Regin, in Norse mythology, the youngest son of Hreidmar (also spelled Hreithmar or Rodmar), and brother of Fafnir and Otter. Regin coveted the cursed gold hoarded by the Nibelungen gold, which his father had obtained the gold from the gods. Regin engaged the young hero Sigurd to win back the gold from Fafnir, who had killed their father Hreidmar to obtain the gold and turned himself into a dragon to better guard the treasure. Regin plotted to kill Sigurd for the gold once the hero had slain the dragon, but Regin was slain instead by Sigurd, who had discovered his treachery. The tale was chronicled with slightly different details in the Scandinavian 'Volsunga Saga' and in the Icelandic 'Poetic Edda' and 'Prose Edda'. In some versions of the saga, Regin and Fafnir together murdered their father to obtain the gold; in others, Fafnir did the deed alone. But afterward, when Regin demanded his half of the hoard, Fafnir threatened to kill him rather than give up any of the treasure. Regin fled, and Fafnir took the shape of a dragon and perpetually guarded the gold in Gnitaheid (Glittering Heath).
Regin bided his time, becoming a master craftsman, sometimes described as a wizard, and serving King Hialprek in Thiod. The young hero Sigurd (also spelled Sigurth), whose mother had married Hialprek's son, became Regin's apprentice. When Sigurd was old enough, Regin incited him to slay Fafnir. He told Sigurd about the gold, and then fashioned a magic sword, called Gram, for Sigurd to use in the slaying. The sword was so sharp that when Sigurd put it down in running water, it cut in two a tuft of wool that drifted against its edge; and it was so strong that Sigurd was able to split Regin's anvil down to the base with it.
Regin brought Sigurd up to Gnitaheid, and then disappeared into the woods to hide among the heather while Sigurd attempted to kill Fafnir. According to the "Fafnismal" (The Lay of Fafnir) in the 'Poetic Edda', as Fafnir lay dying from the wound inflicted by Sigurd's sword, he warned the hero that Regin would betray Sigurd as Regin had betrayed him. Regin reappeared as Sigurd was wiping the blood of the dead dragon from his sword and hailed him as the bravest of all heroes. Sigurd reproached the wizard for running off to hide during the fight.
Regin cut out Fafnir's heart and drank the blood that flowed from the wound. He told Sigurd to cook the dragon's heart over a fire because he wanted to eat it. Sigurd did as he was told, and when he thought the meat might be done, he touched it with his finger. The hot juice burned him, and he instinctively put his finger in his mouth. When the heart's blood touched Sigurd's tongue, he was suddenly able to understand the speech of birds. He overheard some titmice remarking that Regin was about to betray the boy who trusted him, and if Sigurd cut off Regin's head, he himself would be the sole owner of the gold, and if Sigurd himself ate Fafnir's heart, he would become wise. So Sigurd hacked off Regin's head as the old wizard slept. In another version of the story, Sigurd overheard one of the birds describe how Regin was sneaking up behind Sigurd to kill him. Enraged by this betrayal, Sigurd turned and slew Regin.
Sigurd took the birds' advice. He ate Fafnir's heart and drank both Fafnir's and Regin's blood, transferring their strength and skill to himself.