A Brythonic Celtic kingdom that arose sometime in the mid fifth century AD stretching from Hadrian's Wall above Carlisle all the way down the north west coast of Britain down to Chester in the south.
The name of Rheged seems to have derived from the name of the Celtic tribe of the Brigantes that inhabited the region. The Brythonic Breged seems to have mutated over time into Rheged.
The first recorded king is one Gwrast Lledlwm in around 450, who was the son of Ceneu and the grandson of Coel Hen. He was followed by Meirchion Gul (the thin) who reigned between 470 and 535. It is then believed that Rheged was divided into two in 535, with North Rheged ruled by Cynfarch Oer (the cold) and South Rheged by Elidyr Llydanwyn (the stout).
South Rheged was roughly modern Lancashire and Cheshire; Llywarch Hen (the Old)seems to have been the last king of South Rheged. Sometime around 613 South Rheged fell victim to Northumbrian expansionism and Llywarch Hen fled into exile in Powys.
North Rheged has a king list as follows (though all the dates are of course, approximations;
It was eventually absorbed into the kingdom of Northumbria but at what exact date is uncertain. Some believe it was Edwin that conquered the kingdom; others suggest it was Edwin's successor as king of Northumbria, Oswald who arranged a marriage with the heiress Rhiainfellt and therefore a peaceful absorption of Rheged into his kingdom.
Certainly Rheged disappears sometime in the early seventh century; the Brythonic language spoken by the inabitants (akin to Welsh or Cornish) survived as late as the ninth or tenth centuries. Although, of course both Cumberland and Cumbria derive their name from the same root as Cymru, the land of the Welsh, and serve as a reminder of the Celtic past.
Also the name of a modern Cumbrian tourist attraction (who have incidentally bagsed the name rheged.com) known as the The Upland Kingdom Discovery Centre in Penrith, Cumbria that apparently features Rheged:The Movie.