Guthorm Sigurdsson
Guthormr or Guttormr was according to traditions current in Iceland in the eleventh and twelfth centuries, a noble of Danish origin operating in Norway and the maternal uncle of Norway's first king, Harald Fairhair. By various sources he is said to have been the son of Sigurd Hart, brother of Ragnhild Sigurdsdotter and the descendant of Ragnar Lodbrok. Several sources gives him the title dux and hertỏga both meaning "duke".
Family
Sources on Guthorm describes him as the son of Sigurd Hart and younger brother of Ragnhild Sigurdsdotter. Ragnhild Sigurdsdotter's parentage is controversial as two differing accounts appear in medieval genealogy varies in her parentage. He is alternatively described as the daughter of his son Sigurd Snake-in-the-Eye, while others describe her as the daughter of Sigurd Hart and Sigurd Snake-in-the-Eye's granddaughter, adding an additional generation.Attempts to fit the chronology of the sagas with history does not work with the additional generation between Sigurd Snake-in-the-Eye and Harald Fairhair.
Heimskringla
In Heimskringla, in the section labelled the "Saga of Halfdan the Black" tells of Sigurd Hart, the grandson of Sigurd Snake-in-the-Eye and great grandson of Ragnar Lodbrok. Sigurd had two children: a daughter named Ragnhild and a son called Guthorm. Haki, a berserker encountered her father in Hadeland and killed him, before kidnapping Ragnhild and Guthorm. Soon afterwards, Ragnhild and Guthorm were kidnapped by Halfdan the Black's sorcerer Horik Gand, so that Halfdan could marry Ragnhild. Fagrskinna does not mention any of these details. However, both sagas agree that Ragnhild and Halfdan the Black were the parents of Harald Fairhair.In the following "Saga of Harald Fairhair" Guthorm is said to have been made the leader of Halfdan's hird and played a instrumental role in the defence of Harald's kingdom during his minority. Halfdan the Black drowned while chasing thieves over a frozen lake. His son Harald was only ten years old at the time and Halfdan's various enemies attacked the kingdom. Through Guthorm's guidance, Harald managed to defeat all the invaders despite his young age, starting Harald's conquest of Norway. Guthorm did not take further part of the conquest, rather ruling Harald's original domains during the campaign.
Harald named one of his sons after Guthorm and subsequently made his uncle the local governor of Upland and Viken, ruling from Tønsberg. He died of old age.
Heimskringla gives Guthorm the title dux, a title which was not known to have been used in Norway in the 9th and 10th century when Guthorm is supposed to have lived.