Three Rings for the Elven-kings under the sky,
Seven for the Dwarf-lords in their halls of stone,
Nine for Mortal Men doomed to die,
One for the Dark Lord on his dark throne
In the Land of Mordor where the Shadows lie
One ring to rule them all, one ring to find them,
One ring to bring them all and in the darkness bind them
In the Land of Mordor where the Shadows lie.
The One Ring
Main Article : The One RingThe One Ring was created by the Dark Lord Sauron following his departure from Eregion in order to enlarge his own might by combining it with the power of the Elven smiths, and thus to give him control over the other Rings of Power, which had been made by Celebrimbor and his people with Sauron's assistance. Sauron forged the One Ring secretly in the fires of Mount Doom. His will was then inside a Ring that could control the other Rings. Thus, he was more powerful than ever before when he wore the Ring, but became much weaker when he lost it. Though it appeared to be made of simple gold, the Ring was virtually impervious to damage, and could only be destroyed by throwing it into the pit of the volcanic Mount Doom in which it had originally been forged. This Ring also basically meant that unless you destroyed the ring you could never truly kill Sauron because a piece of him was inside of the Ring and would never leave the world while the Ring still lived.
When a person wore the Ring, he would be partly "shifted" out of the physical realm into the spiritual realm. There, if he managed to consciously subdue the Ring's will with his own, he could theoretically wield all the powers that Sauron had before he lost the Ring; notably, he could control and enslave the will of others. A side effect (but usually the first effect noticed) of the Ring was that it made the wearer invisible to physical beings like living Men (but highly visible to spiritual beings like the Nazgûl), dimmed the wearer's sight, and sharpened his hearing. This "shadow world" was the world the Wraiths were forced to live in always, but it was also a world in which the Calaquendi (Elves of Light) held great power: therefore Glorfindel was able to stay the Witch-king at the Battle of Fornost and later again at the Ford of Bruinen at Rivendell. Part of the nature of the Ring was that it slowly but inevitably corrupted its wearer, regardless of any intentions to the contrary. Whether this was specifically designed into the Ring's magic or is simply an artifact of its evil origins is unknown. (Sauron might be expected to endow his One Ring with such a property, but he probably never intended anyone besides himself to wear it. It may be a side-effect of the portion of Sauron's will that lies within the Ring, influencing the wearer.)