Tristan makes his first appearance in 1120 in Celtic folklore that circulates in the North of France. Although the oldest stories concerning Tristan are lost, some of the derivatives still exist. The two most famous are the retellings of the Anglo-Norman poet Thomas and of the French poet Beroul. Arthurian romancier Chretien de Troyes claims in his works to have also written down his version of the tale, but, if so, there are no physical remains.
Like other Arthurian knights, Tristan and his tale vary from poet to poet. Even the spelling of his name varies a great deal, with Tristran and Tristan being the two most popular spellings. In one tale, Tristan is an overweight knight, who is constantly teased yet astounds everyone by defeating the Great Serpent guarding the Cave of Micheal. In Tristran and Yseut by Beroul, Tristan is as brave and fit as any knight, but he relies on trickery and doesn't uphold the moral standards expected of a knight.
In this tale, Tristran goes to Ireland to bring back the fair Yseut for his uncle King Mark to marry. Along the way, they accidentally ingest a love potion that causes the pair to be madly in love for three years. Although Yseut marries Mark, she and Tristran are forced by the potion to seek one another out for adultery. Although the typical noble Arthurian character would be shamed from such an act, the love potion that controls them frees Tristran and Yseut from responsibility. Thus Beroul presents them as victims. The king's advisors contstantly try to have the pair tried for adultery, but again and again the couple uses trickery to preserve their facade of innocence. Finally the love potion wears off, and the two lovers are free to make their own choice as to whether they cease their adulterous lifestyle or continue. Beroul's ending is morally ambiguous, which differs greatly from his contemporaries such as Chretien de Troyes and adds a bit of mystique to the legend of Tristan. Also, according to Celtic myth, Tristan owned a horse named Bel Joeor.
In the Arthurian Legend of Tristan and Iseult (alternatively Isolde, Isode, Isotta, etc.), there are actually two Iseults involved with him. The first was called Iseult of Ireland, daughter of King Anguish of Ireland. She loved Tristan deeply but was promised to King Mark of Cornwall. The second Iseult was the one Tristan ended up marrying, called Iseult of the White Hands. Tristan did not love this Iseult, but rather Iseult of Ireland. When Tristan was mortally wounded, he called for Iseult of Ireland in the hopes that she might be able to heal him. When her ship arrived, Tristan asked what color the sails were (white meaning she would come, black meaning she wouldn't). The sails were white, yet Iseult of the White Hands, resentful of his love for the other Iseult, lied and Tristan therefore died.
Iseult was also the name given to Tristan's grand-daughter.