Welcome, Matt! I'm pretty new too, both to these forums and to writing as well.
All that aside...it's rather cold in here. *holds up marshmallows on a stick*
You could consider mind magic of some sort, such as influencing emotions or slipping suggestions/false information into a person's mind. The person being affected wouldn't notice unless, for some reason, they question the validity of the suggestion and why they're thinking that. People usually...
Since it's a shadow creature, I figured it wouldn't have a physical form; at least, not the kind that you can hack and slash at with a sword. It would just be the absence of light.
You know how when you're standing in total darkness, the darkness seems almost palpable? How you can't see...
Oh, that just sparked an idea. I remember watching an episode of...something...in which a spirit would attach itself to a person and slowly take their place in life - so that eventually, the person would become the invisible spirit that could only helplessly follow around what was once their...