King Mark had associations with Cornwall which are strong and King Mark may well have been an historical monarch in the area during the early 6th century. In the Welsh legends, he is known as March ap Meirchiawn, and may have been Conomorus, the father of the "DRUSTAVS" inscribed on the Tristan Stone. King Mark of Cornwall is also the jealous husband who killed his nephew Tristam for having an affair with Mark's wife Isolde.
Castle Dore has been widely seen as the chief residence of King Mark of Cornwall from the 12th century poem by Beroul, which locates the fort outside Golant in southern Cornwall. Mark also lived at Tintagel Castle. King Mark of Cornwall is shown in King Arthur literature as a treacherous, cowardly rogue. King Mark is said to have murdered his own brother to get the throne.
His father was killed during a raid on Tintagel by the King of Ireland and Mark inherited the throne. Ireland then demanded tribute for Cornwall’s protection. Tristram, nephew of King Mark was the champion who defended the Cornwall against the Irish champion, Sir Morholt, who came to pick up the required tribute on behalf of King Anguish. King Mark then entered into a peace treaty with the Irish, and sealed it with a marriage to Princess Isolde. Tristram was dispatched to escort her back to Cornwall, but on the voyage back from Ireland the two mistakenly drank a love potion intended for the King Mark and Isolde.
Hence Tristram became the new Queen’s lover. King Mark discovers this and Sir Tristram fled Cornwall with his lover a number of times - hiding out for months with Sir Lancelot - but King Mark always managed to get his wife back. Tristram eventually forced fled to Brittany.
King Mark then dreamt up a number of wicked plans to finally kill Tristam, even forging letters from the Pope to persuade him to go off on Crusade. King Mark of Cornwall killed many of his own knights and also those of King Arthur’s Court, often while in a great rage.
One strange little story is that King Mark had horse's ears, which could be seen only every time he had a hair-cut. To keep the secret of his horses' ears from the world, he murdered each of his barbers. Then one day a local man made a set of pipes from reeds growing on one of the barbers' graves. When he played a tune on them them, they would sing, "March ap Merion has horse's ears"
There are various versions of King Mark’s death, but the commonest is that he was killed killed by the revenge-seeking Sir Bellangre le Beuse, a grandson of the murdered brother of King Mark.
Full references to more about King Mark of Cornwall at the Rochester site