The magical ring central to Tolkien’s The Lord of the Rings is the One Ring.
Sauron forged the One Ring in the fires of Mount Doom during the Second Age. He placed much of his power into it so that he could dominate the other Rings of Power and control their bearers. Its inscription, written in the Black Speech of Mordor, expresses its role in ruling the other rings.
The Ring changes hands repeatedly before the main events of the novel. Isildur cuts it from Sauron’s hand, but the Ring is later lost in the Anduin. Déagol finds it, and Sméagol takes it from him, eventually becoming the creature called Gollum. Bilbo finds it in Gollum’s cave and later gives it to Frodo.
The quest of The Lord of the Rings is to destroy the One Ring in Mount Doom. Although Frodo carries it most of the way, its destruction ultimately depends on Gollum’s intervention at the Cracks of Doom.