Grimsdell Forest is a large forest near Castle Macindaw and the Araluen-Picta border. It is home to Malcolm the healer and his outcasts. Will was told that he would "know it when he saw it" by a Macindaw guard, suggesting that it is recognizable as grim in appearance.
Grimsdell is a seemingly ancient forest, with gnarled trees and vines. There is a mere by which the apparition of the Night Warrior is often made to appear by Malcolm's lantern shows. The forest is surrounded by myths and fear, few people go there and those that do usually come out again in a hurry.
At the end of The Siege of Macindaw, when offered a position at the castle, Malcolm chooses to stay in his clearing with his people.