One Ring
The One Ring, also called the Ruling Ring, the Master-ring, and Isildur's Bane, is the central object of J.R.R. Tolkien's The Lord of the Rings. It was forged in secret by the Dark Lord Sauron in the fires of Mount Doom during the Second Age, and into it he poured a great part of his own native power and malice, so that through it he might rule and dominate the other Rings of Power and the wills of those who bore them.
The Ring appears to be a plain band of pure gold, without gem or ornament, but when heated it reveals a fiery inscription in the Black Speech of Mordor: "One Ring to rule them all, One Ring to find them, One Ring to bring them all, and in the darkness bind them." It is the chief instrument of Sauron's design and the object whose destruction is the goal of the great quest.
Powers
The One Ring grants its wearer invisibility, as the bearer is drawn partly into the shadow world, but this comes at a grievous cost. The Ring corrupts and consumes those who use it, prolonging their lives unnaturally while wearing them thin, as it does to Bilbo Baggins and, over centuries, to the creature Gollum. It exerts a will of its own, seeking always to return to its master, and it tempts even the wise and good with visions of the power to do great deeds, which power would in truth turn to evil.
The Ring cannot be safely wielded against Sauron, for anyone who claimed its full power would in the end become a new dark lord. It can be unmade only in the fires of Mount Doom where it was forged, in the heart of Mordor.
History
Forged by Sauron in the Second Age, the Ring was lost to him when Isildur cut it from his hand at the end of the war of the Last Alliance. Isildur was slain soon after and the Ring fell into the Great River, where it lay for ages until found by the river-folk and passed to the creature who became Gollum. Centuries later it was found by Bilbo Baggins in the events told in The Hobbit.
In The Lord of the Rings, Bilbo passes the Ring to his heir Frodo Baggins, and the wizard Gandalf discovers its true nature. The Council of Elrond resolves that it must be destroyed, and Frodo takes up the burden of carrying it to Mount Doom as the Ring-bearer.
Significance
The One Ring is the engine of the entire narrative of The Lord of the Rings, embodying Tolkien's theme that the lust for domination corrupts absolutely and that the truest heroism may lie in renouncing power rather than seizing it. Its destruction brings about the fall of Sauron and the end of the Third Age.