The basement is a place in your house where you keep many different objects most people don't even use anymore, mostly because they have grown out of them. Basements are usually places where people go to do laundry, or mostly clean seeing how they wanted to get rid of boxes of old stuff they had from their past.
One of those objects was a magic mirror that could lead anyone to a mystical realm that no one has seen before, but exists if you take a couple of steps through it. Many people say that this mystical land can give you an appearance of someone you aren't as you arrive at the location, and hope to see what you might find while you are there. However if you step through the mirror, some stuff might not what you would expect, for your humanity would fade away as well.