The Silmarillion

Valandil took up his abode in Annúminas, but his folk were diminished, and of the Númenóreans and of the Men of Eriador there remained now too few to people the land or to maintain all the places that Elendil had built; in Dagorlad, and in Mordor, and upon the Gladden Fields many had fallen. And it came to pass after the days of Eärendur, the seventh king that followed Valandil, that the Men of Westernesse, the Dúnedain of the North, became divided into petty realms and lordships, and their foes devoured them one by one. Ever they dwindled with the years, until their glory passed, leaving only green mounds in the grass. At length naught was left of them but a strange people wandering secretly in the wild, and other men knew not their homes nor the purpose of their journeys, and save in Imladris, in the house of Elrond, their ancestry was forgotten. Yet the shards of the sword were cherished during many lives of Men by the heirs of Isildur; and their line, from father to son, remained unbroken.

In the south the realm of Gondor endured, and for a time its splendour grew, until it recalled the wealth and majesty of Númenor ere it fell High towers the people of Gondor built, and strong places, and havens of many ships; and the Winged Crown of the Kings of Men was held in awe by people of many lands and tongues. For many a year the White Tree grew before the King's house in Minas Anor, the seed of that tree which Isildur brought out of the deeps of the sea from Númenor; and the seed before that came from Avallónë, and before that from Valinor in the Day before days when the world was young.

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.

And in the days of Eärnil they made their first stroke, and they came by night out of Mordor over the passes of the Mountains of Shadow, and took Minas Ithil for their abode; and they made it a place of such dread that none dared to look upon it. Thereafter it was called Minas Morgul, the Tower of Sorcery; and Minas Morgul was ever at war with Minas Anor in the west. Then Osgiliath, which in the waning of the people had long been deserted, became a place of ruins and a city of ghosts. But Minas Anor endured, and it was named anew Minas Tirith, the Tower of Guard; for there the kings caused to be built in the citadel a white tower, very tall and fair, and its eye was upon many lands. Proud still and strong was that city, and in it the White Tree still flowered for a while before the house of the Kings; and there the remnant of the Númenóreans still defended the passage of the River against the terrors of Minas Morgul and against all the enemies of the West, Orcs and monsters and evil Men; and thus the lands behind them, west of Anduin, were protected from war and destruction.

Still Minas Tirith endured after the days of Eärnur, son of Eärnil, and the last King of Gondor. He it was that rode alone to the gates of Minas Morgul to meet the challenge of the Morgul-lord; and he met him in single combat, but he was betrayed by the Nazgûl and taken alive into the city of torment, and no living man saw him ever again. Now Eärnur left no heir, but when the line of the Kings failed the Stewards of the house of Mardil the Faithful ruled the city and its ever-shrinking realm; and the Rohirrim, the Horsemen of the North, came and dwelt in the green land of Rohan, which before was named Calenardhon and was a part of the kingdom of Condor; and the Rohirrim aided the Lords of the City in their wars. And northward, beyond the Falls of Rauros and the Gates of Argonath, there were as yet other defences, powers more ancient of which Men knew little, against whom the things of evil did not dare to move, until in the ripening of time their dark lord, Sauron, should come forth again. And until that time was come, never again after the days of Eärnil did the Nazgûl dare to cross the River or to come forth from their city in shape visible to Men.