Sinfjötli (Old Norse) or Fitela (Anglo-Saxon) was born out of the incestuous relationship between Sigmund and his sister Signy. He had the half-brothers Sigurd, Helgi Hundingsbane and Hamund.

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

Sinfjötli's mother, Signy, had married the Geatish king Siggeir who treacherously murdered her whole clan until only Sigmund was left. She dressed up as a young Völva (witch, female shaman) who visited Sigmund and slept with him. Then she gave Sigmund a son, Sinfjötli, who would avenge their clan together with Sigmund by killing Siggeir.

Sigmund and Sinfjötli went to Hunaland where Sigmund was proclaimed king of the Huns. Sigmund married Borghild and had the sons Helgi Hundingsbane and Hamund. Borghild was jealous and hated Sinfjötli, which Sinfjötli knew. In order to dispose of him, she gave Sigmund three cups of wine of which the last contained poison. After having seen his father drink two of the cups, Sinfjötli drank the third and died.

Sigmund brought his son's dead corpse to the fjords, where he met Odin disguised as a ferryman. Odin said that he could only take one passenger at a time and took Sinfjötli's body first. Out on the water, Odin and Sinfjötli disappeared, and went to Valhalla.

Sigmund went home and banished Borghild.