The Nine Nazgul or Ringwraiths were the most loyal and terrible servants of the Dark-lord. In origin they were all mortal men, great and powerful lords in their own right, who ruled many lands throughout Middle-earth. Each was seduced by the gift of one of the Nine rings of Sauron, which imbued its wearer with great powers of sorcery, and an almost unlimited lifespan.
But like all of the Dark-lord's gifts it was tainted, for each of the Nine rings was ruled by the One, and thus the nine wearers became slaves of the overbearing will of Sauron. Ultimately, by the end of the Third-age the Nazgul were terrifying undead creatures, existing in a terrible twilight world between life and death, with no thought or desire save that of the Dark-lord himself. Their fate was by this time completely bound up with that of their master and that of the One ring, whose recovery and restoration they arduously sought.