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.
Progress:
Torwa Pathfinder: The flow of time does not stop or slow, human. While my life is eternal, yours is not. Waste not the time that you have...
Completion:
Torwa Pathfinder: This amulet will grant you the access that you seek. The lair of Onyxia can be found in Dustwallow Marsh. As long as you hold the amulet, her wards cannot prevent you from entering.