The Nazgûl1, terrible servants of Sauron in J.R.R. Tolkien's Lord of the Rings, are known by many names: the Nine, the Úlairi, (Black) Riders, Ringwraiths.
Sauron, for a time, in the guise of Annatar, the Lord of Gifts, dwelt among the men and elves of Middle-earth and guided the forging of the Rings of Power. Nine of these rings were given to mortal men. As stated in Of the Rings of Power and the Third Age in The Silmarillion:
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 then. 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 under the domination of the One, which was Sauron's. And they became for ever invisible save to him that wore the Ruling Ring, and they entered into the realm of shadows. The Nazgûl were they, the Ringwraiths, the Enemy's most terrible servants; darkness went with them, and they cried with the voices of death.
Frodo, wearing the Ring, beheld the Nazgûl at the end of Chapter 11, The Fellowship of the Ring (FoR):
In their white faces burned keen and merciless eyes; under their mantles were long grey robes; upon their grey hairs were helms of silver; in their haggard hands were swords of steel.
And again at the end of Chapter 12, FoR:
They appeared to have cast aside their hoods and black cloaks, and they were robed in white and grey. Swords were naked in their pale hands; helms were on their heads. Their cold eyes glittered....
Strider (Aragorn) discusses their nature in Chapter 11, FoR:
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. And at all times they smell the blood of living things, desiring and hating it. Senses, too, there are other than sight or smell. We can feel their presence - it troubled our hearts, as soon as we came here, and before we saw them; they feel ours more keenly. Also...the Ring draws them.
Additionally, the Nazgûl possess the ability to darken, cloud, and overcome the mind of weaker opponents utilizing what Strider (the end of Chapter 10, FoR) refers to as the Black Breath.
During the reign of Malvegil (sixteenth in the line from Elendil), circa 1200 (the Third Age), the lord of the Nazgûl, the Witch-king, established a kingdom in Angmar in the far northern regions of Middle-earth. In 1975, he suffered defeat at the hands of Elrond, and fled far southward to Minas Morgul (The Tower of Sorcery).
In 2951, Sauron sent Khamûl2 and a second3 Nazgûl messenger to Dol Guldur (Hill of Sorcery) in Mirkwood. The Witch-king and the remaining five dwelt at Minas Morgul, until 3018, when their hunt for the Ring forced them - invisible, unclad, and unmounted - out into the open. As stated in The Hunt for the Ring in Unfinished Tales:
They reached the west-shores of Anduin a little north of Sarn Gebir, as they had trysted; and there received horses and raiment that were secretly ferried over the River. That was (it is thought) about the seventeenth of July. Then they passed northward seeking for the Shire, the land of the Halflings.
About the twenty-second of July they met their companions, the Nazgûl of Dol Guldur, in the Field of Celebrant.
Night was waning on the twenty-second day of September when drawing together again they came to Sarn Ford and the southernmost borders of the Shire. They found them guarded, for the Rangers barred their way. But this was a task beyond the power of the Dúnedain.... Some still dared to bar the ford, and held it while day lasted, but at night the Lord of Morgul swept them away, and the Black Riders passed into the Shire.
From this point on, the story is familiar to most readers of The Lord of the Rings.
A Black Rider4 questions Gaffer Gamgee, then follows the hobbits on their path through the countryside. On the first night of their journey, the Black Rider is scared off by a roving band of elves led by Gildor Inglorion. The next day, the Ring-wraith is joined by another, one of whom questions Farmer Maggot of the Marish.
Frodo and his companions manage to elude the Black Riders for a time and safely cross the Brandywine River into Buckland. From there, they continue on through the Old Forest and on to Bree.
In Bree, Merry falls victim to the Nazgûl Black Breath:
I went out for a stroll. I had come back again and was standing just ouside the light of the lamp looking at the stars. Suddenly I shivered and felt that something horrible was creeping near: there was a sort of deeper shade among the shadows across the road, just beyond the edge of the lamplight. It slid away at once into the dark without a sound.
I seemed to be drawn somehow. Anyway, I went, and suddenly I heard voices by the hedge. I did not creep any closer, because I began to tremble all over. Then I felt terrified, and I turned back, and was just going to bolt home, when something came behind me and I...I fell over. I had an ugly dream, which I can't remember. I went to pieces.
In Crickhollow, three Ringwraiths enter the Baggins' home, and Fatty Bolger sounds the Horn-call of Buckland. The Black Riders, empty-handed, ride swiftly to the east.
Gandalf, in his search for Frodo, heads to Weathertop and there encounters the nine Nazgûl. After a bitter fight, he retreats to Rivendell, leaving four Riders at the ford of the River Hoarwell and five in the vicinity of Weathertop.
