It is part of the treasury which at first can't be accessed in Chapter 3, and when first seen in Chapter 5 is guarded by a monster. Baalzack must first be defeated in order to acquire it. Its purpose is not found until much later in the game in the town of Rosehill in which playing it in an exact location in the village grants access to Rose's chambers. The party knows this thanks to a dream that they have in the inn at Strathbaile. They are told of this dream by people at the Burland castle.