In the manuscript of his notes for translators, Tolkien stated that the Witch-king's name and background were not recorded, but that he was probably of Númenórean descent. In the Second Age, the Rings of Power were forged by the Elves of Eregion under Sauron's direction, and nine of these were given to men of the time, one of whom became the Witch-king. The rings gave them immense power, and they "became mighty in their day, kings, sorcerers, and warriors of old." The rings extended their life span, and eventually corrupted them, turning them into the ghastly, undead Nazgûl. The Witch-king became their leader and, as Lord of the Nazgûl, served as Sauron's second in command for over 4,000 years.
Campaign against Arnor
A millennium into the Third Age, the Witch-king reappeared in Angmar, a realm in the far North straddling the Misty Mountains. He quickly dominated Angmar, and turned to wage war against the three splinter kingdoms of Arnor (Arthedain, Rhudaur, and Cardolan); for Sauron, seeing that Gondor remained strong, sought to capitalise on the dissension among the northern kingdoms. It was during these northern wars, prosecuted against the Dúnedain for the next several hundred years, that the King of Angmar became known as the Witch-king. Rhudaur was soon compromised; power there was seized by evil Hillmen allied with Angmar. Argeleb I of Arthedain fortified the border against Rhudaur along the Weather Hills, but was killed in battle with Angmar and Rhudaur. The Witch-king then invaded Cardolan. King Arveleg I of Arthedain was killed defending Weathertop, but the palantír there was saved and moved to Fornost. The last Prince of Cardolan was killed, and most of the Dúnedain of Rhudaur were killed or driven out. Later the Great Plague destroyed many of the remaining Dúnedain of Cardolan, and evil spirits from Rhudaur and Angmar infested the burial mounds in the Barrow-downs.
Only Arthedain remained to resist the Witch-king (though with frequent help from both Lindon and Rivendell). Araval won a victory over Angmar and sought to reoccupy Cardolan, but the barrow-wights terrified all who tried to live near the Barrows. Finally, as it became apparent that Angmar was preparing another attack, Arvedui appealed for help from King Eärnil II of Gondor. But before help could arrive, Angmar overran Arthedain. The Witch-king captured Fornost Erain, the capital of Arthedain. Arvedui fled north, only to drown in the Ice Bay of Forochel early the next year when the ship from Lindon that rescued him sank.
The following summer, arriving too late to save Arvedui, Prince Eärnur of Gondor landed at the harbours of Mithlond with an army from Gondor. The Elves of Lindon and the remnant of the northern Dúnedain joined his army and the combined forces marched against the Witch-king. On the plains west of Fornost Eärnur's army met the army of Angmar, which was forced to retreat toward Fornost. As his army was routed, the Witch-king fled north toward Carn Dûm in Angmar; but Eärnur and Glorfindel, with reinforcements from Rivendell, pursued the retreating party and defeated them. In the process the Witch-king caused the panic of much of Gondor's cavalry, including Eärnur's horse. But with the appearance of Glorfindel the Witch-king fled into the gathering darkness. Eärnur attempted to follow him, but Glorfindel stopped the prince and prophesied, "Do not pursue him! He will not return to these lands. Far off yet is his doom, and not by the hand of man will he fall."
The Witch-king returned to Mordor and led the Nazgûl in the siege of Minas Ithil. The city soon fell to the Nazgûl, and was known afterward as Minas Morgul, the Tower of Black Sorcery. Here the Witch-king made his stronghold, and he was called "the Lord of Morgul".
When King Eärnil II of Gondor died, his son Eärnur, the Witch-king's old enemy, inherited the throne. The Witch-king challenged him to single combat, but Eärnur refused. Seven years later, the Witch-king again challenged him; this time the king accepted. Eärnur rode out of Minas Tirith to meet the Witch-king in Minas Morgul. He entered the city's gates and was never seen again. From this time the Stewards of Gondor ruled the kingdom on behalf of the absent line of kings.
Pursuit of the Ring
During the time of the events of The Lord of the Rings, Sauron learned from Gollum that the One Ring was held by a hobbit named Baggins in a land called "Shire". Sauron sent the Ringwraiths forth to search for the Ring. The Riders did not at first know the location of the Shire, and were dispelled by Saruman from Isengard, but when they came by chance upon Gríma Wormtongue in Rohan, he told them what he knew of Saruman's plans, including his interest in the Shire and the Shire's location.
The Witch-king and the other Nazgûl rode from Mordor and Dol Guldur searching for the Shire. Four entered the Shire, and found that "Baggins" had moved to Buckland. Several Nazgûl attacked Gandalf on Weathertop and tried to ambush Frodo Baggins in Buckland and at Bree. Five, including the Witch-king, finally found Frodo on Weathertop with the other hobbits, accompanied by the Ranger Aragorn. The Ringwraiths attacked the party, and the Witch-king wounded Frodo with a Morgul-blade. Frodo's wound threatened to turn him into a wraith under the control of the Nazgûl.
As the company made for Rivendell, the realm of Elrond Half-elven, they met Glorfindel, who loaned Frodo his horse, Asfaloth. Pursued by all nine Nazgûl, the horse carried Frodo across the Bruinen. From the far bank, Frodo defied the Nazgûl. When the Witch-king rode into the water, Elrond, who controlled the river, released a flood that caught three Nazgûl and their horses. Glorfindel advanced and drove the terrified horses of the remaining Nazgûl into the flood. The horses drowned and all nine Nazgûl were swept away.
Campaign against Gondor
On their return to Mordor, the Nazgûl were remounted on great winged beasts. The Witch-king returned to Minas Morgul to prepare the assault upon Gondor. His forces attacked Faramir's Rangers in the Battle of Osgiliath and drove them back across the Anduin.
The Witch-king soon led large numbers of Orcs, Haradrim, and Easterlings to besiege Minas Tirith in the Battle of the Pelennor Fields. After the gates of the city were broken, he rode to enter the city but was prevented from entering by Gandalf.
Recalled to the battle by the unexpected advance of the Rohirrim, the Witch-king attacked Théoden, who had outrun his own riders. Snowmane, Théoden's horse, was struck by a dart and fell upon Théoden. As the Witch-king approached him for the kill, Éowyn, the king's niece, barred his way. She decapitated his mount, and the Witch-king replied with a powerful blow from his mace, breaking her arm and her shield. As he prepared to finish her off, the hobbit Meriadoc Brandybuck stabbed the back of the Witch-king's knee with a Dúnedain dagger which bore enchantments deadly to the Witch-king. Éowyn then thrust her sword into the void between the Witch-king's crown and torso. Her sword shattered but the Witch-king's clothing fell to the ground and he vanished with a wailing cry.
This fulfilled the prophecy of Glorfindel, for the Witch-king fell not by "the hand of man", but by a woman and a hobbit.