During the full moon fall asleep by the hearth in the cabin to go to the dream world. Follow the Grizzled Boor when he appears to the titanic tree and enter it to fight him. Defeat him (do not kill him) and take the key and return to the 'Real' world. Go past the wall blocking the road to Mingi Taw, cross the stolid log bridge, and search for the hidden door below the trail in the next area. Use the key to open the door, once inside you will be able to manipulate the 'moods of the moon' at the bench.