Fáfnir (Frænir) was a son of the dwarf king Hreidmar and brother of Regin and Ótr. As told in the Völsunga saga, Fáfnir slew his father Hreidmar to obtain the gold which he had demanded of Odin, as a compensation for the slaughter of Ótr. Greedy Fáfnir changed into a dragon to guard his treasure, but was later slain by the young hero Sigurd. Sigurd was the foster-son of Regin, who plotted revenge against his brother for the death of their father.
After Sigurd, under the advice of Odin, had killed Fáfnir, Regin asked him to cook the dragon’s heart for him. Sigurd touched the heart as it was cooking to test if it was done and burned his thumb. He put his thumb into his mouth, and thus gained the ability to understand the language of birds. The birds told Sigurd that it was Regin’s intention to kill him, so Sigurd killed Regin and left with Fáfnir’s treasure.