Mirror of Unseeing

A large magic prismatic mirror that renders any viewer of themselves invisible, along with any sentient consciousness in its view (or in the view of any in its view) that is in the company of one that's aware of the mirror. Objects are also rendered invisible. 
The mirror will reflect back only that which is plain or indistinct to any conscious viewers, i.e. in a room full of objects and furniture with people and animals, it will only show the plain floor and the walls of the room; in a field with circus activity, it will only show the plain grass, the sky, and the horizon.
The mirror was created as an artifice for war in cloaking armies.