People who travel through mirrors. And dont always have any other existence.

The mirror travel is via the dead lands* as a conduit, though one can get trapped in it.

Donons can travel through space from literal point to point Transit to another mirror, or for inter-planer travel.

Note that if one encounters a mirror in the dead lands, unless one is a Donon or otherwise experienced wizard, one will encounter a random or tangled web of destinations.

The mirror could also be a barrier. Whatever it is it will probably be opposite of what it is in its material plane.