Void stones are rare materials found only in the ancient ruins sprawled across the second Earth. Their only known purpose was to stabilize gates. However, in the future Oliver knew of, the demons used them to open gates to other worlds by destroying them.
The otherworld gate opened through the destruction of void stones was often random and unpredictable. You could never know what will come out of it.
If you are lucky, it might be some random monsters that could be killed easily. However, luck was not always on the side of humanity.
Most of the time, demonic creatures rush out of the gates that were created this way.
There were many theories as to why it was that way. The most accepted one was the mana-linked-many-world-theory.
This theory goes in depth about the relationship between intelligent life and mana. And the flow of mana from one world to another.
