.25:41). Locusts are what the released unclean spirits are like; they are given strict authority and limited power, under the angel of the Abyss, called destruction (Psalm78:49).
Sixth trumpet
The voice from the four horns of the altar is probably of the Lord Jesus, and provides a connection with the clamour of the martyrs of the fifth seal and the prayers of the saints. Four angels of destruction are released from the Euphrates - this river is in Babylon, a place often associated in the Bible with demons - and use 200,000,000 horsemen (probably spiritual creatures) for their mission of destroying a third of mankind by fire, smoke and brimstone (Joel2:1-11). The word sorcery is a translation of the Greek pharmakeia, from which comes pharmacy, drug-making: hallucinatory drugs were already in use in Old Testament times.
Seventh Seal: Prelude to the Seven Trumpets
1 When He opened the seventh seal, there was silence in heaven for about half an hour.
2 And I saw the seven angels who stand before God, and to them were given seven trumpets.
3 Then another angel, having a golden censer, came and stood at the altar. He was given much incense, that he should offer it with the prayers of all the saints upon the golden altar which was before the throne.
4 And the smoke of the incense, with the prayers of the saints, ascended before God from the angel's hand.
5 Then the angel took the censer, filled it with fire from the altar, and threw it to the earth. And there were noises, thunderings, lightnings, and an earthquake.
6 So the seven angels who had the seven trumpets prepared themselves to sound.
First Trumpet: Vegetation Struck
7 The first angel sounded: And hail and fire followed, mingled with blood, and they were thrown to the earth. And a third of the trees were burned up, and all green grass was burned up.
Second Trumpet: The Seas Struck
8 Then the second angel sounded: And something like a great mountain burning with fire was thrown into the sea, and a third of the sea became blood.
9 And a third of the living creatures in the sea died, and a third of the ships were destroyed.
Third Trumpet: The Waters Struck
10 Then the third angel sounded: And a great star fell from heaven, burning like a torch, and it fell on a third of the rivers and on the springs of water.
11 The name of the star is Wormwood. A third of the waters became wormwood, and many men died from the water, because it was made bitter.
Fourth Trumpet: The Heavens Struck
12 Then the fourth angel sounded: And a third of the sun was struck, a third of the moon, and a third of the stars, so that a third of them were darkened. A third of the day did not shine, and likewise the night.
13 And I looked, and I heard an angel flying through the midst of heaven, saying with a loud voice, "Woe, woe, woe to the inhabitants of the earth, because of the remaining blasts of the trumpet of the three angels who are about to sound!"
Rev 8:1-13 (NKJV)
Fifth Trumpet: The Locusts from the Bottomless Pit
1 Then the fifth angel sounded: And I saw a star fallen from heaven to the earth. To him was given the key to the bottomless pit.
2 And he opened the bottomless pit, and smoke arose out of the pit like the smoke of a great furnace. So the sun and the air were darkened because of the smoke of the pit.
3 Then out of the smoke locusts came upon the earth. And to them was given power, as the scorpions of the earth have power.
4 They were commanded not to harm the grass of the earth, or any green thing, or any tree, but only those men who do not have the seal of God on their foreheads.
5 And they were not given authority to kill them, but to torment them for five months. Their torment was like the torment of a scorpion when it strikes a man.
6 In those days men will seek death and will not find it; they will desire to die, and death will flee from them.
7 The shape of the locusts was like horses prepared for battle. On their heads were crowns of something like gold, and their faces were like the faces of men.
8 They had hair like women's hair, and their teeth were like lions' teeth.
9 And they had breastplates like breastplates of iron, and the sound of their wings was like the sound of chariots with many horses running into battle.
10 They had tails like scorpions, and there were stings in their tails. Their power was to hurt men five months.
11 And they had as king over them the angel of the bottomless pit, whose name in Hebrew is Abaddon, but in Greek he has the name Apollyon.
12 One woe is past. Behold, still two more woes are coming after these things.
Sixth Trumpet: The Angels from the Euphrates
13 Then the sixth angel sounded: And I heard a voice from the four horns of the golden altar which is before God,
14 saying to the sixth angel who had the trumpet, "Release the four angels who are bound at the great river Euphrates."
15 So the four angels, who had been prepared for the hour and day and month and year, were released to kill a third of mankind.
16 Now the number of the army of the horsemen was two hundred million; I heard the number of them.
17 And thus I saw the horses in the vision: those who sat on them had breastplates of fiery red, hyacinth blue, and sulphur yellow; and the heads of the horses were like the heads of lions; and out of their mouths came fire, smoke, and brimstone.
18 By these three plagues a third of mankind was killed - by the fire and the smoke and the brimstone which came out of their mouths.
19 For their power is in their mouth and in their tails; for their tails are like serpents, having heads; and with them they do harm.
20 But the rest of mankind, who were not killed by these plagues, did not repent of the works of their hands, that they should not worship demons, and idols of gold, silver, brass, stone, and wood, which can neither see nor hear nor walk.
21 And they did not repent of their murders or their sorceries or their sexual immorality or their thefts.
Rev 9:1-21 (NKJV)