Nightshade is a place, not unlike our own world, but much more ancient. A parallel plane described in ancient texts as purgatory. It was believed that the Fallen walked freely there, and could traverse between our world and theirs. Human souls also occupy this plane, though the nature of how they arrive, or why, is obscure. As I understand, they are not free to come and go. Given the information and what is known, I would venture to say these souls belong to those bound for the infernal realm, or those which are easily corruptible with unfinished business. Lost souls.
