Norbert had once harbored desires of taking Magdeburg, having been its archbishop in life. When he had joined the ranks of the undead, the Brujah Cedric had still been Prince of Magdeburg and Jürgen his occasional advisor (though still in service to Hardestadt).
After a fire claimed Cedric, Norbert and Jürgen had both been contenders for the city's throne, but with Hardestadt's support, Jürgen easily emerged victorious. Eventually, Norbert took the princedom of Braunschweig, becoming a thorn in the side of the ruler of Magdenburg.
The generation of von Xanten's sire is not explicitly given, but can inferred by his command of Obtenebration. If Gotzon was indeed 5th generation, von Xanten would be 6th. See here for more information.
For the historical figure, see Norbert of Xanten.