The two armies meeting at Arfderydd belonged to Brittonic kingdoms of what is now southern Scotland and northern England - the world the Welsh later called Yr Hen Ogledd, the Old North. The defeated lord was Gwenddoleu ap Ceidio, a king whose name survives only in fragments of poetry and triad. The opposing force is variously named in different sources, which is one of the maddening pleasures of working with this material; some texts identify Gwenddoleu's enemies as the princely brothers Peredur and Gwrgi, others as Rhydderch Hael of Strathclyde. The likeliest reading is that several allied armies came against him. Gwenddoleu was killed. The Welsh Triads remember his warband as one of the 'Three Faithful Warbands of the Island of Britain', who 'continued to battle for a fortnight and a month after their lord was slain' - an act of devotion so unreasonable, in the medieval Welsh sense, that it stuck in the cultural memory.

The same Welsh Triads list Arfderydd as one of the Three Futile Battles of the Island of Britain - alongside the Battle of Camlann, where Arthur fell, and the legendary Battle of the Trees. Futile not because nothing happened, but because so much was lost for so little gain. The triadic categories of medieval Welsh learning are not history in the modern sense; they are a way of organising what matters, a memory grid stretched across centuries. To name a battle 'futile' was to mark it as a wound. The Dialogue of Myrddin and Taliesin, the first song of the Black Book of Carmarthen, returns to Arfderydd again and again, naming warrior after warrior who fought there - Cedfyl, Cadfan, Maelgwn, Erith, Gwrith, Bran, Melgan, Rhys, Cynelyn, Cyndur, the sons of Eliffer, and Dywel fab Erbin. The names are mostly all we have. Behind each one was a man who rode out one morning and did not ride back.

The story that has carried Arfderydd through fourteen centuries belongs to the bard rather than the king. Myrddin Wyllt - 'Myrddin the Wild' - is described in the poems as having worn a golden torque at the battle and then fled, mad with grief and terror, into the Caledonian Forest. There he talks to apple trees. He speaks with Taliesin. He becomes a prophet, a hermit, a man who cannot bear human company. Centuries later, Geoffrey of Monmouth would weave him into a Latin biography of a wizard called Merlin Caledonius, and from there the figure migrated south and west into the high Arthurian romances. The Merlin of the round table grew from a different soil than people often realise. He is, in his oldest layer, a survivor of a real or remembered battle in the Solway lowlands, traumatised into prophecy. The poem Apple Trees gives him a refuge among the orchards. The dialogue with Gwyn ap Nudd, the mythological psychopomp who collects the souls of warriors, puts him 'at the place where was killed Gwenddoleu, the son of Ceidaw, the pillar of songs.'

If you stand near Arthuret church today, west of Longtown, you are looking at flat farmland threaded by the River Esk as it bends toward the Solway. There is no monument; there is no plaque large enough to do the story justice. The plain looks ordinary. Yet this part of Cumbria - the Old North, the territory of Rheged and Strathclyde, the contested ground between the Romans' wall and the early Scots - was where the Brittonic culture that produced these poems lived and fought and remembered. Skene's identification of Arfderydd with Arthuret is not certain; some scholars have suggested other sites. What is certain is that somewhere in this country, on a day in 573, a battle took place that the Welsh thought worth grouping with Camlann among the wounds of the island. The Triads do not give numbers. They give names. They name Gwyn ap Nudd standing where Gwenddoleu fell, and the ravens screaming, and a young bard running into the trees with a golden torque around his neck.