Sanfew requires a rare herb for the next part of the purifying ritual. Travel back to the Troll Stronghold to find this herb that the trolls use as an ingredient for their favourite dish.
To start this quest speak to Sanfew in Taverley. You must have finished the Troll Stronghold and Druidic Ritual quests and you will need level 31 herblore.
For full information on the quest, view the manual page here