In 1550 Francis I sought to exercise influence to compel the cathedral chapter of the neighbouring Prince-Bishopric of Ratzeburg to elect his seven-year-old son Magnus as the next prince-bishop.
On this occasion Magnus took Katarina Stenbock, the step-mother of his wife Sophia, and her half-sister Princess Elizabeth of Sweden by boat from the royal palace of Stockholm.
As a Swedish commander, and with Sophia's dowry at his disposal, Magnus had gained a considerable fortune and pursued a new prize.
This ignited a conflict between Magnus on the one hand and his father and brothers, Francis (II) and Maurice, as well as the estates of the duchy.
Opposition to Magnus was also driven by a temperament notoriously prone to drink and the infliction of violence on dissenters and inferiors.
So next year his brother-in-law, the new King John III of Sweden, enfeoffed Magnus with Sonnenburg castle in Orissaare on Ösel island, recently conquered from Denmark.
The Danes complained, meanwhile Magnus fell into John's disfavour for his mistreatment of Sophia, who separated from him and stayed with their son Gustav in Sweden.
In 1578 Magnus started a second attempt to conquer Saxe-Lauenburg, but was repelled by his brother Francis (II), whom - for his military success - their father rewarded with the rank of viceregent.
Their marriage was unhappy and in 1578 Sophia's brother, King John III of Sweden, expelled Magnus from the kingdom.