In the fiction of J.R.R. Tolkien, the Nazgūl (Black Speech: Ringwraiths, sometimes written Ring-wraiths), also known as the Nine Riders or Black Riders (or simply the Nine), are evil servants of Sauron in Middle-earth. The rarely used Quenya name for them is Ślairi.
The nine Nazgūl arose as Sauron's most powerful servants in the Second Age of Middle-earth. It is said that three of the Nine were lords of Nśmenor corrupted by Sauron. They were all powerful mortal Men to whom Sauron gave nine Rings of Power. These proved to be their undoing:
Those who used the Nine Rings became mighty in their day, kings, sorcerers, and warriors of old. They obtained glory and great wealth, yet it turned to their undoing. They had, as it seemed, unending life, yet life became unendurable to them. They could walk, if they would, unseen by all eyes in this world beneath the sun, and they could see things in worlds invisible to mortal men; but too often they beheld only the phantoms and delusions of Sauron. And one by one, sooner or later, according to their native strength and to the good or evil of their wills in the beginning, they fell under the thralldom of the ring that they bore and of the domination of the One which was Sauron's. And they became forever invisible save to him that wore the Ruling Ring, and they entered into the realm of shadows. The Nazgūl were they, the Ringwraiths, the Enemy's most terrible servants; darkness went with them, and they cried with the voices of death.—The Silmarillion: "Of the Rings of Power and the Third Age," p. 289
For many years the bearers used the rings to gain great wealth, prestige and power. The corrupting effect of the rings caused their bodily forms to fade over time until they had become wraiths entirely. Given form only through the attire of black cloaks and hauberks of silver mail, their original form was completely gone and invisible to mortal eyes. Their hypnotic eyes could be plainly distinguished from their dark clothing, and in a rage they appeared in a hellish fire. Untouchable to mortal men, (unless blessed by Elvish magic), they had many weapons, which included long swords of steel and flame, daggers with magical venomous properties and black maces of great strength.
Their arsenal of deadly armaments was not confined to physical means; they also had magical weapons of devastating power. They were surrounded by an aura of terror, which affected all living creatures; their breath (called the Black Breath) was poisonous, and their cries caused terror and despair in all who heard them. Some of the Nazgūl appear to have been accomplished sorcerers and used magic to devastating effect. According to Tolkien, though, it was the fear they inspired that was the chief danger:
"They have no great physical power against the fearless," he wrote, "but what they have, and the fear that they inspire, is enormously increased in darkness" (Letters, 210)
The Nazgūl first appeared around 2251 of the Second Age and were soon established as Sauron's principal servants. They were dispersed after the first overthrow of Sauron in 3434 at the hands of the Last Alliance of Elves and Men, but their survival was nonetheless assured while the One Ring persisted. They re-emerged around 1300 of the Third Age, when the Lord of the Nazgūl, the Witch-King of Angmar, led Sauron's forces against the human kingdom of Arnor. He was eventually defeated in battle in 1975 and returned to Mordor, gathering the other Nazgūl in preparation for the return of Sauron to that realm. In 2000, they besieged Minas Ithil and captured it after a two-year siege. The city thereafter became the stronghold of the Nazgūl, from where they directed the rebuilding of Sauron's armies.
In 2942 Sauron returned to Mordor and declared himself openly in 2951. Two or three of the Nazgūl were sent to his fortress at Dol Guldur to garrison that outpost.
In 3017, near the beginning of the story told in The Lord of the Rings, Sauron commanded the Ringwraiths to recover the One Ring of Power from "Baggins of the Shire". Disguised as horse riders clad in black (hence the term Black Riders), they sought out Bilbo Baggins who, as Gollum had revealed, had the One Ring in his possession.
The Nazgūl at this point were dependent on their black horses (stolen from Rohan) for transportation. When they were swept away by the waters of the river Bruinen, their horses were killed and the Ringwraiths were forced to return to Mordor to regroup. They reappeared later mounted on flying creatures, at which point they were referred to as Winged Nazgūl.
By the conclusion of the War of the Ring, all of the Nine Nazgūl were destroyed. The Lord of the Nazgūl himself was slain by Éowyn, the niece of the Théoden (with help from Merry, known as the Magnificent thereafter) during the Battle of the Pelennor Fields. The remaining eight Ringwraiths attacked the Army of the West during the last battle at the Black Gate. However, when Frodo Baggins put on the ring in the fires of Mount Doom, Sauron ordered the eight remaining Nazgūl to fly with all possible speed to Mount Doom to intercept Frodo. They arrived too late, with the Ring falling into the fire along with the hapless Gollum. The Nazgūl were caught in the firestorm of the erupting mountain and were destroyed.
Only a few of the Nazgūl are named or identified individually in Tolkien's works. Their leader was the Witch-king of Angmar, and his second in command was named Khamūl. At least three of them were of Black Nśmenórean race. Khamūl was a lord of Easterlings, and was the only Nazgūl known by his name, although Gothmog, Lieutenant of Morgul, may have been a Nazgūl.
The early Middle-earth Role Playing games name the eight, other than Khamul, Er-Murazor (the Witch-king, of Nśmenórean race—note that the canonical Witch-king is specifically not Nśmenórean), Dwar, Ji Indur, Akhorahil, Hoarmurath (Nśmenórean), Adunaphel (female Nśmenórean), Ren and Uvatha1, but none of these names are considered canon (it is particularly unlikely, in the context of the books, that any of the Nazgūl would have been female).
The term Nazgūl has been used to refer to IBM's cadre of lawyers, with whom it has been said that IBM can blacken the sky - particularly with reference to the SCO v. IBM lawsuit because they supposedly never sleep, are utterly ruthless, and are completely loyal servants to their master. In addition it has been said that they are "probably really nice people. They would be nicer too if they had (say) blood or souls like normal people."
This usage appears to have originated in a comment on Slashdot:
"Not long ago, the Black Gate of Armonk swung open. The lights went out, my skin crawled, and dogs began to howl. I asked my neighbor what it was and he said, 'Those are the nazgul. Once they were human, now they are IBM's lawyers.'"
(IBM is headquartered in Armonk, New York.)
It also has been suggested that this usage can be traced back to the 1969–1982 IBM antitrust suit with the United States Department of Justice, but this has not been substantiated.
Tolkien himself was known to use the term figuratively. In a 1945 letter to his son, he compared his reaction to the aircraft of World War II to how Frodo might have felt if he had discovered Hobbits "learning to ride Nazgūl-birds" (Letters, 100).
Note 1: "What were the names of the nine Nazgūl?" (http://www.glyphweb.com/arda/faq/nazgul.html) at The Encyclopedia of Arda