This page of the guide to GreedFall contains the walkthrough for Quest for Panacea main quest.
Talk to Daren about Tierna harch cadachtas in Magasvar. You will encounter guards - if Siora is with you, you will pass without any consequences. Talk to Daren - the conversation may lead to a fight resulting in -2 reputation with the Naut. After the conversation, go to Frasoneigad.
Talk to the locals in the village. They can give you quests that will allow you to win their trust. You have to, e.g. repair four bells and find Peddir. The man asks you to collect berries - you can find them on the glade nearby (look for bushes with blackberries). To repair the bells, you need Craftsmanship level 1 or have Kurt in the team.
Then, return to the village and report the tasks. You can reveal Peddir's secret or suggest that he can become an apprentice (+1 reputation with the Naut). Talk to another native standing on the roadside - you will receive an objective to find a holy seal. You will find it a little further in the forest - find it to get another + 1 reputation with the Naut.
After completing the smaller tasks, find and try to talk to tierna harh cadachtas. The shaman will escape - defeat a group of salamanders. Run after the woman - you will reach a closed gate. The mission is complete - you get 2,000 XP. You will immediately begin In Pursuit of the Tierna Harh Cadachtas quest.