Malachi 4:5-6
“Behold, I will send you Elijah the prophet before the coming of the great and dreadful day of the Lord. And he will turn the hearts of the fathers to the children, and the hearts of the children to their fathers, lest I come and strike the earth with a curse.”

The last word in our Old Testament becomes the first word in our New Testament.
You can read the story of Elijah in 1 Kings 17 – 2 Kings 2.
Elijah is a great prophet that confronted one of the most evil kings of Israel – King Ahab and Queen Jezebel.
This prophecy from Malachi finds a fulfillment once in the days preceding the ministry of Jesus.

And His disciples asked Him, saying, “Why then do the scribes say that Elijah must come first?”
Jesus answered and said to them, “Indeed, Elijah is coming first and will restore all things. But I say to you that Elijah has come already, and they did not know him but did to him whatever they wished. Likewise the Son of Man is also about to suffer at their hands.” Then the disciples understood that He spoke to them of John the Baptist. Matt 17:10-13

Another fulfillment comes in the days preceding the Second Coming of Christ.

In Rev 11:6 it speaks of two witnesses in the days of the tribulation. These have power to shut heaven, so that no rain falls in the days of their prophecy; and they have power over waters to turn them to blood, and to strike the earth with all plagues, as often as they desire.
I believe these two are Moses and Elijah – the Law and the Prophets. They appear together in Matthew 17.
The Jews today still set out a glass of wine at an empty seat during a Passover seder for the arrival of Elijah.
He will come.
And so Jesus return!