With a little more information from Clara, Walker felt that he had made a few theories about what might be causing issues nearby the new ruins discovered.
He had originally believed it might be another bridge to the elemental planes opening up. It would make sense if the dark elemental plane was on the other side of it causing a disturbance with the dark mana. Or that a monster had already passed in to the world and had manipulated the dark mana because of its strong affinity for dark mana over others.
However, even if that was the case, then the odds that a strong enough monster which could take control of dark mana over Walker and Mordant and even Onyx, was very low. They would need the dark elemental mana from the dark elemental plane more than anyone else would so they could not survive within the world without losing mana, not taking it.