The Gall Nuts are a special kind of berry that is used for making ink, which can be found in a tree within The Forest. Brendan goes into the forest in search of the gall nuts so he can help Brother Aidan finish the Book of Iona. He meets Aisling, a spirit of the forest, who at first is supicious of Brendan and his reason for entering through her forest, but she eventually befriends him and takes him to the tree that grows the berries he needs.