John the Baptist
A voice cries out,“In the wilderness clear a way for the Lord; build a level road through the rift valley for our God."
“I am about to send my messenger, who will clear the way before me. Indeed, the Lord you are seeking will suddenly come to his temple, and the messenger of the covenant, whom you long for, is certainly coming,” says the Lord of Heaven’s Armies.
Look, I will send you Elijah the prophet before the great and terrible day of the Lord arrives.
These verses are self-explaining. The fulfilment came in the form of John the Baptist.
John replied in the words of Isaiah the prophet, "I am the voice of one calling in the wilderness, 'Make straight the way for the Lord.'"