Cunomorus was born into the royal house of Dumnonia, the kingdom of the Dumnonii that covered the south-western peninsula of Britain from the River Tamar to the River Exe, a land of granite coasts, deep river valleys, and ancient tin mines that had drawn traders since the Bronze Age. The name Cunomorus means Hound-of-the-Sea in the Brittonic tongue, a title that spoke of maritime power and perhaps of the hunting packs that ranged the moors behind his coastal fortresses. His seat was at Castle Dore, an Iron Age hill fort near Fowey that was reoccupied and fortified in the post-Roman period, its ramparts enclosing a great hall where the king held court. The 9th-century Life of Saint Paul Aurelian, written by the monk Wrmonoc at the Breton monastery of Landevennec, records that Cunomorus ruled not only in Cornwall but also held lands in Brittany, and that his kingdom was so extensive that four languages were spoken within its borders. This was no petty chieftain but a major power broker of the 6th-century British world, a man whose influence reached from the Channel to the Celtic Sea.

The defining evidence of Cunomorus's life is not a chronicle or a poem but a stone. Sometime in the mid-6th century, a granite pillar was carved and erected near Castle Dore, standing almost three metres tall beside the road that led to the king's stronghold. On its face, in Latin letters of the post-Roman insular style, an inscription reads: DRVSTANVS HIC IACIT CUNOMORI FILIVS. Here lies Drustanus, son of Cunomorus. The stone is a memorial or a cenotaph, marking the grave or the memory of a young man named Drustanus, a Brythonic name that would later become Drystan in Welsh and Tristan in French. A now-lost third line, recorded by the 16th-century antiquarian John Leland, added CVM DOMINA OUSILLA - "with the lady Ousilla," a Latinisation of the Cornish name Eselt, the Isolde of later romance. The stone transforms everything we think we know about the Tristan legend. In every medieval romance, from Beroul to Thomas Malory, the English knight who compiled Le Morte d'Arthur in 1485, Tristan is Mark's nephew, the son of his sister Blancheflor. But the stone says son. The Welsh Triads, those ancient catalogues of British heroic lore, also list "March" as the father of "Drystan" in one triad, a tradition that the French poets may have sanitised to avoid the scandal of a son loving his father's wife.

The historical Cunomorus was a warlord of the old British kind, a man who made alliances with Frankish princes and fought wars that are recorded only in fragments. The Frankish chronicler Gregory of Tours, writing in the late 6th century, mentions a Prince Conomor in Brittany who was remembered as an evil conqueror, a figure of tyranny and violence. Some scholars identify this Breton Conomor with the Cornish Cunomorus, while others argue they were distinct but related rulers, cousins or allies across the Channel. What is certain is that Cunomorus's power attracted enemies as well as allies. One tradition holds that he supported Chramm, the rebellious son of the Frankish King Clothar I, in a failed uprising around 560 CE, and that both Cunomorus and Chramm fell in the same campaign. Another tradition, recorded in the Welsh king lists of Dumnonia, places Cynvor - the Welsh form of Cunomorus - in the royal genealogy between Constantine of Dumnonia and Geraint, suggesting that he was the father or grandfather of the Constantine who would one day be remembered as Arthur's successor.

The transformation of Cunomorus into King Mark of Cornwall is one of the most remarkable journeys in Arthurian scholarship. The 9th-century Wrmonoc was the first to make the connection explicit, writing that the king he called Marcus was also known as Quonomorus. By the 12th century, when the French poets Beroul and Thomas of Britain composed their Tristan romances, the name Mark had become fixed as the cuckolded king, the husband of Isolde and the uncle of Tristan. The poets gave him horse's ears - a motif borrowed from Welsh legend about Merchwyn the Mad - and made him a coward, a traitor, and a fool, the opposite of every virtue that the Round Table represented. But the historical Cunomorus was none of these things. He was a sea-king of the Old West who held his kingdom together through the darkest years of British history, who buried his son beside the road to his fortress, and whose name was carved into stone with a care that suggests grief as much as pride.

The legacy of Cunomorus extends far beyond the single stone that bears his name. The Welsh Triads remember him as March son of Meirchion, a king linked with Glamorgan and Gwynedd as well as Cornwall, suggesting that his influence - or his legend - ranged across the whole Brittonic world. The Breton bards sang of him, the Cornish bards remembered him, and the French poets made him the villain of the greatest love story ever told. The Tristan Stone still stands near Fowey, moved from its original position beside Castle Dore but still bearing the inscription that links a father and a son across fifteen hundred years. Whether Drustanus died in battle, in a hunting accident, or by the treachery that the later romances invented, the stone does not say. What it says is simpler and more terrible: that a king named Cunomorus once lived in Cornwall, that he had a son named Drustanus, and that the son died before the father, leaving only granite and letters to mark the place where memory began.