Mennonas

A knight who rivaled his friend, the sorcerer Mabon the Black, for the love of the lady Grysinde.

Mabon sent for Tristan to help him in the situation. Tristan arrived with Isolde, and when Mennonas saw that Grysinde was not as beautiful as Isolde, he killed her. Tristan then fought Mennonas in single combat and beheaded him.