Andvari

In Norse mythology, Andvari (Old Norse "careful one") is a dwarf who lives underneath a waterfall and has the power to change himself into a fish at will. Andvari had a magical ring Andvaranaut, which helped him become wealthy.

Myths & legends
Using a net provided by Rán, Loki catches him as a pike and forces him to give up his gold and Andvaranaut. Andvari cursed the stolen gold which would destroy anyone who possessed it. Loki used this cursed gold to pay the ransom to Fáfnir for the release of Óðinn and Hoenir.

After the deaths of Brynhildr and Sigurðr who later obtained the gold through killing Fáfnir, Gunnarr left Andvari's gold in a cave. Years later, Andvari discovered the cave and his lost gold, although his ring was lost forever.

Modern dpictions
In Richard Wagner's cycle of music dramas, Der Ring des Nibelungen, the character Alberich is based in some part on Andvari, but more on the Frankish sorcerer Alberich.