He was hailed as the heir to the Swedish throne and became embroiled in the power struggles between his father and his uncles, dukes Erik and Valdemar, who had rebelled against King Birger.
However, King Haakon donated them Kungaälv at the Norwegian-Swedish border as a fief, and from there, the dukes waged open war against Birger.
According to Erikskrönikan, the six-year-old Magnus was rescued by a squire from Småland and carried in his back to Denmark where he was taken up by his maternal uncle, King Erik Menved.
The throne of Norway was at stake in this betrothal, since Haakon had changed the order of succession so that Ingeborg's future children and possibly his husband would succeed him.
The formal betrothal took place at the St. Mary's Church in Oslo, with Bishop Peder of Viborg standing in for Magnus in the ceremony.
[1] In 1317, King Birger decided to get his revenge against the dukes and to regain the full control over Sweden, which had been divided between the three brothers in 1310.
[4] The dukes' deaths caused an uproar and King Birger was faced with a rebellion which he tried to control with Erik Menved's support.
According to Erikskrönikan, Magnus arrived from Denmark in early 1318 with 600 Danish cavalrymen which joined Birger's forces in Östergötland.
[3] Birger and Märta fled to Gotland, while Magnus was left behind to command Stegeborg Castle, which was sieged by the dukes' supporters.
[4] In 1319, Birger was deposed and Duke Erik's three-year old son Magnus Eriksson was elected King of Sweden.