The kingdom was previously inhabited by Britons and was probably created in the third quarter of the fifth century when Anglian warriors invaded the Derwent Valley. Deira's territory extended from the Humber to the Tees, and from the sea to the western edge of the Vale of York. It later merged with the kingdom of Bernicia, its northern neighbour, to form the kingdom of Northumbria.
The name of the kingdom is of Brythonic origin from the Proto-Celtic *daru, meaning "oak", (derw in modern Welsh) in which case it would mean "the people of the Derwent", a derivation also found in the Latin name for Malton, Derventio. It is cognate with the modern Irish doire (pronounced [ˈd̪ˠɛɾˠə]); The origin of the name for County Londonderry and city of Derry stems from this word.
According to Simeon of Durham (writing early in the 12th century), it extended from the Humber to the Tyne, but the land was waste north of the Tees. After the Brythonic kingdom centred on Eboracum, which may have been called Ebrauc, was taken by King Edwin, the city of Eboracum became its capital, and Eoforwic ("boar-place") was taken by the Angles.
Archaeology suggests that the Deiran royal house was in place by the middle of the fifth century, but the first certainly recorded king is Ælla in the late sixth century. After his death, Deira was subject to king Æthelfrith of Bernicia, who united the two kingdoms into Northumbria. Æthelfrith ruled until the accession of Ælla's son Edwin, in 616 or 617, who also ruled both kingdoms until 633.
Kings of Deira
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