Plane of Mirrors
The Plane of Mirrors is a collection of demiplanes in Dungeons & Dragons that's really freaking obscure and very difficult to find any information on, mainly because you can't Google it without getting a crapload of useless results about physics and refraction.
Mirror planes were introduced in the Third Edition Manual of the Planes as an optional group of transitive planes. They are small planes that each connect to a group of mirrors that can be located in any other planes throughout the multiverse. A mirror plane takes the form of a long, winding corridor with the mirrors it attaches to hanging like windows along the walls. Mirror planes allow quick travel between the various mirrors that are linked to each, but each plane contains a mirror version of any traveler that enters it. This mirror version has an opposite alignment and will seek to slay their real self to take their place. All mirrors connect to a mirror plane, though each mirror plane usually has only five to twenty mirrors connecting to it.
The pirmary natives are the Nerra. They constantly observe the Material Plane, occasionally making forays to kidnap its inhabitants. They are known to wander the planes on missions of reconnaissance and infiltration for a theoretical future invasion. Nerras resemble humanoids with mirrorlike skin. When killed, nerras shatter into thousands of shards. Nerras wield weapons made out of the mirrolike substance of their home plane. Although they consider themselves superior to Material Plane beings, they try to maintain a neutral outlook on most matters.
Types of Nerras Kalareem: Ferocious warriors that fight with two shard swords, they are the chosen defenders of the Plane of Mirrors.
Sillit: The leaders, planners, and schemers of the nerras.
Varoot: The most common nerras, experts at spying, infiltration, and subterfuge.