King Mark of Cornwall was the son of King Felix of Cornwall. His father died after a raid on his castle at Tintagel his household and, impressed by his prowess of arms, made him his steward. Tristan became a local hero when he defended the kingdom against the Irish champion, Sir Morhaut. With the Irish in retreat, King Mark entered into a peace treaty with them, to be sealed by his marriage to Princess Isolde. Tristram was sent to escort her to Cornwall, but the two mistakenly drank a love potion intended for the bridal couple. Despite Mark and Isolde’s marriage, Tristram became the new Queen’s lover. They tried to keep their affair a secret, making great use of Tristan's natural talent for disguise. Mark , of course, found out however and a bitter feud with his nephew ensued. Sir Tristan fled his uncle’s sphere of influence with his lover a number of times - notably hiding out for months with Sir Lancelot - but Mark always managed to retrieve his wife, and Tristram was eventually forced to leave for Brittany.
To outwit his nephew, King Mark entered into a number of wicked plans. He begged Tristram’s help against Cornwall’s enemies in order to get him killed or, at least, sent overseas; and even forged letters from the Pope to persuade him to go off on Crusade. Mark killed many of his own knights and also those of King Arthur’s Court, often while in a great rage. At one time he invaded Arthur’s own kingdom, but was beaten back by the Knights of the Round Table. There are various versions of King Mark’s death, but he is generally thought to have been killed by the revenge-seeking Sir Bellangre le Beuse, a grandson of Mark’s murdered brother.
King Mark's associations with the county of Cornwall are strong and he may well have been an historical monarch in the area during the early 6th century.