Too Much Headspace

The Dark Mirror appears to be an ornate hand mirror made of ivory, polished hardwood, and precious gems. This evil artifact is actually the product of ongoing rituals by the Spider Clan, a physical manifestation of their stated mission to hold a dark mirror up to Rokugan's culture. A person that looks into the mirror runs the risk of being possessed by the powerful Kensen inhabiting it. This possession is total; the victim's mind and soul are gone, irrevocably. At this date, it is unknown if their spirit is released into the afterlife, trapped within the mirror, or destroyed completely.

The Dark Mirror has a primal, malicious intelligence; it might choose not to possess a specific viewer, waiting for a more powerful victim to come along. Likewise, it will take the opportunity to jump to a higher status victim, but generally prefers to stay with a single host as long as it can, building a home and letting its plots mature.

The person possessed by the mirror becomes a twisted version of their previous self; the virtues they once exemplified are used to evil ends, and  their vices are magnified. A samurai once known for generosity and compassion might start giving away more than he can afford, crippling not just his own effectiveness but his lord's as well. One known for skill in battle might become bloodthirsty, while a skilled courtier uses etiquette and bureaucracy to become a bully. Whatever the specific manifestation, the ultimate goal is to pervert and undermine the principles of Bushido.

If you kill it, you become it
When the host is killed by another samurai, the Mirror immediately attempts to possess the killer; this is the most frequent way it takes a new host, often provoking a murder when it finds a promising victim. The new host may not show an immediate change in personality, as the Mirror lies low and completes its takeover.

For this reason, the Mirror does not fear "defeat"; its driving goal is the subversion of Bushido, so it cares little if a specific plot, or a useful host, is destroyed. It will gladly move onto the next host, and use that body to continue the previous plan, if possible, or "uncover" the plot to gain status and glory which help it in the next plot.

The Mirror can possess any human, but will usually not condescend to take a peasant or other non-samurai unless they have a useful position (such as head of a Ninja clan) or access to someone more powerful. If the host is killed by a non-human, it may choose to use the creature's body to reach another human, or just return to the mirror and wait.

It is unknown what would happen if its host was killed by a powerful Oni, but even the most wicked Spider shugenja are reluctant to find out.

Mechanics
Anyone that touches or looks into the mirror must succeed at a Void roll (TN 30) or become possessed. Once possessed, the mirror disappears, and reforms as an amulet around the host's neck. The amulet looks like the mirror's frame, with the victim's Clan mon where the glass should be (for an unaligned host, it forms the kanji for whichever of the Bushido virtues that host best represents).

The Mirror has full access to all the host's abilities, including Traits, Rings, Void points, Spells, Techniques, Skills, Advantages, Disadvantages, and other special abilities.

The host will usually gain a new Disadvantage, for which it receives no additional character points. The GM should select a thematically appropriate disadvantage that perverts the host's previous sense of Bushido and Honor. Other characters that knew the host's disadvantages previously do not automatically learn of this new one.

While possessed, the host does not loose any Honor for committing dishonorable acts. It still suffers Honor loss brought on by opponents' Techniques and spells, and may choose to loose Honor for acts committed in front of witnesses to maintain its cover.

When the host is killed, the killer must immediately make a Void roll (TN 30) as above or become the new host.

The only way to defeat the Mirror is to defeat it in a duel it without violating any tenants of Bushido. This does not need to be an Iaijutsu duel; Courtier, Artisan, or other contests work as well (even something as simple as a test of wills may be sufficient). If the challenger is successful, the amulet falls to the ground and returns to its original appearance; the host's body simply vanishes (it is just a reflection, after all). Anyone touching the mirror at this point risks artifact possession, as described above.