The Icelandic sources make Björn one of the sons Áslaug bore to Ragnarr Loðbrók, and Ragnarssona þáttr sets him beside Ívarr the Boneless and Sigurðr Snake-in-the-Eye when the brothers kill King Eysteinn Beli and carve up the North between them. Björn's share was Svíþjóð, Sweden proper. Hervarar saga ok Heiðreks carries the line on through two sons, Refill and Eiríkr, and it is from that thin genealogical thread that later historians hung an entire royal house: the Munsö dynasty, named for an island in Lake Mälaren where an eighteenth-century antiquarian identified a barrow as Björn Järnsidas hög. A broken runestone stands on the mound. It says nothing about him.
The voyage that made his name left the Loire in 859 alongside the Danish chieftain Hásteinn with, by one accounting, sixty-two ships. They forced the Strait of Gibraltar, wintered in the Camargue, worked their way up the Rhône, and took Pisa. Then came Luni, a small Ligurian port with marble quarries and a bishop, which the Norsemen are supposed to have taken for Rome itself. William of Jumièges tells the trick: Hásteinn had himself carried to the gate as a dying man begging baptism, was borne into the church in a coffin, and rose out of it armed. Whether any of that happened, the sack did. Homeward in 861 the fleet struck Pamplona and carried off García Íñiguez, king of the Pyrenean Basques, for a ransom reported at seventy thousand gold dinars.
What survives outside the sagas is thinner and stranger. The Annales Bertiniani note a Viking leader called Berno active on the Seine between 855 and 858, and Frankish and Arabic chroniclers between them confirm that a large northern fleet did go into the Mediterranean and did come back gutted: roughly twenty ships of the original sixty, the rest lost to storms off Spain and to the fire-throwing engines of the Andalusian navy. The chronology that would make this Berno a son of Ragnarr refuses to close, and few historians now defend the Swedish kingship. The Mediterranean voyage needs no pedigree. Fewer than a third of the men who rowed out of the Loire saw it again, and those who did came home rich.