Retrieve a Putrid Vine from the guardian at the bottom of the Sunken Temple and return to Torwa Pathfinder.
It is as I feared -- the toxin is simply too weak for such a powerful creature.
If we are to create a toxin of high enough potency, you must venture to the bottom of the Temple of Atal'Hakkar, unlock the secrets of the altar and recover a putrid vine from the guardian there. The temple is dangerous, and it would behoove you to bring friends.
Return to me when this is done.