Answer: The adverb slowly describes the verb walked. It basically says HOW the person walks (like did they walk fast, slow, at an average pace, etc., and in this case, it says that they walked in a slow manner).
Edit: so I’m assuming effect is like the mood of the story created, and I’d say that it creates a somber, suspenseful feeling, opposed to if she was walking quickly, which would create a rushed mood.
