Ragnarssona þáttr informs that he was the son of Helgi the Sharp (the great-great-grandson of king Ring of Ringerike) of the Dagling dynasty and Aslaug. She was the daughter of Sigurd Snake-in-the-Eye (one of Ragnar Lodbrok's sons) and Blaeja, the daughter of king Aelle II of Northumbria. When Sigurd Hart was only twelve years old he slew a berserker named Hildibrand in a duel, and twelve other men. He married Ingeborg, the daughter of the Jutish chieftain Harald Klak. Sigurd and Ingeborg had the children Guttorm and Ragnhild. When his uncle king Fróði of Ringerike died, Sigurd went to Norway to succeed him as king.
Ragnarssona þáttr and Halfdan the Black's saga relate that a berseker from Hadeland named Haki killed Sigurd, but lost a hand in the fight. Then Haki went to Sigurd's residence at Stein and took Sigurd's children Ragnhild and Guttorm. Haki returned with the children and all the loot to Hadeland. Before Haki recuperated from his wounds and could marry the 15 or 20 year old Ragnhild, she was captured a second time, by Halfdan the Black. Halfdan and Ragnhild were the parents of Harald Fairhair.