Language is Cumbric Britannic
Religion is Anglian-Germanic.
Currently ruled by Ethelfrith of house Leodwaldings, who united his kingdom of Bernicia with the neighboring kingdom of Deira. The heir to the throne of Deira has fled south to East Anglia, where he plots his revenge against Ethelfrith.
The lands that make up Northumbria were among the first conquered by the Angles when they settled in the country. The early Angles and Britons fought ferociously against each other with the Britons finally being conquered and pacified, with Angle and Briton intermarriages used to cement the bond of the two peoples. Most Britons now consider themselves Angles, or at the least, see no difference between the two.
Northumbria is an expansionist realm, always seeking to expand its borders.