Stab a Devilsaur with the Devilsaur Barb. Return to Torwa Pathfinder in Un'Goro Crater when you have completed this task.
The toxin you helped me create is for a special purpose. The great devilsaurs of the crater make it dangerous to hunt and difficult for me to survive. Even if I possessed the necessary strength, killing a devilsaur simply allows another to move into its territory.
Take this poisoned barb and stab it deep into a living devilsaur. The toxin should pacify the creature, though I do not know how quickly...