"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 falling into darkness. Now they are slaves to his will. They are the Nazgul, Ringwraiths, neither living nor dead. At all times they feel the presence of the Ring, drawn to the power of the One."
With the aid of the Elves of the Havens some of the folk of Nargothrond built new ships, and they went forth and explored the great Isle of Balar, thinking there to prepare a last refuge, if evil came; but it was not their fate that they should ever dwell there. ~ The Silmarillion, Chapter 14 (High Elf Ship by David Nash)