Lyonesse first appears in Arthurian literature as the kingdom of Tristan's father, a real place that later became a lost one. In the earliest French romances of the 12th century, the poet Beroul named Tristan's father as King Rivalen and placed his kingdom in a region called Lyonesse, though he was vague about its exact location. In the Prose Tristan of the 13th century, the kingdom was called Leonois, ruled by Meliodas, and Sir Thomas Malory in his Le Morte d'Arthur of 1485 called the kingdom Liones and the king Meliodas. The name Leonois was originally the French form of Lothian, a region in Scotland, but English writers relocated it to the Cornish coast, where a genuine tradition of submerged land already existed. The Cornish name for the Seven Stones Reef, the notorious wrecking ground between Land's End and the Isles of Scilly, was Lethowsow, meaning "the milky ones" from the white water that perpetually surrounds the rocks. This was the site of the legendary capital, where the cathedral stood before the flood, and where the Torrey Canyon would later run aground in 1967.

The sinking of Lyonesse is the central event of its legend. The land was said to have been a paradise of orchards and farmland, where the climate was perfect year-round, the cattle gave thick cream, and the beehives dripped with honey. The people were tall and handsome, the women serene and noble, and even the poorest lived in neat cottages amid charming gardens. Some were practitioners of white magic, but the black arts were unknown. Despite this virtue, the kingdom was destroyed. In one version, the people committed a crime so terrible that God sent a storm in vengeance, and a single enormous wave swallowed the land in one night. The date given in folklore is 11 November 1099, though some versions place it ten years earlier. In another version, connected to Arthur's final battle, Mordred pursued the remnants of Arthur's army to Lyonesse after the disaster at Camlann. Arthur's men reached the Isles of Scilly and survived, but the ghost of Merlin appeared and caused the land to sink, destroying Mordred's forces in the deluge.

The sole survivor of the catastrophe was Trevelyan, the last governor of the kingdom. He had been out hunting all day and fell asleep beneath a tree. A horrible noise woke him, and he saw the sea rising across the land. He mounted his white horse and galloped toward Cornwall, reaching higher ground at Land's End with only moments to spare, his horse losing a shoe in the flight. From this ancestor, the Cornish families of Vyvyan and Trevelyan trace their lineage, and both families bear crests that show a white horse, commemorating the escape. The Vyvyans claim that Trevelyan was the last governor, and their ancient arms showed a lion rampant standing upon the waves of the sea, the beast of Lyonesse surviving the flood that took the kingdom.

The legend of a sunken kingdom is not unique to Cornwall. In Brittany, the tale of Ker Ys or the City of Ys tells of a city drowned because of its debauchery, with only King Gradlon escaping on his horse. In Wales, the kingdom of Cantre'r Gwaelod in Cardigan Bay was submerged through the drunken negligence of its prince Seithenyn, who left the floodgates open. These Celtic parallels suggest that the Lyonesse story belongs to a deep pattern of lost lands in British and Breton folklore, a memory of the sea's power to reclaim what was once dry ground. The archaeological evidence supports this memory. The Isles of Scilly were once a single island, and Roman records describe them as such. Sea levels rose during the Bronze Age, submerging forests and farmland, and at extreme low tide in Mount's Bay, petrified tree stumps become visible where a forest once stood. John of Worcester, writing in 1099, recorded that St Michael's Mount was then five or six miles from the sea, enclosed in thick woodland, a description that matches the submerged landscape.

Lyonesse entered the broader Arthurian world most dramatically through Alfred Tennyson's Idylls of the King, published between 1859 and 1885. Tennyson made Lyonesse the site of the final battle between Arthur and Mordred, describing it as "a land of old upheaven from the abyss by fire, to sink into the abyss again," a place of "ever-shifting sand" and "phantom circle of a moaning sea." In Tennyson's vision, the land rose from the deep to host the last clash of the Round Table, then sank back beneath the waves, taking the age of chivalry with it. This poetic transformation turned Lyonesse from a Cornish local legend into a universal symbol of the passing of golden ages, the place where every great kingdom must eventually meet its end. The fishermen of Cornwall still say that on calm days, when the Atlantic rollers sweep in from the west, the church bells of Lyonesse can be heard tolling beneath the waves, a sound that carries across two thousand years of memory and loss.