According to "Ecclesiastical History of the English People", a history of the English church written by the 8th-century monk Bede, there were seven early Anglo-Saxon rulers who held "imperium", or overlordship, over the other kingdoms. The fifth of these was Edwin of Northumbria, who was killed at the battle of Hatfield Chase by a combined force including Cadwallon, a British king of Gwynedd, and Penda. After Edwin's death, Northumbria briefly fell apart into its two subkingdoms of Bernicia and Deira. Within a year Oswald, Edwin's nephew, killed Cadwallon and reunited the kingdoms, subsequently re-establishing Northumbrian hegemony over the south of England. In 642 Penda killed Oswald at the battle of Maserfield, and Northumbria was again divided. Oswald's son Oswiu succeeded to the throne of Bernicia, and Osric's son Oswine to Deira, the southern of the two kingdoms.