Malachi

Elijah's final mention in the Hebrew Bible is in Malachi, where it is said that Elijah will come again before "the great and terrible day of the Lord." That day is described as the burning of a great furnace, "... so that it will leave them neither root nor branch (Malachi 3:19)". Traditionally, in both Judaism and Christianity, this is taken to mean the return of Elijah will precede the Messiah.

"Behold, I will send you Elijah the prophet before the great and terrible day of the Lord comes. And he will turn the hearts of fathers to their children and the hearts of children to their fathers, lest I come and smite the land with a curse."
— Malachi 4:5-6