A Master or Mountain God is a protectorate of a natural place (i.e. a mountain). The "Master" can be human, animal, or mushi. Their job is simply to keep everything in balance. The Masters are found in places that are in path of the Light Vein.
A Master maintains the balance of that place with the help of the Mugura.The Masters are the only mushi besides the Kouki who have appeared in more than one episode in the anime series Mushishi.
A Master can be replaced by another living being if the being is powerful enough to kill the Master and then eats the Master itself. A Kuchinawa, a large serpent type mushi usually hunts for these Masters and takes their place after it consumes them.
A Kuchinawa, or any thing powerful enough to kill a Master and take its place