"The Nazgûl (Black Speech: Ringwraiths, sometimes written Ring-wraiths), also known as the Nine Riders or Black Riders (or simply the Nine), were Sauron's "most terrible servants" in Middle-earth. They were mortal Men who had been turned into wraiths by their Nine Rings of power.." from tolkiengateway.net
Video from Realms Unraveled
The Fate of the Ringwraiths: What Happened to the Nazgûl after the Witch King Fell in Battle?
"Dive deep into the shadowy realms of Middle-earth with our latest exploration: "The Fate of the Ringwraiths: What Happened to the Nazgûl after the Witch King Fell in Battle?" from video introduction