If these "shadow beings" are so dangerous, what's protecting YOU from them? I don't suppose you're putting yourself at risk every day on the off chance you find someone lost here, are you?
Reiyuu: I believe you are mistaken. That being said, let me show you something.
[Reiyuu goes to a nearby shadow infected tree]
[Reiyuu spells out a weird incantation]
[The tree seems to return back to normal for a brief while before the tree goes back to purple again]
Reiyuu: What you just saw is me using my essence to force the shadow beings lurking inside this tree out, it only works if you have a gift given to you by a legendary capable of doing so.
If you are worried about me being transformed into a shadow pokemon, I am a human soul, this effectively means I am immune to corruption from a shadow being.