§ Of the Rings of Power and the Third Age The Silmarillion

Yet at the last, in the wearing of the swift years of Middle-earth, Gondor waned, and the line of Meneldil son of Anárion failed. For the blood of the Númenóreans became much mingled with that of other men, and their power and wisdom was diminished, and their life-span was shortened, and the watch upon Mordor slumbered. And in the days of Telemnar, the third and twentieth of the line of Meneldil, a plague came upon dark winds out of the east, and it smote the King and his children, and many of the people of Gondor perished. Then the forts on the borders of Mordor were deserted, and Minas Ithil was emptied of its people; and evil entered again into the Black Land secretly, and the ashes of Gorgoroth were stirred as by a cold wind, for dark shapes gathered there. It is said that these were indeed the Úlairi, whom Sauron called the Nazgûl, the Nine Ringwraiths that had long remained hidden, but returned now to prepare the ways of their Master, for he had begun to grow again.