Explore the dark forest zone and there are a couple of ways to get hints. You might come across a ruined stone tower: the rune-stone at the base will have the location. Also the crypts in the area will have a similar rune-stone in the treasure room. (the layouts of the crypts are randomized, and it is possible for them to have no treasure room, but that seems to be the exception, I just got unlucky with the first one I found)