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

Thus the Exiles of Númenor established their realms in Arnor and in Gondor; but ere many years had passed it became manifest that their enemy, Sauron, had also returned. He came in secret, as has been told, to his ancient kingdom of Mordor beyond the Ephel Dúath, the Mountains of Shadow, and that country marched with Gondor upon the east. There above the valley of Gorgoroth was built his fortress vast and strong, Barad-dûr, the Dark Tower; and there was a fiery mountain in that land that the Elves named Orodruin. Indeed for that reason Sauron had set there his dwelling long before, for he used the fire that welled there from the heart of the earth in his sorceries and in his forging; and in the midst of the Land of Mordor he had fashioned the Ruling Ring. There now he brooded in the dark, until he had wrought for himself a new shape; and it was terrible, for his fair semblance had departed for ever when he was cast into the abyss at the drowning of Númenor. He took up again the great Ring and clothed himself in power; and the malice of the Eye of Sauron few even of the great among Elves and Men could endure.