Frodo: What are they?
Strider: They were once men. Great kings of men. Then Sauron the deceiver gave to them nine rings of power. Blinded by their greed, they took them without question. One by one, they fell into darkness and now they’re slaves to His will. They are the Nazgûls, Ringwraiths. Neither living nor dead. At all times, they feel the presence of the Ring. Drawn to the power of the One, they will never stop hunting you.
The Lord of the Rings.