Nazgûl

"They were once men. Great kings of men. Then Sauron the deceiver gave to them nine rings of power. Blinded by their greed, they took them without question, one by one falling into darkness. Now they are slaves to his will. They are the Nazgul, Ringwraiths, neither living nor dead. At all times they feel the presence of the Ring, drawn to the power of the One. They will never stop hunting you."

- Aragon telling Frodo about the Nazgûl.

The Nazgûl or Ringwraiths or The Nine (also known as Black Riders) are servants of Sauron in J. R. R. Tolkien's classic Middle-earth writings and the quaternary antagonists of The Lord of the Rings Trilogy alongside their leader, the Witch-King of Angmar.

History
They originally were leaders of Men who were given the Nine Rings of Power by Sauron, which enslaved them. Over time they changed into ghostly beings, invisible to the normal eye; they often wear visible dark cloaks and other garments, enabling other beings to see them. They are usually mounted, either on horseback or riding great winged beasts (though unnamed, they are often called "fell beasts", after a descriptive term used by Tolkien). They serve Sauron in various fuctions, as scouts and spies as well as leaders of his armies.

Only two Nazgûl are known as individuals:
 * The Witch-King of Angmar, leader of the Nazgûl. His name is not known, and he is instead referred to by various epithets.
 * Khamûl, the Witch-king's second in command. He was a former Easterling lord, and is the only named Nazgûl. He became the new leader of the Nine after the death of The Witch-King after the Battle of Gondor.

In the book The Lord of the Rings, the Nazgûl are sent to the Shire on horseback to retrieve the One Ring, and subsequently chase the Ring-bearer Frodo Baggins and his companions on their way to Rivendell. Later in the book they appear at various times riding the winged fell beasts. The Witch-king of Angmar leads Sauron's armies in the Siege of Minas Tirith and the Battle of the Pelennor Fields, but there is killed by Lady Éowyn of Rohan and the hobbit Merry Brandybuck.

The other Nazgûl reappear at the Battle of the Black Gate, from where they speed to Mount Doom after Sauron becomes aware of the Ring's location. They burn in the flames of the erupting volcano after the One Ring was destroyed.