Start the quest by speaking to Sanfew (if boosting herblore use before talking to him), who lives on the top floor of the herb shop in Taverley. He will ask you to find out the secret of the herb Goutweed, in order to perform the next part of the purifying ritual on the Druid's circle. Only the trolls in the Troll Stronghold know about Goutweed, so Sanfew will suggest you speak to his friend Eadgar who lives near there at the top of Trollheim.