Also known as Black Riders or Nazgul. They were once kings of man who were given nine rings of power but when Sauron made the great rings he gained control over the nine who were then formed into the Ringwraiths. Instead of being men they became the faceless, formless black cloaked beings riding great steeds with fiery eyes (Did you know their costumers had over 50 meters of fabric???). They have a keen sense of smell and can feel the power of the one ring, so Sauron sent the nine out to the Shire to find the 'Baggins' from the tortured words of the creature Gollum. The nine stop at nothing to find the ring and carry blades of Mordor that have the dark power of turning those who they maim into mindless wraiths themselves. Only Elven blades are able to slay the Ringwraiths when all others perish before they strike, they also don't like fire or water as "They are neither living nor dead".

We get a brief taste of the Ringwraiths in the Two Towers  - though now they are on wings, one flys around the Dead Marshes searching for the ring whilst the second one nearly takes Frodo (in his "suicidal lemming" instance) in Osgiliath (though it could actually be the same Wraith - they all kinda look the same. 

The Nazgul make an even greater appearance on their Fell Beasts in RotK where they are lead by the sinister WitchKing of Angmar. As Frodo, Sam and Gollum make their way up the stairs of Minas Morgul, the WitchKing leaves his lair as he starts to command the forces of Mordor. The Nazgul attack the retreating men from Osgiliath and are only hampered by Gandalf on the plains. During the siege of Minas Tirith the Nazgul attack both men and catapult alike, laying waste to whatever they can get their claws on. The WitchKing himself knocks Théoden off of his horse and steps down to make the final blow but is stopped by Éowyn, "Do not get between the Nazgul and his prey." When Merry stabs him from behind after almost killing Éowyn with his mace he is distracted enough to give her a chance, "No man can kill me" where by Éowyn replies "I am no man" and proceeds to thrust her sword into his head where he dissolves. 

The some of the Nazgul survive to the battle at the Black Gate but are attacked by the great Eagles and retreat to Mount Doom when Sauron learns that the ring has been taken there, but when the ring is destroyed, so are the Nazgul, finally moving on into death.

Any of the thumbnails

Amun Sul	Menacing	WitchKing	Scary	More Running (Poor Horses)	Lightning	Twilight Wraiths	Race to the Ford

WitchKing 1	WitchKing 2