According to
Íslendingabók,
Heimskringla and
Fagrskinna, Halfdan was the son of the Yngling King
Gudrød the Hunter.
Heimskringla also names his mother, as
Åsa, daughter of King Harald of
Agder, and his half-brother as
Olaf Geirstad-Alf. Heimskringla relates that when Halfdan's father was killed, Åsa took the 1 year-old Halfdan and returned to
Agder, where Halfdan was raised. When he was 18 or 19 years old, Halfdan became king of Agder. He quickly began adding to his kingdom, through political negotiation and military conquest. He divided the kingdom of
Vestfold with his brother Olaf and, through military action, persuaded
King Gandalf of
Vingulmark to cede half his kingdom. Based on the formulaic nature of his ties to his predecessors, his strong affiliation with Agder, and the failure of an early saga dedicated to him to name any family connections, some scholars have suggested that the linkage to the earlier Yngling dynasty of Vestfold was a later invention, created to associate a conquering Halfdan and his son
Harald Fairhair with the family glorified in the
Ynglingatal, whom he had displaced.{{cite web|url = https://nbl.snl.no/Halvdan_Svarte|title= Halvdan Svarte |publisher =Norsk biografisk leksikon|author= Claus Krag Halfdan next is said to have subdued an area called
Raumarike. To secure his claim to Raumarike, Halfdan first defeated and killed the previous ruler,
Sigtryg Eysteinsson, in battle. He then defeated Sigtryg's brother and successor
Eystein, in a series of battles. This established Halfdan's claim not only to Raumarike, but also to half of
Hedmark, the core of Sigtryg and Eystein's kingdom. These details are only mentioned in
Heimskringla.
Fagrskinna and
Heimskringla both agree that Halfdan's first wife was Ragnhild, daughter of King Harald Gulskeg (Goldbeard) of
Sogn. Halfdan and Ragnhild had a son named "Harald" after his grandfather, and they sent him to be raised at his grandfather's court. Harald Gulskeg, being elderly, named his grandson as his successor, shortly before his death. Ragnhild died shortly after her father, and the young king Harald fell sick and died the next spring. When Halfdan heard about his son's death, he travelled to Sogn and laid claim to the title of king. No resistance was offered, and Halfdan added Sogn to his realm. The narrative in
Heimskringla then adds another conquest for King Halfdan. In Vingulmark, the sons of Gandalf of Vingulmark,
Hysing,
Helsing, and Hake, attempted to ambush Halfdan at night, but he escaped into the forest. After raising an army, he returned to defeat the brothers, killing Hysing and Helsing. Hake fled the country, and Halfdan became king of all of Vingulmark. According to
Heimskringla, Halfdan's second wife, also named Ragnhild, had been kidnapped from her home by Hake, a "
berserker" who encountered her father in
Hadeland and killed him. Halfdan had her kidnapped from Hake, so that he could marry her.
Fagrskinna does not mention any of these details. However, both sagas agree that Ragnhild and Halfdan had a son who was also named
Harald. (Among the more unlikely claims in
Fagrskinna and
Heimskringla are that this woman was
Ragnhild Sigurdsdotter, daughter of
Sigurd Hjort, king of
Ringerike. This would make Ragnhild the granddaughter or even great-granddaughter of
Sigurd Snake-in-the-Eye – an impossibility, given that most sources suggest that Sigurd Snake-in-the-Eye was active only in the
late 9th century, which would mean that he was born a generation or two
after Halfdan the Black.) ==Halvdanshaugen==