Willow is a recurring story element from the Cotton series.
Details
Willow is a solid substance made by collecting the juice of willow grass (a cultivation example of unknown composition). It was edible in the olden days, but it is said that many mysterious phenomena occur due to the combination. However, it is a valuable and expensive food because its manufacturing method is special, being considered a delicacy.
Many different varieties of Willow are known to exist, all of which have different magical properties, with the various kingdoms using them as wards to give them protection and keep the magical balance of the land.