Birger came of age when there was a conflict within the Church of Sweden over interpretation of the Privileges of 1280, which had been the cost of the support of the Church for his father's usurpation. The king's brothers Erik Magnusson, Duke of Södermanland and Valdemar Magnusson, Duke of Finland took advantage of this conflict. Duke Eric tried to establish an independent kingdom around Bohuslän, which he had received as part of his marriage to the Norwegian princess Ingeborg, and Halland at the boundary between Sweden, Norway and Denmark. A civil war broke out, but by 1306 emotions had cooled to the point where the dukes acknowledged the son of Birger, Magnus Birgersson, as the successor to the throne. Torkel Knutsson, who was Duke Valdemar's father-in-law, was executed in 1306 as a token of reconciliation between King Birger and his brothers. The same year, in an event known as the Håtuna games ("Håtunaleken"), Birger was taken captive by his brothers on the Håtuna royal estate in Uppland and taken as prisoner to Nyköping Castle ("Nyköpingshus").