Nazgûl, Black Riders, Nine Riders, etc.

“The Nazgûl (from Black Speech nazg, ‘ring’, and gûl, ‘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 Lord of the Nazgûl or the Witch-king of Angmar, had once been the King of Angmar in the north of Eriador. 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, 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 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; at the romantic level as the enemies of the heroic protagonists; and at the mythic level, where the Lord of the Nazgûl calls himself Death and bursts the gates of Minas Tirith with magical spells. … 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. …”
Wikipedia
Identity and Origins of the Nazgûl
WIRED: 9 Facts About Tolkien’s Nines
YouTube: The Nazgûl of Middle-earth 12:21