From the introduction:
The mirror can be found in any inn, manor, or castle in your game. The room the mirror is in is slightly chill, just a few degrees colder than any other room in the building, and upon inspection, there is the slight imprint of frost along the edges of the mirror. The mirror itself is long, about 5ft by 3ft, and is encased in an ornate, but slightly tarnished, silver frame. There appears to be a segment of the frame which is slightly out of place. It pushes back into place easily with a click and the frost grows, covering the entire surface of the mirror. At this point, the players can now move through the mirror as a portal, though they still cannot see what is on the other side. Stepping through the mirror places the players at the far end of a throne room. Turning back, they can see through to the room where they came from, but cannot interact with it in any way.