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. However, the
capitular canons refused. Magnus then spent his youth at the Swedish royal court of the
House of Vasa. He befriended his cousin Prince Eric and, after his ascension to the throne as King
Eric XIV, Magnus fared well. Eric XIV waged war against their common cousin King
Frederick II of
Denmark. Magnus advanced in this conflict, which became the
Scandinavian Seven Years' War (1563–1570), to the Swedish supreme command in 1566. Magnus married Eric's half-sister
Princess Sophia of Sweden in a discreet ceremony on the eve of Eric's own marriage on 4 July 1568. However, Eric came to dismiss the rather unsuccessful Magnus as supreme commander. Later Magnus swung over to Eric's half-brothers
Charles and John, and Magnus invaded Stockholm with them on 29 September of that year, overthrowing Eric. 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. About 1570 Magnus prevented Katarina's plans to remarry with his brother Francis. As a Swedish commander, and with Sophia's
dowry at his disposal, Magnus had gained a considerable fortune and pursued a new prize. His father Francis I agreed to resign in favour of Magnus in 1571, in return for which Magnus promised to redeem the pawned ducal
demesnes. Rather than redeeming the estates, however, Magnus, further alienated ducal possessions, for instance selling the expectancy to the pawned estates of the bailiwick (Amt) of
Tremsbüttel to Duke
Adolphus of Holstein-Gottorp. 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. In October 1573 Francis I deposed Magnus and re-ascended the ducal throne. The following year Magnus hired, among others, Dutch troops to take Saxe-Lauenburg by force. He invaded, ravaged and plundered
Ratzeburg in early October but withdrew when his brother Francis (II), an experienced military commander, and Duke Adolphus of Holstein-Gottorp,
Circle Colonel (
Kreisobrist) of the
Circle of Lower Saxony, arrived with troops. In return Saxe-Lauenburg had to cede the bailiwick of
Steinhorst to
Holstein-Gottorp in 1575. Magnus fled to his estates in
Uppland in 1574, there displaying violence, wantonness and brutality. 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. Magnus fell out with
Klaus von Ungern, then the local Danish
stadholder in
Arensburg, the Danish part of Ösel. Magnus claimed the Danish island
Mön as part of his estates and occupied it. Further, he robbed
burghers in
Pernau. His atrocities also included abuse of his wife Sophia. 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. In 1581 - shortly before he died and after consultations with his son, the Prince-Archbishop
Henry of Bremen and
Emperor Rudolph II, but not negotiated with his other sons Magnus and
Maurice - Francis I made his third son Francis II, whom he considered the ablest, his sole successor, violating the rules of
primogeniture. The violation of primogeniture, however, gave grounds for the estates to consider the upcoming duke as illegitimate. Francis II, though, only officiated as administrator of Saxe-Lauenburg, while Magnus II appealed to Rudolph II to endow him with the throne. On 31 January 1585 Rudolph II finally ruled in favour of Francis II, as agreed with Francis I in 1581. Meanwhile, Francis II had won over his brother Maurice by sharing rule with him and with the estates. On 16 December 1585 Francis II accepted, by the constitutional act of the "Eternal Union" (), the establishment of the representatives of Saxe-Lauenburg's nobility and cities, Lauenburg upon Elbe and Ratzeburg, as the estates of the duchy; a permanent institution with a crucial say in government matters. In return the estates accepted Francis II as legitimate and rendered him homage as duke in 1586. Francis II lured Magnus into a trap in
Hamburg and captured him later in 1588. Magnus remained imprisoned for the rest of his life, mostly in the castle of
Ratzeburg, where he died in 1603. ==Marriage and issue==