Magnus, Louis and the latter's father-in-law William II, Duke of Brunswick-Lüneburg (Celle), and their troops beleaguered Morris in the prince-archiepiscopal castle in Vörde and forced him to sign his resignation.

After the death of his brother Louis in 1367, Magnus became the designated heir of both ducal principalities, Wolfenbüttel and Celle (colloquially also Lüneburg).

When both his father and William II, who ruled over Celle, died in 1369, Magnus gained both ducal principalities.

But already in 1370, he lost Celle to the Ascanian dukes of Saxe-Wittenberg (Albert and his uncle Wenceslas, Elector of Saxe-Wittemberg), who had been given the principality by Charles IV, Holy Roman Emperor, who had also banned Magnus.

The Lüneburg War of Succession continued for several years after Magnus died in the Battle of Leveste (a part of today's Gehrden), near the Deister, on 25 July 1373.