The History of the Ring During the Third Age
At the end of the Second Age, when Sauron was overthrown, it was Isildur, the High King of the United Kingdom of Gondor and Arnor, who cut the One Ring from his hand. At the time, this was deemed a righteous act and the only means of destroying the powr of the Dark Lord; however once Isildur himself seized the One Ring, a part of him was corrupted by its evil power. For strong and virtuous though he was, Isildur could not resist its promise of power.
Though he stood on the volcanic slopes of Mount Doom itself, in whose fires the Ring was forged and the only place where it could be unmade, he could not bring himself to destroy it. Isildur succumbed to temptation and took the One Ring as his own, and thus its curse soon fell upon him. While marching back north, he with his sons who had accompanied him to the Great War, were waylaid by Orcs and slain. But Isildur put on the Ring and became invisible, as such was its power, and ran away. But the Orcs followed him by scent. Isildur plunged himself into the River Anduin, and there, the Ring betrayed him. It slipped off his finger and the Orcs seeing him, shot him with their arrows. Thus did the Ring fall out of history, and it would not be part of important events in the world for 3000 years.
However, many years later, but still many years before the Return of Sauron, a fisher-community nearby the banks of the river had sprung up. And one day, two of them, their names Smeagol and Deagol were fishing. It was Deagol who while in the River, found the Ring. Smeagol demanded it be given to him, as it was his birthday. Deagol refused and Smeagol killed him.
The Power of the Ring lengthened Smeagol's life, yet it warped him beyond recognition. Thereafter, he was called Gollum because of a nasty gutteral sounds he made when trying to speak. He became a ghoullish being who shunned the light and lived by foul murder and eating unclean meat. He found comfort in the dark pools in deep caverns.
For nearly five centuries, Gollum lived hidden beneath the Misty Mountains. Then 2941 of the Third Age, the Hobbit Bilbo Baggins made a fateful visit to his cavern and found the One Ring. In 3019, Gollum at last hunted down Frodo Baggins, the new Ringbearer, but try as he might, could not overcome him. For a time, Frodo almost seemed able to tame Gollum, but Gollum lived by treachery.
So it was at the final moment, when the power of the Ring overcame Frodo upon Mount Doom, Gollum attacked Frodo on the edge of the Cracks of Doom. Summoning all his evil strength, Gollum won the Ring by biting off Frodo's finger, but at the moment of victory, he toppled backwards with his precious prize down into the fiery bowels of the Earth.
