Cumbria is a county located in the northwest of England, bordering Scotland to the north.
In the generation after Arthur, it was ruled by Rhydderch the Generous.
Cumbria | 400-600 AD
The region of Cumbria was part of the Roman province of Britannia. Roman influence waned during the early fifth century as Roman legions withdrew from Britain, leaving the area vulnerable to incursions from various groups.
The Early Medieval period was marked by the migration and settlement of various Germanic tribes, including the Angles, Saxons, and Jutes. These groups, collectively known as Anglo-Saxons, began to establish their kingdoms in different parts of Britain.
During the sixth century, the Kingdom of Rheged emerged as a notable kingdom in the area that roughly corresponds to modern-day Cumbria and parts of southern Scotland. Rheged is mentioned in later medieval poetry and Welsh genealogies, and it is believed to have been a significant center of power and influence. The early centuries of the Early Medieval period saw struggles for dominance among various British and Anglo-Saxon kingdoms. The Kingdom of Rheged likely had interactions and conflicts with neighboring kingdoms, such as Northumbria to the east.
Christianity began to spread in Britain during this period, and it is believed that missionaries and religious communities played a role in the conversion of the local population to Christianity. Saint Kentigern, also known as Saint Mungo, is said to have been active in missionary work in the region, including areas around Cumbria.
Source
Vita Merlini | Geoffrey of Monmouth, c. 1150