This page in our guide to GreedFall contains a walkthrough for The Suffering of Constantin quest.
Find a healer in Wenshaveye and contact Cecillus, a healer in San Matheus. The man will help you only if you use the Intuition talent in the conversation. Go to Wenshaveye and find the hunter.
Talk to him, and then talk to the relatives of the missing villagers. Your task is to find and destroy the nests of the beasts. To destroy a nest, you need Alchemical Preparation of Elemental Damage. In the meantime, talk to the missionaries and convince them to leave the place.
After that, inform relatives of the victims to receive +2 reputation points with the Naut. Go to Castach's hut and bring him to Constantin. You will gain 2,000 XP and +1 reputation point with the Congregation of Merchant.