Nennius - Historia Brittonum- Arthur's 12 Battles
St. Germanus, after his death, returned into his own country. At that time, the Saxons greatly increased in Britain, both in strength and numbers. And Octa, after the death of his father Hengist, came from the sinistral part of the island to the kingdom of Kent, and from him have proceeded all the kings of that province, to the present period.
Then it was, that the magnanimous Arthur, with all the kings and military force of Britain, fought against the Saxons. And though there were many more noble than himself, yet he was twelve times chosen their commander, and was as often conqueror. The first battle in which he was engaged, was at the mouth of the river Gleni. The second, third, fourth, and fifth, were on another river, by the Britons called Duglas, in the region Linuis. The sixth, on the river Bassas. The seventh in the wood Celidon, which the Britons call Cat Coit Celidon. The eighth was near Gurnion castle, where Arthur bore the image of the Holy Virgin, mother of God, upon his shoulders, and through the power of our Lord Jesus Christ, and the holy Mary, put the Saxons to flight, and pursued them the whole day with great slaughter. The ninth was at the City of Legion, which is called Cair Lion. The tenth was on the banks of the river Trat Treuroit. The eleventh was on the mountain Breguoin, which we call Cat Bregion. The twelfth was a most severe contest, when Arthur penetrated to the hill of Badon. In this engagement, nine hundred and forty fell by his hand alone, no one but the Lord affording him assistance. In all these engagements the Britons were successful. For no strength can avail against the will of the Almighty.
The more the Saxons were vanquished, the more they sought for new supplies of Saxons from Germany; so that kings, commanders, and military bands were invited over from almost every province. And this practice they continued till the reign of Ida, who was the son of Eoppa, he, of the Saxon race, was the first king in Bernicia, and in Cair Ebrauc (York).
Genealogy of the Kings of Bernicia
Woden begat Beldeg, who begat Beornec, who begat Gethbrond, who begat Aluson, who begat Ingwi, who begat Edibrith, who begat Esa, who begat Eoppa, who begat Id. But Ida had twelve sons, Adda, Belric, Theodric, Ethelric, Theodhere, Osmer, and one queen, Bearnoch, Ealric. Ethelric begat Ethelfrid: the same is Ædlfred Flesaur. For he also had seven sons, Eanfrid, Oswald, Oswin, Oswy, Oswudu, Oslac, Offa. Oswy begat Alfrid, Elfwin, and Egfrid. Egfrid is he who made war against his cousin Brudei, king of the Picts, and he fell therein with all the strength of his army, and the Picts with their king gained the victory; and the Saxons never again reduced the Picts so as to exact tribute from them.