Gira
According to an ancient legend, giants inhabited Myriaden at the same time as humans. I won't go into details, but according to the stories of your people, a titan was defeated and buried under the area now known as Tarasquan.
You may ask how I could make the link between the sample and the titans? It seems to be a contamination related to therianic energy and his concoction reminds me of texts I read about techniques used by mages before the Cataclysm. Although I could certainly be mistaken, if it is only the question of a particularly talented poisoner, he certainly has some rare knowledge and extremely dangerous motives.
You must now discover his hiding place. If, like I guess, he wishes to destroy a portion of the swamp to have access to its underground, he is probably not hiding in the Tarasquan. However he is probably not far, or he has agents watching over the situation for him. Go to Farlamburg and make inquiries.