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Ragnarssona þáttr informs that he was the son of Helgi the Sharp (the great-great-grandson of king King of Ringerike) of the Dagling dynasty and Aslaug. Helgi was the son-in-law 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 eleven 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.
Excerpt from Halfdan the Black's saga: Sigurd Hjort was the name of a king in Ringerike, who was stouter and stronger than any other man, and his equal could not be seen for a handsome appearance. His father was Helge Hvasse (the Sharp); and his mother was Aslaug, a daughter of Sigurd the snake-eyed, who again was a son of Ragnar Lodbrok. It is told of Sigurd that when he was only twelve years old he killed in single combat the berserk Hildebrand, and eleven others of his comrades; and many are the deeds of manhood told of him in a long saga about his feats. Sigurd had two children, one of whom was a daughter, called Ragnhild, then twenty years of age, and an excellent brisk girl. Her brother Guthorm was a youth. It is related in regard to Sigurd's death that he had a custom of riding out quite alone in the uninhabited forest to hunt the wild beasts that are hurtful to man, and he was always very eager at this sport. One day he rode out into the forest as usual, and when he had ridden a long way he came out at a piece of cleared land near to Hadeland. There the berserk Hake came against him with thirty men, and they fought. Sigurd Hjort fell there, after killing twelve of Hake's men; and Hake himself lost one hand, and had three other wounds. Then Hake and his men rode to Sigurd's house, where they took his daughter Ragnhild and her brother Guthorm, and carried them, with much property and valuable articles, home to Hadeland, where Hake had many great farms.