In the aftermath of the Saxon Wars, Emperor Charlemagne in 804 established a missionary diocese at Osterwieck (then called "Seligenstadt") in Eastphalia, in the course of the Christianisation of the pagan Saxons and Polabian Slavs. Under its (supposed) first bishop Hildegrim of Châlons the capital was moved to Halberstadt, confirmed by Charles' son Louis the Pious in an 814 deed. The bishopric's boundaries originally reached the Elbe and Saale rivers in the east, nevertheless, when Emperor Otto I founded the Archbishopric of Magdeburg in 968, Halberstadt lost the eastern half of its district to it. Halberstadt diocese was a suffragan of the Archdiocese of Mainz.