the Nazgūl arose as Sauron's most powerful servants in the Second Age of Middle-earth. They were once mortal Men, three being "great lords" of Nśmenor. Sauron gave each of them one of the nine Rings of Power. Sauron also gave seven Rings of Power to the Dwarves, and Celebrimbor forged three, untainted by Sauron's evil, for the Elves. It was Sauron's design to control all these rings and their bearers through the power of his One Ring, forged in secret for this purpose, but only the Nine succumbed completely to its power:
"Those who used the Nine Rings became mighty in their day, kings, sorcerers, and warriors of old. They obtained glory and great wealth, yet it turned to their undoing. They had, as it seemed, unending life, yet life became unendurable to them. They could walk, if they would, unseen by all eyes in this world beneath the sun, and they could see things in worlds invisible to mortal men; but too often they beheld only the phantoms and delusions of Sauron. And one by one, sooner or later, according to their native strength and to the good or evil of their wills in the beginning, they fell under the thraldom of the ring that they bore and of the domination of the One which was Sauron's. And they became forever invisible save to him that wore the Ruling Ring, and they entered into the realm of shadows. The Nazgūl were they, the Ringwraiths, the Ulairi, the Enemy's most terrible servants; darkness went with them, and they cried with the voices of death"