Three days later, Strider, Frodo, Sam, Merry, and Pippin arrive at Weathertop and encounter the remaining Nazgûl. Frodo is terribly wounded with an enchanted Morgul-knife, but the band of five manage to continue their journey eastward.
At the Ford of Rivendell, the band encounters the Nine in full force. Frodo crosses the river on Glorfindel's steed, and swears, "By Elbereth and Luthien the Fair, you shall have neither the Ring nor me!".
Then the leader, who was now half across the Ford, stood up menacing in his stirrups, and raised up his hand. Frodo was stricken dumb. He felt his tongue cleave to his mouth, and his heart labouring. His sword broke and fell out of his shaking hand.
At that moment there came a roaring and a rushing: a noise of loud waters rolling many stones. The three Riders that were still in the midst of the Ford were overwhelmed: they disappeared, buried suddenly under angry foam. Those that were behind drew back in dismay.
Later, Gandalf remembers the encounter at the Ford:
Caught between fire and water, and seeing an Elf-lord [Glorfindel] revealed in his wrath, they were dismayed, and their horses were stricken with madness. Three were carried away by the first assault of the flood; the others were now hurled into the water by their horses and overwhelmed.
Their horses must have perished, and without them they are crippled. But the Ringwraiths themselves cannot be so easily destroyed.
In The Two Towers, Frodo and Sam watch in horror as the lord of the Nazgûl exits Minas Morgul:
The earth groaned; and out of the city there came a cry...a rending screech, shivering, rising swiftly to a piercing pitch beyond the range of hearing. As the terrible cry ended, falling back through a long sickening wail to silence...an army came.... At their head was one greater than all the rest: a Rider, all black, save that on his hooded head he had a helm like a crown that flickered with a perilous light.
This very same lord of the Nazgûl, once King of Angmar, leads his forces to Gondor and makes his final appearance in The Return of the King at the Battle of the Pelennor Fields:
Suddenly in the midst of the glory of the king (Théoden)...the new morning was blotted from the sky. Dark fell about him. The great shadow descended like a falling cloud. And behold! it was a winged creature.... Down, down it came...upon it sat a shape, black-mantled, huge and threatening. A crown of steel he bore, but between rim and robe naught was there to see, save only a deadly gleam of eyes.... He was come again, bringing ruin, turning hope to despair, and victory to death. A great black mace he wielded.
Dernhelm cries out:
'Begone, foul dwimmerlaik, lord of carrion! Leave the dead in peace!'
A cold voice answered: 'Come not between the Nazgûl and his prey! Or he will not slay thee in thy turn. He will bear thee away to the houses of lamentation, beyond all darkness, where thy flesh shall be devoured, and thy shrivelled mind be left naked to the Lidless Eye....'
'Do what you will; but I will hinder it, if I may.'
'Hinder me? Thou fool. No living man may hinder me!'
At this point, Dernhelm reveals herself to be Éowyn
, Lady of Rohan
'But no living man am I! You look upon a woman.... Begone, if you be not deathless!'
The winged creature screamed at her, but the Ringwraith made no answer, and was silent, as if in sudden doubt.... Suddenly the great beast beat its hideous wings and...leaped into the air, and then swiftly fell down upon Éowyn.... A swift stroke she dealt, skilled and deadly. The outstretched neck she clove asunder....
Out of the wreck rose the Black Rider, tall and threatening, towering above her. With a cry of hatred that stung the very ears like venom he let fall his mace. Her shield was shivered in many pieces, and her arm was broken; she stumbled to her knees. He bent over her like a cloud, and his eyes glittered; he raised his mace to kill.
But suddenly he too stumbled forward with a cry of bitter pain, and his stroke went wide, driving into the ground. Merry's sword had stabbed him from behind, shearing through the black mantle, and passing up beneath the hauberk had pierced the sinew behind his mighty knee.
'Éowyn! Éowyn!' cried Merry. Then tottering, struggling up, with her last strength she drove her sword between crown and mantle, as the great shoulders bowed before her. The sword broke sparkling into many shards. The crown rolled away with a clang. Éowyn fell forward upon her fallen foe. But lo! the mantle and hauberk were empty. Shapeless they lay now on the ground, torn and tumbled; and a cry went up into the shuddering air, and faded to a shrill wailing, passing with the wind, a voice bodiless and thin that died, and was swallowed up, and was never heard again in that age of this world.
1 Nazgûl (literally 'Ring-wraith') is derived from nazg, the word for 'ring' in the Black Speech of Mordor.
2 Khamûl, the Shadow of the East, second in command of the Nazgûl.
3 A third Nazgûl was present for a time.
4 Said to be Khamûl, in Unfinished Tales