"In SA 1693 began the War of the Elves and Sauron, during which the Dark Lord recovered the Nine Rings and gave them to mighty lords and rulers of Men. The mightiest of all of them was likely a king and a powerful sorcerer, possibly of Númenórean heritage, but little is known of his earthly life, except that over the coming centuries, they prospered as their power, wealth and mortal lifespans extended under the corrupting influence of their rings.
The only certainty is that eventually he and all the others slipped into the Shadow-world and became one of the Nazgûl, or Ringwraiths, who were enslaved wholly to Sauron's will.
For the next 1200 years the greatest among them, known simply as the Lord of the Nazgûl, would serve Sauron as one of the commanders of his armies." Courtesy of The One Ring Wiki