His existence is attested by the nearly contemporary account Vita Ansgari, written by the missionary Rimbert, who visited Sweden alongside Ansgar in c. 852.
[1][2] Although the genealogy of early Swedish local rulers is not known, Björn is assumed to have belonged to the House of Munsö, which would later produce Eric the Victorious, generally considered to be Sweden's first king.
Among other matters they told the Emperor that many people in their country wished to embrace the Christian religion, and that their king endorsed the arrival of proper teachers.
[5] After traveling a long distance, mostly by foot, they arrived to the port of the kingdom which was called Birka, where they were friendly received by the king, whose name was Björn ("Tandem ad portum regni ipsorum, qui Byrca dicitur, pervenerunt, ubi benigne a rege, qui Bern vocabatur, suscepti sunt").
[12] When speaking of Ansgar's second visit to the Swedes in about 852, the Vita states that "almost the entirety of that land consists of islands", suggesting a mainly coastal and insular polity.
[13] The Vita Karoli Magni, written around the time of Ansgar's visit, has a short description of the Baltic region: "There is a bay which stretches from the western ocean in eastern direction, of unknown length but with a breadth that may not exceed 150 kilometers, while it is less at many sites.
[14] This account, together with other sources from the 9th century, seems to indicate that the Danes and Swedes were the main Nordic peoples known to the continental authors, and that Björn's sphere of power was not merely local.