Fafnir/Frænir

Fafnir is a mythological figure from Norse mythology. He was a Dwarf or Jotunn and one of the sons of Hreidmar the magician. He desired his father’s magic ring Advarinaut, which could magically produce treasure. Eventually, he became so corrupted by his desire that he killed his father for the ring. He gained a great pile of treasure and turned into a dragon to protect it, though the corruption of his greed forced him to remain in that form. Fafnirs brother Regin also desired the ring but waited too long to act on his greed. To get the ring from his brother, he sent his foster son, the hero Sigurd, to go and kill him. He repaired the sword of Sigurd’s father, Nothung, and advised him to stab the Fafnir’s underbelly since the rest of his body was covered in impenetrable scales, and he could attack with poisonous breath. He dug a tunnel underneath Fafnir and killed him by striking his heart. 

Citations:

Bane, Theresa. Encyclopedia of Beasts and Monsters in Myth, Legend and Folklore. United States, McFarland, Incorporated, Publishers, 2016.