How to unlock: Talk to the guards standing next to the assassin's body (M1,8).
You can't accept this quest during your first visit in the village. It will become available once you get back from Iron Flail Fort, which was essential to The Iron Flail.
First, you should investigate the assassin's body by "touching" his soul. A new location called Mowrghek Ien will be unlocked (M12). It is located within the Dyrwood, and not the White March. When you get there, head to the northwest part of the location. Unfortunately, you will have to go around: first south, then west, and finally north. You will reach Clearing (M12,6). After a longer conversation, you will begin a fight with three opponents: Llengrath (a very powerful mage) and two dragons (Turisulfus and Gafonercos). Due to the fact that this is the most difficult battle in the entire game, it is highly recommended that you reach level 16 and obtain the best available armor and weapons.
After the combat your quest will be over, and you will gain some experience points. Don't forget to search the fallen bodies as you will find Llengrath's Grimoire that contains a number of powerful spells.