Character group from J. R. R. Tolkien's legendarium
The Nazgûl (from Black Speech , "ring", and , "wraith, spirit"), introduced as Black Riders and also called Ringwraiths, Dark Riders, the Nine Riders, or simply the Nine, are fictional characters in J. R. R. Tolkien's Middle-earth. They were nine Men who had succumbed to Sauron's power through wearing Rings of Power, which gave them immortality but reduced them to invisible wraiths, servants bound to the power of the One Ring and completely under Sauron's control.
"The Lord of the Rings" calls them Sauron's "most terrible servants". Their leader, known as the Witch-king of Angmar, the Lord of the Nazgûl, or the Black Captain, was Sauron's chief agent for most of the Third Age. At the end of the Third Age, their main stronghold was the city of Minas Morgul at the entrance to Sauron's realm, Mordor. They dress entirely in black. In their early forays, they ride on black horses; later they ride flying monsters, which Tolkien described as "pterodactylic". Their main weapon is terror, though in their pursuit of the Ring-bearer Frodo Baggins, their leader uses a Morgul-knife which would reduce its victim to a wraith, and they carry ordinary swords. In his final battle, the Lord of the Nazgûl attacks Éowyn with a mace. The hobbit Merry Brandybuck stabs him with an ancient enchanted Númenórean blade, allowing Éowyn to kill him with her sword. 
Commentators have written that the Nazgûl serve on the ordinary level of story as dangerous opponents of the Company of the Ring; at the romantic level as the enemies of the heroic protagonists; and finally at the mythic level. Tolkien knew the "Lacnunga", the Old English book of spells; it may have suggested multiple features of the Nazgûl, the Witch-King, and the Morgul-knife.
The Nazgûl appear in numerous adaptations of Tolkien's writings, including animated and live-action films and computer games.
Fictional history.
Second Age.
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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 downfall. 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 thraldom 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 Nazgûl were they, the Ringwraiths, the Úlairi, 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"
The Nazgûl or Ringwraiths (Quenya plural: "Úlairi") first appeared in the Second Age. The Dark Lord Sauron gave nine Rings of Power to powerful mortal men, including three lords of the once-powerful island realm of Númenor, along with kings of countries in Middle-earth. The rings enslaved their bearers to the power of Sauron's One Ring, into which he had put much of his own power. The corrupting effect of the Rings greatly extended the bearers' lives. 
The Nazgûl had a sharp sense of smell. Their sight worked differently, too: "They themselves do not see the world of light as we do, but our shapes cast shadows in their minds, which only the noon sun destroys; and in the dark they perceive many signs and forms that are hidden from us: then they are most to be feared." Their chief weapon was terror; it was so powerful that Sauron faced one disadvantage when using them: they could not easily travel in secret. The terror they spread was greater when they were unclad and invisible; and when they were gathered together.
Only two of the Nazgûl are named or identified individually in Tolkien's works. Their chief, also known as the Lord of the Nazgûl and the Black Captain, appears as the Witch-king of Angmar during the Third Age, instrumental in the destruction of the North-kingdom of Arnor. In "Unfinished Tales", his second-in-command is named as ;templatestyles src="Template:Visible anchor/styles.css" / ;Khamûl, the "Black Easterling" or the "Shadow of the East". Three of the Nazgûl were great Númenórean lords; in his notes for translators, Tolkien speculated that the Witch-king of Angmar, ruler of a northern kingdom with its capital at Carn Dûm, was of Númenórean origin.
The Nine soon became Sauron's principal servants. They were dispersed after the first overthrow of Sauron late in the Second Age at the hands of the Last Alliance of Elves and Men, but their survival was assured by the power of the One Ring.
Third Age.
The Nazgûl re-emerge over a thousand years later in the Third Age, when the Lord of the Nazgûl leads Sauron's forces against the successor kingdoms of Arnor: Rhudaur, Cardolan, and Arthedain. He destroys all three but is defeated by the armies of Gondor and the Elf-lord Glorfindel, who prophesies that "not by the hand of man will he fall". He escapes, and returns to Mordor. There, he gathers the other Nazgûl to prepare for the return of Sauron.
The Nazgûl besiege Minas Ithil, a Gondorian fortress in the Ephel Duath, capture it, and acquire its "palantír" for Sauron. The city becomes Minas Morgul, the Nazgûl's stronghold, and the valley is known as Morgul Vale (Imlad Morgul). Sauron returns from Dol Guldur to Mordor and declares himself openly. He sent two or three of the Nazgûl, led by Khamul, to garrison Dol Guldur.
Sauron learns from Gollum that a hobbit, Bilbo Baggins of the Shire, has acquired the One Ring. Sauron entrusts its recovery to the Nazgûl. They reappear "west of the River", riding black horses that were bred or trained in Mordor to endure their terror. They learn that the Ring has passed to Bilbo's heir, Frodo, and hunt him and his companions across the Shire; the hobbits hear snuffling, and sometimes see them crawling. The hobbits escape, via Tom Bombadil's realm where they are not pursued, to Bree. A Ranger of the North, Aragorn, arrives ahead of them and for some days leads them on paths not closely followed by the Ringwraiths.
