Magical ring of extreme and potentially world-controlling power.
Crafted by the Nibelung Dwarf Alberich out of the Rhinegold he stole from the Rhinemaidens by runic spell. Alberich was able to learn how to forge it by cursing love. He uses it to take control of the Nibelung. Alberich with the ring can divine gold, and forces the Nibelung to mine it for him. He plans to eventually enslave the world using the ring. If he attacks Valhalla with the ring, he will defeat the Gods. However if it is returned to the Rhinemaidens the curse will be lifted.
The Gods Wotan and Loge take the ring from Alberich, who curses it so that all who do not possess it covet it - often to a violent extent - for themselves. At various times, in the possession of Alberich, Wotan, Fasolt, Fafner, and Siegfried. Wotan sires the Walsungs to recover it and Alberich sires Hagen on Queen Grimhilde. Finally it is returned to the Rhinemaidens by the lover of Siegfried and former Valkyrie Brunnhilde during her immolation on his funeral pyre, though Hagen is drowned by the Rhinemaidens while trying to take it.
The actual Power of the Ring is ambivalent: the only one who seems to derive any physical power from it is Alberich (who had cursed love) and nobody else seems to derive any power from it: Brunhilde seems to think she can overpower Siegfried (in the guise of Gunther) with its power, but it does not avail her in the slightest. In fact, it could be said that the power of the Ring - not unlike the power of the Curse Alberich puts on it later - is purely the power of suggestion. In this case, the suggestion that the Ring will grant its owner that which they most keenly desire: Fafner, who only cares about his own survival, thinks it will grant him everlasting life. Brunhilde later sees it as the symbol of Siegfried's love.