Fafnir

Fafnir was a dwarf who turned into a dragon after stealing Andvari's ring.

The Hammer of Thor
Hearthstone tells Magnus that Fafnir was a dwarf who went mad after stealing Andvari's ring. He murdered his father and guraded the treasure in a cave, eventually turning into a dragon. From then on, the curse of Andavari's ring was known as The Curse of Fafnir.