Find Mudbarrow with Spirit Vision. Discover the location of the treasure by questionning Slumberjack in his sleep. Tell Mudbarrow you found the treasure and to go rest now.
If you cast Spirit Vision at the sawmill at the left-most building, you will find a spirit named Mudbarrow. He wants you to question a sleeping Lone Wolf in order to locate a chest that she hid after she killed him.
Question Slumberjack at sawmill for the location of the loot and then kill her. (If you had already killed her before talking to Mudbarrow, this quest cannot be completed normally.)
Afterwards, head to the revealed location with high Wit (18+) and then dig up the chest and loot it. Return to Mudbarrow after you're done to tell him to move on, and he will reward you with EXP and gold.
Note: If you've killed Slumberjack prior to questioning her, the location of the chest is found along the shore on the water just west of Mudbarrow's building.