His earliest known ancestor is thought to be Folke the Fat, a powerful Swedish leader of the early 12th century, who married Ingegerd Knudsdatter, daughter of Canute the Saint of Denmark and Adela of Flanders, a descendant of Charlemagne.
[1] Ingegerd and her sister Cecilia both went to Sweden after the death of Adela and married there, and Folke and his kin were therefore close to the ruling elite of the Kingdom of Denmark.
[3] While his older brother Birger Brosa held the office of Riksjarl between 1174 and 1202, the younger Magnus lived at the family estate Bjälbo, in the current Mjölby municipality, Östergötland, Sweden.
[5] He married the noblewoman Ingrid Ylva, of possible royal descent, and fathered several sons who would influence early Swedish history, most notably Birger Jarl.
The first was the Battle of Älgarås, (now a place in the Swedish community Töreboda in Västergötland) in November 1205, where King Sverker II defeated Erik Knutsson.
A medieval Liber Daticus (Book of Donations) from Lund states that an aristocrat called Magnus fell at the Battle of Lena ("II Kal.