"'See, I will send you the prophet Elijah before that great and dreadful day of the Lord comes.'" NIV translation
This prophecy ends the Old Testament and it is fulfilled in the beginning of the New Testament. The prophet Elijah was taken up to heaven in a whirlwind (see 2 Kings 2) and this verse speaks to the fact that God was going to send him back before the coming of the Lord.
There have been those that do not understand this and they have concluded that the prophecy will be fulfilled in Revelation 11 as they identify Elijah as one of the two witnesses that we see there. The prophecy was fulfilled with John the Baptist as Jesus said that he was the Elijah that was to come.
"He will turn the hearts of the fathers to their children, and the hearts of the children to their fathers; or else I will come and strike the land with a curse." NIV translation
We live in a world with broken lives and broken families but, through Jesus Christ, the relationships can be healed. This can only be done when our hearts are in fact turned to our heavenly father.