Void magic requires a soul to be sacrificed to it every time it is used. Which is why it is so rare and hard to use.
Often Val and Sno are the only two who end up not using it too often due to the high emotional state you need to be in and the sacrifice of a soul.
It requires you to abandon your morals and emotions for the darker side of emotions, malice, hatred, anger and revenge. As such it can only really truly be triggered when someone is pushed to the brink of their emotional state. It’s said this power at its peak can even cause the most powerful of divine creatures to struggle.
Only Kitsunes have a slight chance to control this power even among them only two have mastered this power one being Sno and the other his father.
Val is working on controlling this power but it’s exceedingly difficult similar to another ability in this world.