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 having to sift through a crapload of useless results about physics and refraction.
It all starts with Lewis Carroll's "Beyond the Looking Glass", probably. In D&D, the main plot of the module X12: Skarda's Mirror takes place in a demiplane carved out of onyx and featuring many reflective surfaces, which Skarda subsequently discovered and terra-formed. This cannot be THE plane of mirrors - it is only a cubic mile in size! - but since the module was good, this plane's ideas lingered around the fandom, notably that Skarda's Mirror was not the only mirror linking to this moribund plane.
Mirror planes were again referenced 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 Plane of Mirrors has a role in the Shackled City Adventure Path.
The primary 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 mirrorlike substance of their home plane. Although they consider themselves superior to Material Plane beings, they try to maintain a neutral outlook on most matters.
However there're also these folks called the Kamerel, who were the original inhabitants of the Outlands before the Rilmani came around. The kamerel fled here after a war with the rilmani, and may or may not have created the nerra.
Surprisingly, the plane of mirrors has made an appearance in 5e, on page 158 of Tasha's Cauldron of Everything. Though in this incarnation it exists only as an in-universe theory to explain certain magical zones with weird mirror-like effects.
|The Cosmology of Planescape|
|Inner Planes||Ethereal Plane||Prime Material||Astral Plane||Outer Planes|
|Elemental Planes||Energy Planes||Demiplane of Dread||Plane of Shadow||Plane of Mirrors|
|World Serpent Inn||Tu'narath||Sigil||Demiplanes||Ordial Plane?|