Offa 's victory over Wessex at Bensington , Mercian pressure resumed Process_start and Kent was absorbed Conquering into Mercia .

An alliance between British and Mercian forces followed Motion , secured Assistance by Cadwallon 's marriage Forming_relationships to Alcfrith , Penda 's sister , and they marched Motion_directional north to face the armies of Northumbria ( who were occupying Gwynedd ) at the Battle of Cefn Digoll .