With a great amount of mana, Walker managed to heal many of the injuries that had been caused by being so close to the light purifying arrow that he and Gil had made. But he could still feel the places where splinters of wood from that arrow had hit him. He even still felt a few splinters till in his arms.
The blinding effect that they had all been hit with due to the arrow's explosion also left them seeing spots, but the healing had done its job. Walker could see the entire room and the damage done.
Because the arrow had exploded with light mana in a place where lots of darkness mana had been, there was a clear effect. Many stones had been eroded over time and now lacked the darkness mana that had ebbed in to the stone becoming a part of it. losing that, had caused them to crumble.
Creation is hard, cheer me up!