D&C 2

The first chronological revelation included in the Book of Commandments as well as our current Doctrine and Covenants, was one delivered by Moroni and the night of his first visit to the Prophet Joseph Smith, repeating with some variation of the words previously delivered by Malachi -


"BEHOLD, I will reveal unto you the Priesthood, by the hand of Elijah the prophet, before the coming of the great and dreadful day of the Lord.
2And he shall plant in the hearts of the children the promises made to the fathers, and the hearts of the children shall turn to their fathers.
3If it were not so, the whole earth would be utterly wasted at his coming."  (Doctrine and Covenants | Section 2:1 - 3)

What is this Priesthood that was to be revealed or restored by the hand of Elijah?  It is the keys and authority associated with with binding or sealing spiritual things up on earth and simultaneous in heaven for as taught in the LDS Bible Dictionary -

"We learn from latter-day revelation that Elijah held the sealing power of the Melchizedek Priesthood and was the last prophet to do so before the time of Jesus Christ. He appeared on the Mount in company with Moses (also translated) and conferred the keys of the priesthood on Peter, James, and John (Matt. 17:3). He appeared again, in company with Moses and others, on April 3, 1836, in the Kirtland (Ohio) Temple and conferred the same keys upon Joseph Smith and Oliver Cowdery. All of this was in preparation for the coming of the Lord, as spoken of in Malachi 4:5–6 (D&C 110:13–16). As demonstrated by his miraculous deeds, the power of Elijah is the sealing power of the priesthood by which things bound or loosed on earth are bound or loosed in heaven. Thus the keys of this power are once again operative on the earth and are used in performing all the ordinances of the gospel for the living and the dead."  (Bible Dictionary | E Elijah:Entry)

Of that event, we read -

"...another great and glorious vision burst upon us; for Elijah the prophet, who was taken to heaven without tasting death, stood before us, and said:
14 Behold, the time has fully come, which was spoken of by the mouth of Malachi—testifying that he [Elijah] should be sent, before the great and dreadful day of the Lord come—
15 To turn the hearts of the fathers to the children, and the children to the fathers, lest the whole earth be smitten with a curse—
16 Therefore, the keys of this dispensation are committed into your hands; and by this ye may know that the great and dreadful day of the Lord is near, even at the doors."  (Doctrine and Covenants | Section 110:13 - 16)

It is this keys and power that links exalted families and saints into one great family of the House (or family) of Israel.  This sealing power is essential for such.  Obviously, without such authority, the world or plan of salvation would be frustrated, cursed, or as Moroni said "utterly wasted at his coming."