The herb is in Jogre dungeon. The dungeon's entrance is found to the north along the coast. Search the rocks there and enter the cave. Beware of the aggressive Jogres that inhabit the dungeon!
All caves in the dungeon contain the herb, but the southernmost one has 6 herbs on walls. Search a wall with green spots (fungus covered wall) to find the herb. This may take some time. Clean the herb. (The Jogres can drop grimy rogue's purse, but Trufitus will not accept one of these). To exit the dungeon, climb up the rocks where you entered it.