Magnus is the son of Kwenthrith of Mercia, who claimed that Ragnar was his father. Having no reason to doubt her, Magnus believes it to be true, though Ragnar stated that he did not lay with Kwenthrith. Magnus is not able to convince Lagertha nor Ubbe, though, since Ragnar told Lagertha
the truth. Bjorn, however, believes Magnus. He is driven by revenge for Ragnar's fate at the hands of the King and attempts to convince Lagertha, Ubbe, and Bjorn to join with King Harald against King Alfred.
Magnus has come across as an annoying character that does not provide real substance to the series. Viewers who know that he is not the son of Ragnar are more likely to dismiss scenes with him, as they do nothing for the overall story.