The County developed out of the possessions of the Brunonen dynasty centered on the town of Brunswick and was enlarged by the inheritances of Henry the Fat of Northeim around Northeim and Göttingen and a part of the Billung inheritance around Lüneburg, which fell to the House of Welf in 1106.
By 1068, the Brunonen margrave Egbert I of Meissen had acquired several Frisian counties, his son an successor Margrave Egbert II, however, was involved in the Great Saxon Revolt against Emperor Henry IV and killed in 1090.
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The bishop was killed by a Frisian merchant in 1099, and the Brunonen heir, the Saxon count Henry of Northeim installed himself as a "margrave" of Frisia, before he himself was ambushed and killed two years later.
She was the daughter of Henry the Fat of Northeim (d. 1101) and Gertrud of Brunswick of the Brunonen dynasty.