Dark reflections lead to a very dark reality.
The Mirror Realm exists alongside the other ones. Nearly impossible to even access, most living things are not even aware the realm exists, even though they technically interact with it every time they look into a reflective surface, such as a mirror or water.
The realm itself lacks much detail, consisting of large amounts of flat surfaces, most of which are covered in mirrors. Effort is made by some of the more eccentric denizens of the plane to make parts of it personal, but these are mostly "passing" fancies that only last for a couple years or even just months at a time before the dweller gets bored and undoes their work.
There are cities in the Mirror Realm, where the mirror folk live and imitate life on other realms, changing their homes in the above mentioned ways. However most of the homes in these settlements are empty, implying that the Mirror World was once much more populous than it is today.
The main denizens of the Mirror World are the Changelings, strange shapeshifters who spend most of their long lives watching mortals through reflective surfaces. They are said to be ruled in their own way by some sort of "mirror king".