At some point later in the game, the lover of her sister comes to see Mazzy and tells her how he tried to acquire an elixir of love from a merchant for Mazzy's sister but that once she drank it she was immediately poisoned. He will ask Mazzy's help and of course you'll have the option to come with her and help as much as you can. Once you get to the Fentan's house in Trademeet, you'll learn that the name of the merchant is Wallace and you'll be able to find him in the southern part of the town. He tells you that he was unaware of the poison and that he now gets his potions from a new priest of Waukeen named Barl.

Barl is an arrogant fool you meet at the temple early in the story. If you suspected him of something, well you were right. When you get to the temple, he's nowhere to find but if you ask the senior priest, he will look into Barl room and find a symbol of Talona, Goddess of poison. When he summons Barl, he will attack you with his 2 Poison Mists but the fight is quite easy. When he's dead, Mazzy grabs a potion you won't have access to. Now you just have to go back to the Fentan's house so that Mazzy can give it to her sister. She'll also ask you to remain there for the day until her sister is better, so if you want to explore the region and get the Mana Bow +4 from the Forest of Thetyr, feel free to do so.