Sir Tristram was the nephew of King Mark. His uncle was married to Iseult, whom Tristram was in love with due to a potion they accidentally drank together. (1)

Sir Tristram was the greatest knight of King Mark. He had been wounded killing Marhault in Ireland, and thus had to return to Ireland to heal. When he did so, he fell in love with the King's daughter Isode. Unfortunately, he was forced to win her for King Mark to fulfill a mission. (2) In fact, Tristram and Isode's tragic love story is now a movie of its own, called Tristan and Isolde.(3)





Here is Sir Tristram in his armor. (5)


Here is an illustration of Sir Tristram and La Belle IIsolde. [6]

After winning Iseult for King Mark, Iseult and Tristram drank the love potion asccidentally. The two became lovers even after the marriage of Mark and Iseult. He married, and died after being told Iseult could not come to cure an injury of his. Iseult commited suicide, and they were buried together. (7)


Sources:
(1) http://legends.duelingmodems.com/kingarthur/tristan.html
(2)http://www.cliffsnotes.com/WileyCDA/LitNote/id-182,pageNum-22.html
(3)http://movies.yahoo.com/movie/1808672940/details
(4)http://faculty.arts.ubc.ca/sechard/GRAPHICS/TRISTISW.JPG
(5)http://www.sacred-texts.com/neu/eng/crt/img/10000.jpg
[6]http://www.nashfordpublishing.co.uk/portraits/arthurian_scenes.html
7) http://www.earlybritishkingdoms.com/bios/tristram.html