David Jones, in his long essay 'The Myth of Arthur' asserts that "the tradition of Arthur was, for the Welsh, an authentic part of their historical mythus, whereas for the English it was a literary convention mixed with local traditions as at Glastonbury ...". It is easy to lose sight of this fact. 'King Arthur and his Knights' are all too familiar. It is likely that the source material for much of the medieval Arthurian romances was Geoffrey of Monmouth's History of the Kings of Britain which was written in Latin in the Twelfth Century (Breton sources have been suggested for the French romances of Chretien de Troyes but these are not extant). Geoffrey drew upon an older 'historical' source in the work of the ninth century chronicler Nennius. It seems that he also drew upon Welsh material, some of which has survived and some has not. It is likely that he filled in both the historical and the fictional gaps by making things up. Certainly he drew things together that had not previously belonged together, bringing in Merlin from a quite different set of literary and legendary material. But his Arthur is at least a Brythonic war lord in a country that was divided both within itself and by invasion. The medieval romance tradition ignores this context and makes him very much a contemporary figure (to them) ruling a medieval court whose knights get mixed up in the Holy Grail legend. As for the great English Arthurian cycle, it is based very much on this pan-European material in spite of its sinewy engagement with the realities of knightly conflict (Sir Thomas Malory wrote his Arthurian cycle while in prison having fought in some the the decisive battles of the Wars of the Roses). Malory's constant refrain is "so the Frensshe book maketh mencyon". The last books of the cycle (based in this case partly on the earlier English alliterative Morte Arthur) are a fast-moving and tragic account of illicit love and its consequences. But in the hands of Tennyson in The Idylls of the King they became a Romantic and dreamy expression of Victorian neo-medievalism. This is this 'English' Arthur that survived into the cinema age, though some more recent films have put back the harder edge of Malory's gritty realism.
But still all this is a million miles away from How Culhwch Won Olwen, or the Arthurian references in early Welsh poetry. But what did survive into the later medieval tradition was the idea that, as Malory puts it:
"Som men say in many partys of Inglonde that Kyng Arthure ys nat dede, ...... Yet I woll nat say that hit shall be so, but rather I wolde sey: here in thys worlde he chaunged hys lyff. And many men say that there ys wrytten uppon the tumbe thys:
HIC IACET ARTHURUS, REX QUONDAM REXQUE FUTURUS"
The idea of a grave for Arthur in the Englynion y Beddau is 'anoeth' a word that has caused some problems of interpretation. In modern Welsh it usually means 'unwise' but can also mean 'strange'. In earlier Welsh it also had the meaning 'marvel'. So 'An unwise thought ,a grave for Arthur' or 'A great marvel, a grave for Arthur' are both possible. But the general idea is that Arthur doesn't need a grave because he is not dead but sleeping and will come again to re-claim the sovereignty of the island of Britain in a time of of need. It was an idea that the Welsh adhered to but it was firmly knocked on the head by Edward I when he had two bodies removed from a tomb at Glastonbury in 1278 in the middle of a period of conflict with Llywelyn ap Gruffudd the last native Prince of Wales. Here are Arthur and Guenevere says Edward, so he's not coming back. In such ways do myth and legend become the stuff of politics, which I suppose gets as close to the world of a Brythonic warlord as we can get.