The randomness of a wanderer being sat right at the entrance to the bridge of the elemental planes experiencing dark mana connection from the dark elemental plane was an interesting way to start the quest off. Even more so that the true dark sage had literally pulled the shadow mana wanderer back in to the planes themselves.
Walker knew that there were monsters called shadow people or shadow men, however, compared to the race he had just met, it was clear that they were exceptionally different. That there had to be a change in name for the monsters that were known. Something that would most likely begin to appear as time went on.
The idea that any monsters might have developed differently and even become intelligent races within the elemental planes was nothing new. It was something that had already been proven as true.