As your eyes move from door to door you glance down the hallway at the mirror and something doesn't look right. You walk down the hall toward the mirror, your reflection slowly growing in the glass. It is a full-size mirror apparently mounted to the wall at the end of the hallway. The frame is silver, but otherwise very plain in design. The glass seems strange somehow, like it has a sort of watery shimmer to it.
The reflection is yours, but not quite. Your chest seems a little fuller, your body slightly more toned, your hair a little longer and a slightly different shade, your eyes are a different color. You think of funhouse mirrors that make people appear taller/shorter, fatter/thinner, or all sorts of distorted reflections. Is this just some trick of a clever mirror designer?
You blink and suddenly you're staring a different reflection. You look younger, like you are a teenager again instead of in your mid-20s. How on Earth?
You close your eyes and shake your head. Mirrors can't play tricks like that; they aren't magical.
When you open your eyes again the reflection has changed again, this time more drastically. In the reflection, the hallway is gone and replaced by an underwater scene. You're still there, but naked and with a fishtail where your legs would be. The mirror is showing a reflection of you as a mermaid! You look down and see your legs intact. You look at the walls and back down the hallway but it seems unchanged from before. First the mirror showed what you thought was a more attractive self-image. Then a younger you. Then it showed one of your fantasies: to be a mermaid. Maybe this mirror really is magical, somehow tapping into your desires, not maker how deep or dark.
Your gaze returns to the mirror and it shows you once again in the hallway, but still naked. Instead of a fish tail, your reflection has horns, dark wings, and a slender pointed tail. Tapping into your darkest desires indeed: now it shows your reflection as a succubus! But it's only a reflection, right? As you did for the mermaid reflection, you check your actual body and confirm the only changes are in your reflection.
The mirror must be magical and if it can show your changed reflection then it can probably change your real body to match that reflection. But how? And you should you try to have it change you? You don't know the consequences, but the possibilities are very tempting. It seems like the reflection will be something you consciously (or maybe unconsciously) desire so you can probably control the reflection by focusing. But for the reflection to become reality what must you do?