הִנֵּה אָנֹכִי שֹׁלֵחַ לָכֶם אֵת אֵלִיָּה הַנָּבִיא לִפְנֵי בּוֹא יוֹם ה' הַגָּדוֹל וְהַנּוֹרָא
Behold, I will send you Elijah the prophet before the coming of the great and awesome day of the L-rd.
You may recall that Eliyahu (Elijah) ascended to Heaven alive in 2 Kings chapter 2. But don’t think that he’s still walking around in a physical body! The Radak tells us that upon his ascension, Eliyahu’s soul returned to Heaven and his body returned to the Earth. In order to send Eliyahu to us, G-d will first reconstruct a body for him like his original, after which Eliyahu will return to instruct the people.