The powerful ring sought by Sauron in J.R.R. Tolkien’s Middle-earth is the One Ring.
Sauron forged the One Ring in the fires of Mount Doom to control the other Rings of Power and extend his domination over their bearers. He placed much of his own power into it, making the Ring both a source of strength and a vulnerability.
The Ring passes through several important hands before Bilbo Baggins finds it in The Hobbit. Bilbo later gives it to Frodo, who is charged with taking it back to Mount Doom so it can be destroyed. Its destruction ends Sauron’s power.
The One Ring is sometimes loosely called simply “the Ring,” but Tolkien’s larger mythology contains many other rings. The full name distinguishes Sauron’s master ring from the Elven Rings and other Rings of Power.