Ysonde or Iseult. Daughter of the Queen of Ireland. Sir Tristram, being wounded, was cured by Ysolde, and on his return to Cornwall gave his uncle such a glowing description of the young princess that he sent to ask her hand in marriage. Ysolde married King Mark of Cornwall, but entertained a criminal passion for the nephew. This attachment being discovered by the king, he banished Tristram from Cornwall. Sir Tristram went to Wales, where he performed prodigies of valour, and his uncle invited him back again. The guilty intercourse being repeated, Sir Tristram was banished a second time, and went to Spain, Ermonie, and Britany. In this last place he met with Ysolt of the White Hand, daughter of the Duke of Brittany, whom he married. After many marvellous exploits he was severely wounded, and, being told that no one could cure him but Ysolde, he sent a messenger to Cornwall, and told him if the queen consented to accompany him he was to hoist a white flag. The queen hastened to succour her lover, but Ysolt told her husband that the vessel was coming with a black sail displayed. Sir Tristram, in an agony of despair, fell on his bed and instantly expired. Soon as Ysolde heard thereof, she flung herself on the corpse and died also. King Mark buried the two in one grave, and planted over it a rose-bush and vine, which so intermingled their branches as they grew up that no man could separate them.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894