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.

Now Sauron prepared war against the Eldar and the Men of Westernesse, and the fires of the Mountain were wakened again. Wherefore seeing the smoke of Orodruin from afar, and perceiving that Sauron had returned, the Númenóreans named that mountain anew Amon Amarth, which is Mount Doom. And Sauron gathered to him great strength of his servants out of the east and the south; and among them were not a few of the high race of Númenor. For in the days of the sojourn of Sauron in that land the hearts of well nigh all its people had been turned towards darkness. Therefore many of those who sailed east in that time and made fortresses and dwellings upon the coasts were already bent to his will, and they served him still gladly in Middle-earth. But because of the power of Gil-galad these renegades, lords both mighty and evil, for the most part took up their abodes in the southlands far away; yet two there were, Herumor and Fuinur, who rose to power among the Haradrim, a great and cruel people that dwelt in the wide lands south of Mordor beyond the mouths of Anduin.

When therefore Sauron saw his time he came with great force against the new realm of Gondor, and he took Minas Ithil, and he destroyed the White Tree of Isildur that grew there. But Isildur escaped, and taking with him a seedling of the Tree he went with his wife and his sons by ship down the River, and they sailed from the mouths of Anduin seeking Elendil. Meanwhile Anárion held Osgiliath against the Enemy, and for that time drove him back to the mountains; but Sauron gathered his strength again, and Anárion knew that unless help should come his kingdom would not long stand.

Now Elendil and Gil-galad took counsel together, for they perceived that Sauron would grow too strong and would overcome all his enemies one by one, if they did not unite against him. Therefore they made that League which is called the Last Alliance, and they marched east into Middle-earth gathering a great host of Elves and Men; and they halted for a while at Imladris. It is said that the host that was there assembled was fairer and more splendid in arms than any that has since been seen in Middle-earth, and none greater has been mustered since the host of the Valar went against Thangorodrim.

From Imladris they crossed the Misty Mountains by many passes and marched down the River Anduin, and so came at last upon the host of Sauron on Dagorlad, the Battle Plain, which lies before the gate of the Black Land. All living things were divided in that day, and some of every kind, even of beasts and birds, were found in either host, save the Elves only. They alone were undivided and followed Gil-galad. Of the Dwarves few fought upon either side; but the kindred of Durin of Moria fought against Sauron.

The host of Gil-galad and Elendil had the victory, for the might of the Elves was still great in those days, and the Númenóreans were strong and tall, and terrible in their wrath. Against Aeglos the spear of Gil-galad none could stand; and the sword of Elendil filled Orcs and Men with fear, for it shone with the light of the sun and of the moon, and it was named Narsil.