King Anguish was one of the rebel kings who thought the boy-king, Arthur, was an imposter and rebelled against him at the beginning of his reign. The two made peace, however, after the Battle of Bedegraine. For many years, the Irish had been colonising the Western regions of Britain and Anguish saw himself as overlord of both the Scots (who called him Angusel) and the Cornish. King Mark of Cornwall had given out tribute to the Irish in the past, but eventually decided to assert his independence and refused payment. Anguish retaliated by sending his great champion, Sir Morhaut his brother-in-law, to force the Cornish to obey his demands. Sir Morhaut fought in single combat with King Mark’s own champion, his nephew Sir Tristram. Tristram won the battle, killing Morhaut and freeing Cornwall from Irish domination into the bargain. However, before Morhaut’s death, Tristram was wounded by his opponent’s poisoned sword. King Anguish’s Court was famous for its great healers and so Tristram was sent across the Irish Sea in disguise to seek their help. Anguish became such a great friend of Tristram that, when his deception was uncovered, he forgave him for killing his brother-in-law and sent him safely home. His wife, Isolde, was less tolerant, however, and tried to kill Tristram in his bath!

Later, Sirs Bleoberis and Blamore de Ganis summoned Anguish to King Arthur's Court to be charged with the murder of their cousin. Tristram stepped forward as Anguish's champion and won his acquittal. It was probably at this time that the Irish King was elevated to be a Knight of the Round Table. With the two sides reconciled, Tristram travelled again to Anguish’s Court in order to escort the latter’s daughter, La Belle Isolde, to Tristram’s uncle, King Mark, the Lady’s prospective husband. Considering the misery that ensued, Anguish only wished that Tristram had asked for her hand himself.

His character is probably based on the historical King Oengus mac Nad Friach of Cashel in Munster, who lived in the late 5th century.