One Ring

The greatest of the Rings of Power, forged secretly by Sauron in the fires of Orodruin. The Ring not only transported the wearer into the Wraith-world, making him invisible to those in the real world, but also granted virtual immortality and mastery over the nineteen Great Rings, and all other magical Rings forged in Eregion in the middle of the Second Age. After Sauron's defeat at the end of that Age, the Ring was lost; the story of its recovery and ultimate destruction lie at the heart of The Hobbit and The Lord of the Rings.