In Norse mythology, Fáfnir (Old Norse and Icelandic) or Frænir was a son of the dwarf king Hreidmar and brother of Regin and Ótr. In the Volsunga saga, Fáfnir was a dwarf gifted with a powerful arm and fearless soul. He guarded his father's house of glittering gold and flashing gems. He was the strongest and most aggressive of the three brothers.
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After Ótr was killed by Loki, Hreidmar received the cursed gold of Andvari's as repayment for the loss of his son. Fáfnir and Regin then killed their father to get the gold, but Fáfnir decided he wanted it all, turning into a dragon (symbol of greed). Regin then sent his foster-son, Sigurd, to kill the dragon. Sigurd succeeded by digging a pit under the trail Fáfnir used to walk to a stream and plunging his sword Gram into his heart as he walked past. Regin, however, corrupted by the curse on Andvari's gold, planned to kill Sigurd to take the treasure for himself, but Sigurd, having eaten part of Fáfnir's cooked heart, was warned by birds of Regin's attack and ended up killing him.
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As Fafner, he is featured in Richard Wagner's Der Ring des Nibelungen, although he began life as a giant rather than a dwarf, before once again turning into a dragon to better guard the gold.
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Some versions are more specific about Fafnir's treasure hoard, mentioning the swords Ridill and Hrotti.