Björn Ironside, according to Norse legends, was a Norse Viking chief and Swedish king. According to the 12th- and 13th-century Scandinavian histories, he was the son of notorious Viking king Ragnar Lodbrok and lived in the 9th century, between 855 and 858. Björn Ironside is said to have been the first ruler of the Swedish Munsö dynasty. In the early 18th century, a barrow on the island of Munsö was claimed by antiquarians to be Björn Järnsidas hög or Björn Ironside's barrow. Medieval sources refer to Björn Ironside's sons and grandsons, including Erik Björnsson and Björn at Haugi. Icelandic sagas claim that Björn was the ancestor of the house of Munsö, the line of kings that ruled in Sweden until c. 1060. Björn in Frankish sources "Berno" was a powerful Viking chieftain and naval commander. He appears in contemporary sources such as Annales Bertiniani and the Chronicon Fontanellense. He is first mentioned in the summer of 855. The oldest text that details his origins is the Norman history of William of Jumièges (c. 1070). According to William, the Danes had a custom of requiring the younger sons of kings to leave the kingdom, to reinforce the king's authority; thus, after Ragnar Lodbrok became king, he ordered Björn to leave his realm. Björn left Denmark with a considerable fleet and started to ravage in West Francia. The contemporary annals show that he cooperated with another Viking called Sigtrygg and sailed up the Seine in 855, from which his and Sigtrygg's forces raided the inland. Their combined forces were beaten in Champagne by Charles the Bald of West Francia in the same year, but not decisively. Sigtrygg withdrew in the next year, but Björn received reinforcement from another Viking army and could not be expelled from the Seine area. He and his men took up winter quarters at the so-called Givold's Grave, which served as base for an assault against Paris, which was plundered around the new year 856–857. Björn constructed a fortification on the island Oissel above Rouen which he kept as his stronghold for years. He certainly swore fealty to Charles the Bald in Verberie in 858 but it is not clear if he kept his pledge. King Charles eventually resolved to meet the unruly Seine Vikings with all his available forces and besieged Oissel in July. The siege failed badly, for the pirates defended the fortification with vigour. Moreover,