Elijah will reveal unto men the priesthood – sealing power

Bruce R. McConkie (Quorum of the Twelve)

Elijah will reveal unto men the priesthood. He will bring again the sealing power. He will authorize mortals to use the priesthood to bind on earth and seal everlastingly in the heavens. He will give the same keys to Joseph Smith and Oliver Cowdery that he gave to Peter, James, and John on the holy mount. ( The Millennial Messiah, p. 267)

Vs. 1—"… Elijah the prophet [will be sent] before the coming of the great and dreadful day of the Lord."

http://scottwoodward.org/dc2-commentary-prophetic.html