Five of the Nazgûl corner Frodo and his company at Weathertop, where the Witch-king stabs Frodo in the shoulder with the Morgul-knife, breaking off a piece of it in the Hobbit's flesh. During their assault, they mentally command Frodo to put on the One Ring; while wearing it, he sees them as pale figures robed in white, with "haggard hands", helmets and swords. The Witch-king was taller than the others, with "long and gleaming" hair and a crown on his helmet.
When all Nine are swept away by the waters of the river Bruinen, their horses are drowned, and the Ringwraiths are forced to return to Mordor to regroup. 
The nine members of the Company of the Ring, tasked with the destruction of the Ring, leave Rivendell as the "Nine Walkers", in opposition to the Nazgûl, the "Nine Riders".
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The Nazgûl came again ... like vultures that expect their fill of doomed men's flesh. Out of sight and shot they flew, and yet were ever present, and their deadly voices rent the air. More unbearable they became, not less, at each new cry. At length even the stout-hearted would fling themselves to the ground as the hidden menace passed over them, or they would stand, letting their weapons fall from nerveless hands while into their minds a blackness came, and they thought no more of war, but only of hiding and of crawling, and of death.
"The Return of the King", "The Siege of Gondor"
The Nazgûl reappear mounted on hideous flying beasts. During the Battle of the Pelennor Fields, the Lord of the Nazgûl uses magic, including Grond, a battering-ram engraved with evil spells, to break the gates of Minas Tirith. He is faced by Éowyn, a noblewoman of Rohan; and not far away, Merry, a hobbit of the Company. Éowyn boldly calls the Nazgûl a "dwimmerlaik", telling him to go if he is not deathless. He casts back his hood to reveal a crown, but the head that wears it is invisible. Merry's surreptitious stroke with an enchanted Barrow-blade brings the Nazgûl to his knees, allowing Éowyn, the niece of Théoden, to drive her sword between his crown and mantle. Thus is the Witch-king destroyed by a woman and a Hobbit, fulfilling Glorfindel's prophecy. Both weapons that pierced him disintegrate, and both assailants are stricken with the Black Breath.
After the fall of the Lord of the Nazgûl, command of Mordor's army in the field falls to Gothmog, the "lieutenant of Morgul", of unspecified race.
The remaining eight Ringwraiths attack the Army of the West during the Battle of the Morannon. When Frodo claims the Ring for his own in Mount Doom, Sauron, finally realizing his peril, orders the remaining eight Nazgûl to fly to intercept him. They arrive too late: Gollum seizes the Ring and falls into the Cracks of Doom, destroying the Ring. That ends Sauron's power and everything he had brought into being using it, including the Nazgûl.
Steeds.
The Nazgûl's flying steeds are given various descriptions but no name. The soldier of Gondor Beregond calls them "Hell Hawks". Tolkien describes them as "fell beasts", though he also applies the adjective "fell" ("fierce, cruel") to other creatures throughout "The Lord of the Rings" – even at one point to the wizard Gandalf. In a letter, he calls the winged mounts "Nazgûl-birds". In the absence of a proper name, derivative works sometimes press "fellbeast" or "fell-beast" into service.
In the Battle of the Pelennor Fields, where the Lord of the Nazgûl rode one of the flying beasts against King Théoden of Rohan, his mount is described as: 
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It is said to attack with "beak and claw". Tolkien wrote that he "did not intend the steed of the Witch-king to be what is now called a 'pterodactyl'", while acknowledging "obviously it is "pterodactylic"" and owed much to the "new ... mythology" [of the "Prehistoric"], and might even be "a last survivor of older geological eras."
The medievalist Marjorie Burns compares the fell beast to the "Poetic Edda"'s flying steed Sleipnir, "Odin's eight-legged otherworldly horse". She writes that whereas Gandalf's horse Shadowfax resembles Sleipnir in his miraculous speed and in almost seeming to fly, the Nazgûl's mount actually flies but is a "negative image" of Odin's steed; and, she notes, both Odin and the Nazgûl can cause blindness.
Inspiration.
Development.
Tolkien began writing "The Lord of the Rings" with no conception of Black Riders at all. The horseman in dark clothes in the early chapter "Three is Company" was originally Gandalf; in 1938, Tolkien called the figure's transformation into a Black Rider "an unpremeditated turn". Frodo's ring, too, was simply a magic ring conferring invisibility, both in "The Hobbit" and early drafts of "The Lord of the Rings", with no link to Sauron. However, Tolkien was at the time starting to consider the true nature of the Ring, and the idea that it had been made by the Necromancer, and drew itself or its bearer back to him. The Black Riders became Ringwraiths when the hobbit, at that time called Bingo rather than Frodo, discussed the Riders with the Elf Gildor, later in the same chapter. Over the next three years, Tolkien gradually developed the connections between the Nazgûl, the One Ring, Sauron, and all the other Rings of Power. The pieces finally all came together when Tolkien wrote "The Mirror of Galadriel", some hundreds of pages later, around the autumn of 1941.
"Lacnunga".
The number of the Nazgûl, nine, may be derived from medieval folklore. Edward Pettit, in "Mallorn", states that nine is "the commonest 'mystic' number in Germanic lore". He quotes the "Nine Herbs Charm" from the "Lacnunga", an Old English book of spells:
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against venom and vile things
and all the loathly ones,
that through the land rove, 
against nine fugitives from glory, 
against nine poisons and 
against nine flying diseases. ;/poem ;
Pettit