Topic summary
Black Riders

Extracted from the Wikipedia article Nazgûl.
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.