From the outside it looks like an old-fashioned standing mirror, made of rich oak with beautiful inlays it was clearly crafted by a master carpenter. It has two legs so that it could stand anywhere and be placed anywhere within the room. The large oval mirror is also extremely good given its age and anyone looking into it would be gifted with a great view of themselves.
Only sometimes it isn’t themselves who are looking back at them.
This mirror is magical, it has the strange ability to show the person looking into it “their true self” but it can do more than that. A person looking into the mirror will find themselves becoming their true version and if need be leaving this world behind and going to a place and sometimes time where they can be their true self.
The most common transformation is gender, changing a man into a woman or a woman into a man. Changes like these don’t require a person to be transported to a new world only other memories of their old self changed to fit their true self. More dramatic changes like if a person's true self is a mythical creature require more dramatic solutions be it transported to a remote area on Earth or a whole new version of Earth. Time travel is even possible, if someone has the heart of a knight or a Victorian lady, they will find themselves back in the past. While the person may change their memories of their past self-remain even as new memories arrive to aid them in their new lives.
Not everyone who looks in the mirror with change or the changes are so small goes unnoticed. The mirror has been around for centuries and now a new person is about to look in the mirror.