Mirrorgeists

Mirrorgeists are a type of aberrants believed to be connected to the Ancient of Reflections. They are believed to be born when a mirror reflects a scene of premeditated and violent murder, although as one can expect, not every such event results in a birth of a mirrorgeist. What additional requirements are there is something that varies between the scholar and a bestiary.
They are cunning manipulators that excel in driving people around them into madness by manipulating their perception. Mirrorgeists are capable of creating illusions that are almost impossible to be differentiated from reality and tailormade to what they managed to learn about their victims. Once they (or Initiates that realized what was happening) understand what they are facing, mirrorgeists are rather easy to seal, banish or slay.
The real problem is getting there.

Basic Information

Anatomy

Mirrorgeists lack physical forms. They lack forms as a whole. The closest one can get to witnessing a mirrorgeist is a vague movement in the mirror seen in your peripheral vision. If a mirrorgeist decides to talk with you, it will take the form of your own reflection - this tends to happen when it is cornered by Non-Initiated victims, forcing it to attempt a bargain or some last-ditch manipulation. Initiates know what to expect and most will not fall for it.
The mirror in question is the actual 'body' of the mirrorgeist. The appropriate reaction to a suspicion of a mirrorgeist haunting is immediate destruction of every mirror within the affected area. The correct one will be significantly more resistant to hits (although way from impervious). What's more, when it's shattered, a blood-like liquid will flow from the cracks until the mirrorgeist inside finally dies.
Caution is advised. Mirrorgeist can conjure an illusion of a mirror to shatter while concealing itself elsewhere. Use a blinking and colour-shifting light source to see through mirrorgeist's illusions. False objects will have their shadows (and the colour of their surface lighting) change with a minuscule delay. Mirrorgeist also can't move his mirror on its own. If you know where the mirror was hung before it all begin to happen, approach the right place (ignoring the illusions) and destroy it.
Mirrorgeists appear to grow in strength proportional to the length of their existence, the size of their mirror and how clear their reflection is. There are occasional cases of mirrorgeist manifesting in other reflecting surfaces - such as a glass chandelier - or even transient ones, such as a body of water. Those are way weaker. Many such surfaces may be mirrorgeist-inhabited, but if so, they lack the strength to inform the world about it.

Genetics and Reproduction

Mirrorgeists 'reproduce' through manipulating humans into committing acts of premeditated (if violent and madness-driven) murder in front of reflecting surfaces. Eventually, someone will pick the mirror in question and move it elsewhere, even if that elsewhere is a landfill. This allows mirrorgeists to propagate, although it has to be stated that they are also born on their own.

Ecology and Habitats

Mirrorgeists will establish their zone of influence - typically the house of their victims - and will then proceed to manipulate people trapped within. They are expert manipulators, who will use a multitude of cunning tricks. They understand humans extremely well, including various nuances of their culture (although it takes them a while of observation to learn that).
For example, a married pair might start seeing signs of their significant others having an affair. Naturally, both sides' attempts to explain the truth will be met with a complete lack of understanding. After all, the other side is certain that they saw it.

Additional Information

Perception and Sensory Capabilities

Mirrorgeists do not possess senses as understood by humans. Instead, they know everything that exists within their range of influence (varies between a few meters to a few dozen meters around the infested mirrors). They require that knowledge to warp the perception of anyone inside of it. Their knowledge is limited to what is there, however. Mirrorgeist will not mysteriously know what you know, but it will know what you have sealed in your pocket.

Mirrorgeist

Summary

Type: Aberrant
Subtype: Servitor
Status: Kill-on-Sight
Estimated Numbers: N/A
Censor's Veil: Temporarily Uncensored.

Alignment

Creator Power: Ancient of Reflections
Alignment Status: Servitude
Sanity Damage: N/A
Corporeal Stability: N/A
Magic: N/A

Combat

Combat Behaviour: Ranged/Illusions
Threat Rating: 2
Physical Damage: Vulnerable
Magical Damage: Vulnerable
Glyphs: Vulnerable
Notable Weaknesses: Blinking lights.