Chapter 18 is one of the more complicated chapters of Isaiah, but it is a prophecy made about Ethiopia (the land of Cush).
First notice the promise of God to raise His “banner,” His flag. It is a sign of sovereignty. Though the people of Cush are valiant soldiers, feared far and wide (they are mentioned in chapter 20 as part of Egypt’s army), they are no match for God. He silently watches what they do. Though they may not see Him, He is as present as heat in sunshine and dew in harvest. And at just the right moment, God will send his judgment.
The result will be that people of Ethiopia come to Jerusalem and worship the Lord of all nations, the God of Israel.
This chapter occurs in the section of 13-24 in which God judges the nations. The point of the section is to emphasize that God is the great decider in the affairs and governments of men.
So when did the people of Ethiopia come to Jerusalem to offer sacrifices?
That’s not the point of the text. The point is simply to affirm that God wants all people to worship Him, and one way or another, they will. Christians would do well to keep in mind that the “Great Commission” of Matthew 28:18-20 was in the mind of God eight centuries before in the days of Isaiah. We might also remember a specific fulfillment in Acts 8 when the Treasurer of Ethiopia came to worship — but this is not a specific prophecy of that.