mirror people, vacuum pockets between mirror areas, filled with things that we expect in the dark, real selves get light powers, mirror selves get mirror powers. Living darkness creatures in a shadow ecology.

Mirror: Sufficiently polished metal or stone which can be fashioned into a sufficiently flat surface, or sufficiently still water.
"Sufficiently" is doing some work here but I'm not poking it further until it becomes more directly relevant such as in a thread

Question: Are reflections evil twins?
Answer: Reflections are as evil as the co-authors involved want them to be, yeah. Can be alts, can be related templates, can be facedoubles from different moieties if authors are having fun with it

When an ordinary human (henceforth, ‘illuminator’) looks at a mirror for the first time, they create a mirror-person (henceforth, ‘reflection’) in the world behind mirrors (henceforth, ‘Overshadow’). Whenever that illuminator looks in a mirror, their reflection reflects them exactly. The illuminator gains magic (henceforth, ‘illumination’).

If their reflection makes it to any other mirror besides the mirror they were last reflected in, before the next time their illuminator looks at a mirror, they can escape into IRL (henceforth, ‘Illumination')
In such a case, if their illuminator looks into the mirror they last reflected in, the illuminator is sucked into the mirror and arrives in shadow-world, where they develop new, shadow magic (henceforth, ‘endarkening’). If instead, their illuminator looks in any other mirror, they create a new reflection with the same behavior as described above.
It is difficult for reflections to escape shadow-world for light-world because the shadow-world is full of shadow-monsters who lurk between mirrors

1. In cultures where mirrors feature heavily in religious ritual, getting sucked into a mirror and having an identical doppelganger with different powers show up is considered a good thing.
2. In cultures where mostly nobility have access to mirrors, disappearing happens sufficiently privately that people might think you went for a walk, and let you be replaced with an identical doppelganger without much fuss.
3. It is very difficult to escape, because there are shadow-monsters.
Also sorry 4 which I never specified or implied- mirror technology is very limited- they don't have glass mirrors yet. Oops

Illumination
1. Making light
2. Affixing light to objects/animals (physical sign-posting); navigation, improved visual acuity & observational skills
3. Mental sign-posting
4. Uplifting animals

Overshadowing
1. Making- shadows...they can destroy mirrors...?
2. Affixing shadows to an object or animal- obscuring things
3. Inducing hallucinations, removing- mental processing abilities (mental sign-posting in reverse, removing links to reality)- erasing memories
4. Downlifting/Creating shadow-monsters

Mirroring
Reflections also have powers, which are vaguely as follows: Receptive empathy, projective empathy, mental misdirection, Jedi mind tricks, weak precognition, sensory stuff

There is a society of *overshadowed*, descended from illuminators trapped in Overshadow, who are engaging in a campaign of war against the reflections, because they think they are a danger to illuminators- way i framed it to sage was they think of themselves as the restless ghostly ancestors of the illuminators defending them from reflections.