The Nibelungen Ring was a golden ring, originally named Andvaranaut, that was cursed by its creator: the dwarf Andvari. While traveling the world one day, the god Odin, his brother Hoenir and his blood-brother Loki spotted an otter. Unthinkingly, they killed and skinned the creature, but as a result were captured by a dwarf king, Hreidmar.
The otter, in truth, had been one of Hreidmar's sons, Otr, who had the ability to shapeshift into an otter, and the king demanded either blood-payment (wergeld) for the murder or the death of the three gods involved. Loki offered to get the ransom for Hreidmar, who demanded enough gold to cover and fill the hide of his dead son. Loki went to Andvari and bullied the dwarf into handing over a large amount of gold and treasures. However, angered by Loki, Andvari cursed anyone who took ownership of the golden ring to die, hoping to take revenge on the god of mischief. However, the ring passed to Hreidmar with the rest of the gold, sparing Loki from the curse, and the gods were released to return to Asgard. As Hreidmar's family was known as the Nibelungs, the ring came to be known as the Nibelungen Ring, or Ring of the Nibelungs.
Sometimes after this, due to the ring's influence, Fafnir become murderously jealous of the treasure his father possessed and killed him. Fafnir then transformed himself into a dragon, hiding the treasure in a cave and guarding it constantly. However, despite his effort, Fafnir could not escape the ring's curse: he was later found and slain by the hero Sigurd (renamed Siegfried in the composer Richard Wagner's Ring Cycle), who then claimed the treasure, and, unfortunately, also claimed the ring.
Sigurd would continue on his travels, ultimately coming to an impassible barrier of fire. He leaps over the flames on horseback and finds a magnificent bed, upon which lay a sleeping warrior in full armor. Sigurd removes the armor, revealing and awakening a beautiful woman, the former Valkyrie Brynhildr, who was punished with mortality by Odin for disobeying him. She was additionally punished to love the next human she saw, although due to Sigurd's bravery she is very willing to marry him. The two pledge themselves to each other, but unaware of the ring's curse, Sigurd uses the Nibelungen Ring as an engagement ring, slipping it on Brynhildr's finger. He rides off, promising to return for her.
However, Sigurd is waylaid by a local king, Gjuki, whose daughter, Princess Gudrun, falls in love with Sigurd after hearing of his adventures. She contrives to take him from Brynhildr and succeeds in feeding him a love potion. Sigurd forgets his intended wife and married Gudrun instead; Brynhildr is ultimately rescued from the fire by Gudrun's brother, Prince Gunnar, who disguises himself in Sigurd's armor. However, when taunted by Gudrun, Brynhildr finds out the truth and decides to take revenge on Sigurd for betraying her.
Brynhildr orchestrates the death of Sigurd using her brother-in-law, Guttorm, who slays Sigurd by impaling a spot under his shoulder blade, the hero's one weakness. However, when Sigurd's body is about to be cremated in a funeral pyre, Brynhildr, still wearing the Nibelungen Ring, regrets what she has done and rides at full gallop into the flames of the pyre to die with Sigurd, taking the accursed ring to the underworld with her.
It appears that, due to Poseidon's influence, the Nibelungen Ring renders its wearer physically submissive to the sea god's commands. However, it does not supress the mind of its wearer, and although they cannot control their actions, they are fully aware of what their body is doing under Poseidon's control.