Symbol of Dark Aura and also used to represent aura cost on cards.
As one of the many aura types in MetaZoo Dark Aura is characterized by the black border and the largest amount of beasties and cards with this typing. Being the largest aura typing followed by Forest, Dark is focused one being played during nighttime. Playing a Dark Aura means you have a weakness to Light and Spirit Aura and strengths also against Light and Spirit Aura.