The Mercury Wraiths of Mirrorrim

No city on the continent is more secretive than Mirrorrim, but all who've heard of the isolated metropolis know of the legendary Mercury Wraiths. In Mirrorrim, every mirror and reflective surface functions as a potential monitoring device, and could at any time be the means by which a citizen is being monitored by one of the Mercury Wraiths. Lawbreakers are few and far between in the city, as every room by law has at least one mirror, making the concept of privacy a totally foreign one to the residents of the city.
Though citizens learn to take a certain comfort in their constant guardians, the price of this security is a constant edginess that pervades Mirrorrim society, as even small gestures or remarks that could come off as subversive risk attracting the attention of one of the wraiths. Since they monitor through surfaces seemingly at random, there are a few citizens who are bold enough to sometimes wager that they aren't being watched (some claim to be able to "sense" when the wraiths are near or watching, though this is unverified), leading to the rather bizarre result that even large and obvious offenses can go entirely unpunished by the wraiths if they occur at a time and place that the guardians don't happen to be watching. Another problem with the system is the potential for misinterpretation. Since the wraiths could begin monitoring a conversation at any time, residents are careful to avoid vague language when being critical of someone and to be very specific in every sentence that could otherwise be construed as critical of the city and its mysterious enforcers, as sedition is a very serious crime.
When a Mercury Wraith decides that someone is breaking the rules or simply being excessively subversive or insufficiently devoted to the common good, it emerges from the mirror and tries to seize the individual in question and drag them back through to the other side. These individuals are then trapped inside the mirror for a length of time considered appropriate to their offense, from several hours all the way up to the rest of their lives. Escape from the wraiths is difficult, as ordinary weapons seem to have no effect on them, and they can "reflect" most spells cast at them back at the user. Coupled with their ability to teleport between reflective surfaces, even down to a person's eyeglasses, both evading and fending off one of the city's wardens are unlikely prospects, with nearly all perpetrators ending up trapped in one of the city's many mirrors without anything resembling a trial.
Though being trapped in such a mirror doesn't cause any physical harm, it's reportedly one of the most psychologically harrowing experiences a creature can undergo, as while trapped in the mirror they are in mental contact with everyone else who's confined there, a population that numbers at least in the hundreds of thousands. Since people don't age in the mirror, the extra-dimensional prison contains "lifers" dating all the way back to the original colonization of the planet, and the sustained psychic contact with this vast population can over time drive even the most stalwart soul to the edge of madness and beyond.
Because of these ruthless practices, violent crime is almost unknown in Mirrorrim, as are free speech, relaxation and any sense of privacy or personal space. Those few Mirrorrim citizens permitted to leave the prison-like city often do poorly in the rest of the world, as they've been raised in an environment where it was extremely rare that people took the risk to misbehave. Mirrorrim citizens sometimes leave their belongings unintended in public, fail to lock their doors, wander through dangerous neighborhoods and otherwise take foolish risks until they've spent enough time outside the city to truly understand the potential danger of life without their mysterious mercurial protectors.