On this page of the guide, you will find a walkthrough for one of the main story missions - Face to Face with the Demon.
After a conversation with Derdre, go to the sanctuary in Vedvilvie. Along the way, you will encounter an old hermit. Talk to him, and then start searching the swamps. Reach the subsequent locations marked on the map, and then interact with them. After that, go back to the hermit and talk to him. He tells you to hit the drums.
Hit them in this order, starting from left: the third drum, the first and then the second. Now go to the protruding "altar." Pour in the potion of demonic ritual - you can create it on a workbench, or check the nearby box.
You now have to fight with Nadaig Vedemen. Watch out for his poisonous attacks and try to quickly break the monster's armor. After the fight, talk to the hermit, go to Tir Dob and talk to Derdre. This completes the quest and automatically starts another side mission - Camp of the Ordo Luminis.