Early English Kingdom, in Northumberland, reputedly founded by King Ida in the mid 6th century and centred upon the fortress of Bamburgh on its basalt rock near the sea. The name is not Germanic but British in origin, perhaps the name of those who lost their land to the English.
The Kingdom expanded its territory towards both the Forth and the Tees. King Æthelfrith, Ida’s grandson, advanced against the Welsh in what is now southwestern Scotland, and in 603 fought and defeated the Scots of Dalriada, the later Argyll, who were helping or protecting the Welsh Kingdom of Strathclyde. He thereby established hegemony over much of southwestern Scotland.
In 605 he added the other northern Kingdom, Deira (Yorkshire, more or less) to his lands and married the daughter of his predecessor. The united Kingdom became known as Northumbria. Although Æthelfrith was defeated and killed by his wife’s brother, Edwin, in 616, the union survived. Its unity was briefly disrupted after Edwin’s death in 633, but was soon re-established under princes of the royal house of Bernicia.
After the Vikings had destroyed the Kingdom of Northumbria in the 860s, they created their own Kingdom of York, and Northumbria between the Tees and the Forth was ruled by English earls, a sort of revived Bernicia. In the north it contracted before advancing Scottish power during the 10th century, and the remnant fell under the distant authority of the Kings of England after the Kingdom of York had disappeared, temporarily at first for several years after 927 and finally in 954.
The Earls continued into the Norman era, until William Rufus decided not to appoint a successor to a rebellious Earl after 1095. The Kings of Scotland had a claim on the Earldom and a hope that as they had pushed Bernicia back to the Tweed so they might absorb it at least to the Tyne. That dream largely died in Henry II’s reign.