Pen Rhionydd is named as the location of King Arthur's northern court in a Welsh triad found in Peniarth MS 54, containing pre-Galfridian traditions:
Arthur as Chief Prince in Pen Rhionydd in the North, and Gerthmwl Wledig as Chief Elder, and Cyndeyrn Garthwys as Chief Bishop.[1]
There are no other known references to this location in Arthurian literature. The same triad goes on to say Arthur's other courts were at Celliwig and Mynyw.
A possible location, supported by Rachel Bromwich, the latest editor of the Welsh Triads, is a location somewhere near the Rhins of Galloway and Stranraer.[2] This would match the importance of St Mungo in that area. Both these places would have been in Rheged[according to whom?].