saint after his death. Oswiu (r. 642–670) Oswiu was the brother of Oswald and succeeded him after the latter's defeat in Maserfield. Oswiu succeeded where Edwin and Oswald failed as, in 655, he slew Penda during the Battle of the Winwaed, making him the first Northumbrian King to also control the kingdom of Mercia. During his reign, he presided over the Synod of Whitby, an attempt to reconcile religious differences between Roman and Celtic Christianity, in which he eventually backed Rome. Oswiu died from illness in 670 and divided Deira and Bernicia between two of his sons. Halfdan Ragnarsson (r. 876–877) Halfdan Ragnarsson was a Viking leader of the Great Heathen Army which invaded England in 865. He allegedly wanted revenge against Northumbria for the death of his father, who was supposedly killed by Ælla of Northumbria. While he himself only ruled Northumbria directly for about a year in 876, he placed Ecgberht on the throne as a client-king, who ruled from 867 to 872. Halfdan was killed in Ireland in 877 whilst trying to regain control over Dublin, a land he had ruled since 875. There were no further Viking kings in Northumbria until Guthfrith took over in 883. Æthelstan of Wessex (r. 927–939) Æthelstan ruled as King of the Anglo-Saxons from 924 to 927 and King of the English from 927 to 939. The shift in his title reflects that in 927, Æthelstan conquered the Viking Kingdom of York, previously part of the Northumbrian Kingdom. His reign was quite prosperous and saw great strides in many fields such as law and economics, but was also characterized by frequent clashes with the Scots and the Vikings. Æthelstan died in 939, which led to the Vikings' retaking of York. Æthelstan is widely considered one of the greatest Anglo-Saxon kings for his efforts to consolidate the English kingdom and the prosperity his reign brought. Eric of York (r. 947–948, 952–954) In the early twentieth century, historians identified Eric of York with the Norwegian king Eric Bloodaxe, but more recent scholarship has challenged this association. He held two short terms as King of Northumbria, from 947 to 948 and 952 to 954. Historical documentation on his reign is scarce, but it seems Eric pushed out the joint English-Viking rulers of Northumbria in 947, who then regained the land in 948 or 949. Eric took back the throne in 952, only to be