Revelation 18:21-24
(Rev 18:21 KJV)
And a mighty angel took up a stone like a great millstone, and cast it into the sea, saying, Thus with violence shall that great city Babylon be thrown down, and shall be found no more at all.

Here we have a mighty angel throwing it into the sea. The millstone was a very heavy stone and it was used for pulverizing grain into flour. Here God is telling us that Babylon is going to be pulverized into nothing, just as a heavy millstone pulverizes grain. Just as the grain goes under the millstone as grain and after the stone rolls over it, it becomes like powder or meal, so will Babylon be under the judging hand of God. It will no longer be a great city with much power but it will become as nothing as it will be ground to powder.
(Mat 21:44 KJV) And whosoever shall fall on this stone shall be broken: but on whomsoever it shall fall, it will grind him to powder. The principle the Lord gives us in Matthew 21 is that when someone falls on the kingdom of God they will be broken, which basically means that they will be broken of sin and become saved. When the stone of the kingdom of God falls on someone, it will grind him to powder which means total destruction and that is the image we have here in 18:21. The words “with violence” are used only one time in the New Testament and in the Greek it carries with it the meaning of “sudden rush or violent impulse,” which conveys the meaning that the destruction of Babylon is going to be swift and furious. The millstone is cast into the sea which is the many nations of the earth where the kingdom of Satan dwells.

(Rev 18:22 KJV)
And the voice of harpers, and musicians, and of pipers, and trumpeters, shall be heard no more at all in thee; and no craftsman, of whatsoever craft he be, shall be found any more in thee; and the sound of a millstone shall be heard no more at all in thee;

Here is the reality that at one time Babylon was alive with the music of the world, and it was alive with all kinds of craftsmen being busy at their trades, and even those who used a millstone for baking food, will no longer be heard in her because the day of her judgment has come.
(Eccl 7:4 KJV) The heart of the wise is in the house of mourning; but the heart of fools is in the house of mirth. The wise in heart, that is those who are saved will mourn, for they have knowledge of the impending judgment of the world. The heart of the fool is in the house of mirth because it is these people who fritter the days away in seeking only enjoyment and wealth but care not for the things of God until it is too late when judgment comes upon them. This is the case of Babylon, she sought only the good times of life and cared nothing to mourn for her sins but now the end has come and the time of eternal mourning is upon her.

(Rev 18:23 KJV)
And the light of a candle shall shine no more at all in thee; and the voice of the bridegroom and of the bride shall be heard no more at all in thee: for thy merchants were the great men of the earth; for by thy sorceries were all nations deceived.

Throughout history God has always had a witness in the world. That witness had always been at war with the kingdom of Satan. Here God is telling us that the light of a candle will no longer shine.
(Luke 8:16 KJV) No man, when he hath lighted a candle, covereth it with a vessel, or putteth it under a bed; but setteth it on a candlestick, that they which enter in may see the light. This means no longer will there even be a flicker of the gospel to be taught anymore because the day of salvation is over. Then the bride and bridegroom will no longer be heard which means that the joys of marriage will be absent. This may also refer to the bridegroom who is the Lord Jesus and the bride who is the body of Christ who have now been removed from Babylon as it undergoes total judgment. The body of Christ has ministered and witnessed within the Babylon for thousands of years but now that ministry is done and the time of eternal rest of the Saints has come. Instead of the world embracing the true gospel, it looked to the merchants as their authority but their authority was buying and selling in the beast system which had completely deceived every nation. (Rev 13:17 KJV) And that no man might buy or sell, save he that had the mark, or the name of the beast, or the number of his name. Only those who were of the beast were able to buy and sell and therefore the people of the earth were part of the beast system because they had the mark of the beast to do business with the beast and therein lies the deception. These evil men had caused people to look elsewhere than to the God of the universe and therefore had deceived the nations of the world.

(Rev 18:24 KJV)
And in her was found the blood of prophets, and of saints, and of all that were slain upon the earth.

Here once again is the indictment of the evil, murderous nature of the kingdom of Satan. It was responsible for the murdering of true believers right from the time of Abel to the last one it will kill before the Lord returns. It was also responsible for not only slaying the Christians but also slaying many who got in its way. Evil will attack evil to gain what it wants. For example, one evil nation will rise up against another evil nation and many will die in the war, especially those who are the poor of the land who normally fight the rich man’s wars. The poor soldiers in the armies gain nothing if they win the war and lose everything if they lose the war. As with all wars, only the rich really profit and thereby lies another great deception of the merchants of the earth which we saw in the last verse. God closes this vision with the indictment that the kingdom of Satan is a murderous kingdom and that is why it will be ground to powder because of all its sins against God.
(John 8:44 KJV) Ye are of your father the devil, and the lusts of your father ye will do. He was a murderer from the beginning, and abode not in the truth, because there is no truth in him. When he speaketh a lie, he speaketh of his own: for he is a liar, and the father of it. As the leader, so goes the kingdom!