The Lord of the Nazgûl, also called the Witch-king of Angmar, the Pale King, or Black Captain, is a fictional character in J. R. R. Tolkien's fantasy novel The Lord of the Rings. He is one of the Nine Men that became Nazgûl (Ringwraiths) after receiving Rings of Power from the dark lord Sauron. His ring gives him great power, but enslaves him to Sauron and makes him invisible. As a wraith, he had once established himself King of Angmar in the north of Eriador. In the events of the Lord of the Rings, he stabs the bearer of the One Ring, the Hobbit Frodo Baggins, with a Morgul-knife which would reduce its victim to a wraith. Much later, 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.

In early drafts, Tolkien had called him the "Wizard King", and considered making him either a renegade member of the Istari, or an immortal Maia, before settling on having him as a mortal Man, corrupted by a Ring of Power given to him by Sauron. Commentators have written that the Lord of the Nazgûl functions at the level of myth when, his own name forgotten, he calls himself Death and bursts the gates of Minas Tirith with a battering-ram engraved with magical spells. At a theological level, he embodies a vision of evil similar to Karl Barth's description of evil as das Nichtige, an active and powerful force that turns out to be empty. The prophecy that the Lord of the Nazgûl would not die by the hand of Man echoes that made of the title character in William Shakespeare's Macbeth.

The Witch-king first appears in the Second Age of Middle-earth. The Dark Lord Sauron gave Rings of Power to powerful Men, including kings of countries in Middle-earth. These confer magical power, but also enslave their wearers to the owner of the One Ring, Sauron himself.

The Lord of the Nazgûl appears as the Witch-king of Angmar during the Third Age and is instrumental in the destruction of the Northern kingdom of Arnor. In his notes for translators, Tolkien suggested that the Witch-king of Angmar, ruler of a Northern kingdom with its capital at Carn Dûm, was of Númenórean origin. Nothing is heard of him when Sauron is overthrown by the Last Alliance of Elves and Men late in the Second Age, but his survival is assured by the power of the One Ring.

Over a thousand years later in the Third Age, the Lord of the Nazgûl leads Sauron's forces against the successor kingdoms of Arnor: Rhudaur, Cardolan, and Arthedain. He destroys all of these, but is eventually defeated by the Elf-lord Glorfindel, who puts him to flight, and makes the prophecy 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 Sauron's return.

Towards the end of the Third Age, Sauron sends the Witch-king, leading the other Nazgûl, to the Shire to find and recover the One Ring. He is cloaked and hooded in black; his face cannot be seen; he rides a black horse. At Weathertop, the Witch-king stabs Frodo, the bearer of the One Ring, in the shoulder with the Morgul-knife, breaking off a piece of it in the Hobbit's flesh. Frodo is able to see that the Witch-king is taller than the other Nazgûl, with "long and gleaming" hair and a crown on his helmet. He is swept away by the waters of the river Bruinen and his horse is drowned. He returns to Mordor. He reappears mounted on a hideous flying beast.

During the Battle of the Pelennor Fields, the Witch-king commands Grond, a battering-ram engraved with evil spells, to successfully break the gates of Minas Tirith. Being forced to leave the broken gates he retreats to lead the besieging army against the new threat of the Rohirrim, where he is faced by a single warrior, Dernhelm, actually a disguised Éowyn, a noblewoman of Rohan; and not far away, Merry, a hobbit of the Fellowship. Éowyn boldly calls the Nazgûl a "dwimmerlaik", telling him to go if he was 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 the Witch-king is destroyed by a woman and a Hobbit, fulfilling Glorfindel's prophecy. Both weapons that pierced him disintegrate, and both assailants are stricken by the Black Breath, which causes a cold paralysis, terror, and often death.

Megan N. Fontenot, on Tor.com, writes that in early drafts, Tolkien names him "the Wizard King", so powerful in wizardry that his opponent Gandalf is unable to counter him unaided. In early drafts of "The Council of Elrond", Gandalf explains that his enemy was "of old the greatest of all the wizards of Men". In a later draft, Tolkien adds that the Wizard King was also "a great king of old" and the "fell captain of the Nine [Riders]"; Fontenot glosses "fell" as implying "ravenous cruelty" and "ruthless ... savagery".