Andvari was a Svartálfr that lived in a cave behind a waterfall, and possessed not only gold and riches in abundance, but also had the power to transform himself into a mighty pike, which served him well —the pool in front of his cave being filled with fish.
Another family of Svartálfr lived nearby: Hreiðmarr and his three sons Regin, Fafnir, and Ótr. Each with their own talents and abilities as well.
Regin was a master blacksmith; Fafnir could transform himself; and Ótr was a masterful fisherman, aided by his ability to transform into an otter. He would spend the days fishing in Andvari’s pool and bring his catch back to his father.
One such day, Ótr had caught a sizable salmon, and, after eating his fill, laid down on the banks of the pool to rest. It was while he was asleep that three of the Æsir Óðinn, Hœnir, and Loki came upon the pool and the waterfall.
Loki, spying the sleeping otter, grabbed a nearby stone and flung it at the poor creature, killing it instantly. Not knowing what they had done, the Æsir skinned the creature and, coming upon Hreiðmarr’s house, gave it to him as an offering.
Hreiðmarr, enraged, demanded recompense for the slaying of his son. He ordered them to fill the otter skin with gold and riches and return it to him. Óðinn consented, obliging Loki to fulfill the request.
Loki sought out Ran, the goddess of the sea and, borrowing her net, cast it into Andavir’s pool, snaring him immediately. Loki threatened the Svartálfr, now returned from his pike form, and demanded all the riches he kept in his cave. Powerless, Andvari watched as Loki took everything but one ring.
Loki then seized this as well, leaving Andvari with nothing.
Andvari cursed the ring, and all the gold that it would be a bane to anyone who possessed it. Heedless, Loki delivered his gift to Hreiðmarr who took it greedily, including the cursed ring.
Fafnir, being driven mad by greed, transformed into a fearsome dragon and slew his father and drove out his brother, Regin, so that he alone would be in possession of the hoard of treasure. But the curse of Andvari could not be undone.
Regin fled to the king Sigurd for whom he became chief smith and, being cursed to lust after that same treasure, encouraged the king to slay the dragon and claim the treasure for himself.
Needless to say, Regin dies before he too can lay claim to Andvari’s gold, beheaded by the very sword he smithed for Sigurd.
Over time the gold passes hands, until it is wisely returned to Andvari’s cave.
Upon finding it in his cave, Andvari notices that the ring has gone missing, and spend the rest of his life looking for it.
The Saga of Andvaris Ring is a part of the Vǫlsunga Saga, and the main inspiration for both Lord of the Rings and Der Ring Des Nibelungen.