The servant of the lady Fenice, so named because she came from Thessaly in Greece.
Chrétien de Troyes assures us that Thessaly was a land of traditional charms and devilish enchantments; Thessala was certainly adept at brews. Not only could she cure diseases and heal wounds of all kinds, she also knew how to prepare potions. When Fenice – who was engaged to be married to Alis, the emperor of Greece and Constantinople – fell in love with Alis’ nephew Cligés, she turned to Thessala for help. Thessala made a potion and had Cligés give it to Alis. The potion caused Alis, each night, to think he was making love to his wife when in fact he was sleeping. In this way, Fenice was able to preserve her virginity.
Later, Thessala made Fenice a potion that caused her to seem dead long enough for Cligés to spirit her away. During Fenice’s shammed illness preparatory to this ruse, Thessala also showed plain old human cunning in fooling the doctors by substituting the urine of a woman who really was mortally sick for that of her mistress.
Source
Cligés | Chrétien de Troyes, late 12th century