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

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.