"According to the documents, when a rift is created in the boundary between two different worlds, the energies from each world flood into the other world, creating instability. Of course, we have no way of knowing what kind of effect that has on the other world, but as for our world, the foreign energy has an effect on Monsters, who are especially sensitive to Mana.
To us, Mana is a source of energy to fuel our Magic, but it isn't essential to our survival. Many Commoners hardly have any Mana at all, but of course, they have no trouble staying alive. Monsters are different, though, Mana is as important to them as air is to us, they can't survive without Mana. And the resulting tainted Mana affects them like a poison.
