Ysaive
A niece of King Arthur who married King Caradoc of Nantes, but committed adultery with a sorcerer named Elïavrés. This affair produced a son, Caradoc, who was recognized by King Caradoc as his legitimate heir.
After winning fame as one of Arthur’s knights, Caradoc returned to Nantes and locked Ysave in a tower to punish her for her sins. Ysave and Elïavrés conspired to kill Caradoc with a poisonous serpent, but were unsuccessful.
Source
First Continuation of Chrétien’s Perceval | Attributed to Wauchier of Denain, c. 1200