The Nazgûl, Ringwraiths or Úlairi (also called the Black Riders, the Nine Riders, or simply the Nine) were fictional ghostly servants of Sauron, first appearing in The Lord of the Rings. They originally were human and leaders of Men in the Second Age. Sauron gave to them nine of the Rings of Power, which brought them under his will. Over time they became like undead ghosts, invisible to the normal world. The greatest weapon of the Nazgûl was not swords but the fear they created in mortals.
The leader of the Nazgûl was the Witch-king of Angmar, also known as Lord of the Nazgûl or the Black Captain. He was the commander of Minas Morgul. The only other Ringwraith whose name is given by Tolkien was Khamûl. He was called the Black Easterling, and was the commander of the fortress Dol Guldur in southern Mirkwood.
The Nazgûl were sent to the Shire to find the One Ring. They went as riders, wearing dark cloaks to hide their nothingness. They arrived in Hobbiton just as Frodo Baggins and his companions had set out, and continued to pursue him until his arrival in Crickhollow, Buckland, where they lost him. They took up the search again in Bree, following Aragorn and the hobbits until Weathertop. There they attacked the group, managing to wound Frodo with a Morgul-blade but being forced to retreat. They kept following until the Ford of Bruinen, where they were swept away by the river, called by Elrond. The Nazgûl were unharmed themselves, but their mortal horses drowned, delaying their search for a time. They later reappeared riding great winged creatures, called "fell beasts"; appearing as such they were also called Winged Nazgûl. The Witch-king of Angmar was present at the Battle of the Pelennor Fields, where he was killed by Éowyn and the hobbit Meriadoc Brandybuck. The remaining eight Nazgûl perished when they flew into the fire of the erupting Mount Doom.