The Silmarillion

Then Gil-galad and Elendil passed into Mordor and encompassed the stronghold of Sauron; and they laid siege to it for seven years, and suffered grievous loss by fire and by the darts and bolts of the Enemy, and Sauron sent many sorties against them. There in the valley of Gorgoroth Anárion son of Elendil was slain, and many others. But at the last the siege was so strait that Sauron himself came forth; and he wrestled with Gil-galad and Elendil, and they both were slain, and the sword of Elendil broke under him as he fell. But Sauron also was thrown down, and with the hilt-shard of Narsil Isildur cut the Ruling Ring from the hand of Sauron and took it for his own. Then Sauron was for that time vanquished, and he forsook his body, and his spirit fled far away and hid in waste places; and he took no visible shape again for many long years.

Thus began the Third Age of the World, after the Eldest Days and the Black Years; and there was still hope in that time and the memory of mirth, and for long the White Tree of the Eldar flowered in the courts of the Kings of Men, for the seedling which he had saved Isildur planted in the citadel of Anor in memory of his brother, ere he departed from Gondor. The servants of Sauron were routed and dispersed, yet they were not wholly destroyed; and though many Men turned now from evil and became subject to the heirs of Elendil, yet many more remembered Sauron in their hearts and hated the kingdoms of the West. The Dark Tower was levelled to the ground, yet its foundations remained, and it was not forgotten. The Númenóreans indeed set a guard upon the land of Mordor, but none dared dwell there because of the terror of the memory of Sauron, and because of the Mountain of Fire that stood nigh to Barad-dûr; and the valley of Gorgoroth was filled with ash. Many of the Elves and many of the Númenóreans and of Men who were their allies had perished in the Battle and the Siege; and Elendil the Tall and Gil-galad the High King were no more. Never again was such a host assembled, nor was there any such league of Elves and Men; for after Elendil’s day the two kindreds became estranged.

The Ruling Ring passed out of the knowledge even of the Wise in that age; yet it was not unmade. For Isildur would not surrender it to Elrond and Círdan who stood by. They counselled him to cast it into the fire of Orodruin nigh at hand, in which it had been forged, so that it should perish, and the power of Sauron be for ever diminished, and he should remain only as a shadow of malice in the wilderness. But Isildur refused this counsel, saying: ‘This I will have as were-gild for my father's death, and my brothers. Was it not I that dealt the Enemy his death-blow?' And the Ring that he held seemed to him exceedingly fair to look on; and he would not suffer it to be destroyed. Taking it therefore he returned at first to Minas Anor, and there planted the White Tree in memory of his brother Anárion. But soon he departed, and after he had given counsel to Meneldil, his brother's son, and had committed to him the realm of the south, he bore away the Ring, to be an heirloom of his house, and marched north from Gondor by the way that Elendil had come; and he forsook the South Kingdom, for he purposed to take up his father's realm in Eriador, far from the shadow of the Black Land.

But Isildur was overwhelmed by a host of Orcs that lay in wait in the Misty Mountains; and they descended upon him at unawares in his camp between the Greenwood and the Great River, nigh to Loeg Ningloron, the Gladden Fields, for he was heedless and set no guard, deeming that all his foes were overthrown. There well nigh all his people were slain, and among them were his three elder sons, Elendur, Aratan, and Ciryon; but his wife and his youngest son, Valandil, he had left in Imladris when he went to the war. Isildur himself escaped by means of the Ring, for when he wore it he was invisible to all eyes; but the Orcs hunted him by scent and slot, until he came to the River and plunged in. There the Ring betrayed him and avenged its maker, for it slipped from his finger as he swam, and it was lost in the water. Then the Orcs saw him as he laboured in the stream, and they shot him with many arrows, and that was his end. Only three of his people came ever back over the mountains after long wandering; and of these one was Ohtar his esquire, to whose keeping he had given the shards of the sword of Elendil.

Thus Narsil came in due time to the hand of Valandil, Isildur's heir, in Imladris; but the blade was broken and its light was extinguished, and it was not forged anew. And Master Elrond foretold that this would not be done until the Ruling Ring should be found again and Sauron should return; but the hope of Elves and Men was that these things might never come to pass.