Nazgûl

The Nazgûl – 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.