You can get about half as far in to the forest as Sagi as you could as Kalas, and other than a lack of monsters, nothing seems to have changed. However, if you use the heart link to change to a greythorne, you can get the whole way through the forest. If you approach the spring in the center as a greythorne, the Lord of the Spring from Eternal Wings will speak to you, and you'll be able to trap the magna essence of his voice. The connection between the Spring Lord and greythornes is otherwise unknown.