I gift you: a spooky, ornate gothic mirror. everything it reflects looks dark, covered in cobwebs and dust!
Oh, this is perfect for me and Luma! We'll just set it up in the potion shop real quick, there we go... it looks pretty nice like this, showing a version of it that's all abandoned, and yet still has bottles on display! Although, looking inside, Luma's got some purple fur instead of her usual yellow, and I'm a dark-furred bootleg... let me just touch it...
Oh, oops! There we go, feeling my normal body getting rewired a bit, full of magical energy now. Hehe, and my face is all dark, but I can still see my cute cheek patterns. Well, I guess it's more like being turned into magical energy- letting it infuse me when I'm used to being either normal or glittery- but hey! This isn't the biggest change, sure, but it'll make for a good trick, letting guests turn themselves into darker, scarier versions of themselves! Thanks for the mirror!