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. At all times, they feel the presence of the Ring, drawn to the power of the One. They will never stop hunting you.