Birger Jarl
Magnus, whose birth year has never been confirmed in modern times, was probably the second son of Birger Jarl (1200–66) and Princess Ingeborg, herself the sister of the childless King Eric XI and daughter of King Eric X. Thus, Valdemar Birgersson (1239–1302) was the eldest son and ruled as Valdemar, King of Sweden from 1250–1275, succeeding King Eric, their maternal uncle who ruled until 1250. Birger Jarl had designated Magnus as Jarl, henceforth titled Duke of Sweden, and as Valdemar's successor. Even after Valdemar's coming of age in 1257, Birger Jarl kept his grip over the country. After Birger's death in 1266 Valdemar came into conflict with Magnus who wanted the throne for himself.