Malachi was one of the prophets. The biblical book "Malachi", an oracle. The word of the Lord to Israel through Malachi, concludes the Old Testament.
Nothing is known about the prophet’s life. He supposedly lived in the 5th century BC.
Christian tradition considers him to be a messenger of the new covenant that appears in the Gospel.
“I am about to send my messenger to clear a path before me. Suddenly the Lord whom you seek will come to his temple; the messenger of the covenant in whom you delight is here, here already, says the Lord. Who can endure the day of his coming? Who can stand firm when he appears? He is like a refiner's fire, like a fuller's soap; he will take his seat, testing and purifying...” (Malachi 3:1-3).
Malachi concluded his oracle by saying that Lord had promised to send the prophet Elijah before the Messiah. The three Synoptic Gospels contain references to these verses in Malachi as being fulfilled through John the Baptist and Jesus.
See: Duccio di Buoninsegna. Maestà: The Prophet Malachi.