That is when the kingdom of the world becomes his.No more beast reigning in the world after the seventh trump.
Revelation 11
15 The seventh angel sounded his trumpet, and there were loud voices in heaven, which said:
“The kingdom of the world has become
the kingdom of our Lord and of his Messiah,
and he will reign for ever and ever.”
If you look closely there it says it becomes the kingdom "of our Lord and of his Messiah". So, "our Lord" there is referring to God the Father. He doesn't begin to reign until Jesus delivers His kingdom to the Father when He returns.
1 Corinthians 15:22 For as in Adam all die, even so
in Christ shall all be made alive. 23 But every man in his own order: Christ the firstfruits; a
fterward they that are Christ's at his coming. 24 Then cometh the end, when he shall have delivered up the kingdom to God, even the Father; when he shall have put down all rule and all authority and power. 25 For
he must reign, till he hath put all enemies under his feet. 26 The last enemy that shall be destroyed is death.
You can't have the Father reigning before "the end" when Jesus comes. He has put His Son in charge of the kingdom until then, as passages like Matthew 28:16-18 make clear.
Matthew 28:16 Then the eleven disciples went away into Galilee, into a mountain where Jesus had appointed them. 17 And
when they saw him, they worshipped him: but some doubted. 18 And Jesus came and spake unto them, saying,
All power is given unto me in heaven and in earth.
When Jesus comes it will be the end and He will deliver the kingdom to the Father at that point. And, at that point, the last enemy, death, will have been destroyed and defeated.
And He will come at the seventh and last trumpet to accomplish that.
1 Corinthians 15:51 Behold, I shew you a mystery;
We shall not all sleep, but we shall all be changed, 52 In a moment, in the twinkling of an eye,
at the last trump: for the trumpet shall sound, and the dead shall be raised incorruptible, and we shall be changed. 53 For this corruptible must put on incorruption, and this mortal must put on immortality. 54 So
when this corruptible shall have put on incorruption, and this mortal shall have put on immortality, then shall be brought to pass the saying that is written, Death is swallowed up in victory.
Premils don't have the last enemy, death, being defeated until 1,000+ years after Christ returns, but scripture says death will be defeated and "swallowed up in victory" when Jesus returns at the last trumpet, which is the last prophetic trumpet and therefore is the same as the seventh trumpet of Revelation.