The Nazgûl,also called Ring-wraiths, Black Riders, Dark Riders, the Nine Riders, or simply the Nine were once men of Middle-earth before they were corrupted by their nine rings of power, and transformed into Wraiths who were Sauron's greatest servants.
Back when Sauron was in hiding, he resurrected the old nine kings to defeat the White Council before he fled into the east with his minions still alive. He later sent them to hunt for Frodo Baggins and his band of friends. The Witch-king was killed in battle by Lady Éowyn, and her hobbit friend, Meriadoc Brandybuck, along with his Fell Beast. They were later destroyed when their master Sauron fell.