Sinfjotli

Sinfjǫtli (Old Norse: [ˈsinˌfjɔtli]) or Fitela (in Old English) in Norse mythology was born out of the incestuous relationship between Sigmundr and his sister Signý. He had the half-brothers Sigurðr, Helgi Hundingsbane and Hámundr.

Orthography
In the standardized Old Norse orthography, the name is spelled Sinfjǫtli, but the letter 'ǫ' is frequently replaced with the Modern Icelandic ö for reasons of familiarity or technical expediency.

Life
In Beowulf, Fitela is the nephew of Sigmundr, whereas the Vǫlsunga saga describes him as both Sigmundr's son and nephew due to incest.

In the Vǫlsunga saga, Sinfjǫtli is the grandson of King Vǫlsungr. Signý, King Völsung's daughter, despises her husband King Siggeir, and begs "that she may not be made to return to King Siggeir." Völsungr denies her request to leave, reminding her of the commitment their family must maintain. Despite a warning from Signý, King Völsungr and his 10 sons engage in a battle as King Siggeir’s unbeatable army treacherously murders King Völsungr. Signý pleads with her husband not to dispose of her 10 brothers, but rather Siggeir grants her request "because [he thinks] it better that they suffer more and are tortured longer before they die."

Through 9 long nights, a returning she-wolf (thought to be King Siggeir’s mother) kills 9 of Signý's brothers in turn. A message is passed to the last brother remaining, Sigmundr, to smear honey on his face and bite the tongue out of the she-wolf’s mouth, thus killing her and saving himself. Since Sigmundr has lost his father and brothers, Signý wishes to support him in avenging their family by sending her son to be his companion in his hideout in the forest. However, each of her sons by King Siggeir proves cowardly: they cannot withstand the pain of having the cuffs of their kirtles sewn to their skin. So Signý has Sigmundr kill them as they are no longer of any use. Sigmundr being of the Völsungr line, Signý believes her brother will produce a son worthy of claiming a place in that line, and one night she "exchange[s] shape" with a völva, goes to Sigmundr in his underground dwelling, and spends three nights in his bed with Sigmundr unaware that she is his sister in another form. Very unlike her previous sons born of the devious and unpleasant King Siggeir, Signý then bears a son who is no less strong, handsome, and fearless than Völsungr himself. This is Sinfjǫtli, who together with Sigmundr will avenge their clan by killing Siggeir. Sinfjǫtli ignores the pain, saying that "such pain would seem trifling to Völsungr." The incestuous product of the coalesced Völsungr line, he is capable of the great deeds Signý requires.

Sigmundr and Sinfjǫtli go to Hunaland where Sigmundr is proclaimed king of the Huns. He marries Borghildr and has the sons Helgi Hundingsbane and Hámundr. Borghild is jealous and hates Sinfjǫtli, which Sinfjǫtli knew. In order to dispose of him, she gives Sigmundr three cups of ale, of which the last contains poison. After having seen his father drink two of the cups, Sinfjǫtli drinks the third and dies.

Sigmundr brings his son's corpse to the fjords, where he meets Óðinn disguised as a ferryman. the ferryman says that he can only take one passenger at a time and takes Sinfjǫtli's body first. Out on the water, he and Sinfjǫtli disappear, and go to Valhǫll.

Sigmundr goes home and banishes Borghild. This is roughly the tale told in J.R.R. Tolkien's The Legend of Sigurd and Gudrún.

In Helgakviða Hundingsbana I, from the Poetic Edda, Gudmund accuses Sinfjǫtli of being a werewolf.