They were once Men. Great Kings of Men. Then Sauron the Deciever gave them Nine Rings of Power. Blinded by their greed, they took them without question, one by one, falling into darkness. And now they are slaves to his will. They are the Nazgûl, Ringwraiths, neither living nor dead.
“In rode the Lord of the Nazgûl. A great black shape against the fires beyond he loomed up, grown to a vast menace of despair. In rode the Lord of the Nazgûl, under the archway that no enemy ever yet had passed, and all fled before his face.”