He is named, and his parentage given, in the Annalista Saxo. Morkinskinna names “Magnus” as son of “King Magnus…sister…Úlfhildr…daughter of King Óláfr” and “a powerful duke named Otto south in Saxony”. He participated in the rebellion of Otto von Northeim Duke of Bavaria and was imprisoned by Heinrich IV King of Germany in 1070.
He succeeded his father in 1072 as MAGNUS Herzog von Sachsen although still in captivity.
The Annales Rosenveldenses records the death "1106 VIII Kal Sep in Ertheneburg" of "Magnus dux" and his burial "Luneburg in monasterio". The necrology of Lüneburg records the death "23 Aug" of "Magnus dux". 1
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