A Loyal, Undying, Reflection

It had been a bad week

for the Appropriation Syndicate, and their boss Purloin Pete. Recently, the town guard of XIV  (the northernmost settlement of the hyper-effiecent, wizard controlled state "Magi Lamentorious") had made thirteen arrests and foiled four separate robberies (well-planned robberies, too.) It was all because of one man, Sargent "Mud" Malcolmson. The good Sargent was the worst kind of guard for a gang of thieves; the type who wouldn't take bribes, worked too hard, and was really fucking lucky.
The problem with Sgt. Malcomson, however,  is that he was supposed to retire last week, but he's still making arrests. Worse, he's more efficient than ever. Sgt. Malcomson has been personally involved in every single of the above arrests. The thieves who have gotten away have told stories of the old soldier being around every corridor, leading every patrol route.
Purloin Pete is fucking scared, and he'd love to write it off as made-up stories from his shitty employees. When the ruler of your town is a Magus Vizier, however, you can never be too sure.

So Pete assassinates the guy, puts a knife right into Mud's neck himself.

But he's back on patrol the next day. More specifically, he's leading every single patrol group in the city. At the same time.

Plot hook: So Purloin Pete has put the word out, looking for anyone who would be willing to find out the fuck is happening. He'd pay a handsome reward, and double it if whatever is going can be stopped, plus you'd gain the favor of a major CRIME KING.

What's really going on:

The Magus Vizier of Settlement XIV doesn't have time to fuck around, he wants to do wizard stuff. So when crime is on the rise and the captain guard is going to retire, he's in a decidedly non-wizard spot. So he found a solution. He made Sgt. Malcomson into a Mirror Man. Basically turning one loyal and effective warrior into eight mirrored copies, who can be reborn every day.

Artist: Araki, JJBA

Creation

The process of making Mirror people is this:
Feed the poor bastard a mixture of deadly poisons and vital mimic essences. This will kill him over the course of a single night. As he does, strap him down in front of a large mirror, shine a night’s worth of constant moonlight onto them, and then letting them dissolve in agony. Spread the person-juice onto the mirror, and begin casting commanding spells to sustain and control the beings which will come out.

 Then, depending on the purity of all the components, (poisons, moonlight, and the victim’s loyalty to their master) anywhere from three to hundreds of Mirror People will pour forth.
They are perfect copies of the original, with whatever basic materials they were wearing when they died. (Anything too magical or sturdy won’t melt with the corpse, however). They can be slain, but will re-appear the next night, bleeding out from the mirror.

Strengths

Mirror Men are perfect copies:

They have the same stats as their original version, in Sgt. Mud's case, a 3rd level fighter. They have the same thought processes, skill with their gear, etc. 

Mirror March

Mirror People are incorporeal to each other: They can occupy the same space as each other, and as their weapons count as part of their form, they can fire and attack through each other as well. This can lead to some interesting tactics, they can dog pile on top of you while their copies shoot crossbow bolts right into you, five of them can hold a single door frame, attacking from the same exact spot, looking like a multi-armed god of death. (When they do this, successfully attacking one will hit them all, however.)

Mirror People don't need to eat, breath or sleep. 
They simple persist, like a semi-tangible illusion. This makes for excellent guards that you don't have to pay any sort of upkeep for.  The laws of gravity only sort-of apply, they walk through liquids like air, unless it would cause immediate bodily harm (acid or lava).

Weaknesses 

Mirror People are chained to the mirror which spawns them. 

If the mirror is broken, the Mirror People explode into a shower of glass. Due to the specific natures of the ritual, the mirror cannot be altered in size, shape or durability, so the creator needs to guard it well. The drawback is, if a Mirror Man is killed he reforms the next night from the mirror, so you can't really have it locked away too much, or you're gonna be running back and forth to let them out, giving them instructions, etc. Safe but accessible are two things which are difficult to have together. In our example, the Magus Vizier just keeps his in his central lobby, disguised as a decorative full-length mirror. 

Mirror Men stay the same:
While they can remember orders and instructions as well as anyone else, they do not gain levels, and when killed come back from the mirror with the same memories. Any non-original gear as well as assignments/instructions need to be handed out over and over. The Magus Vizier particularly hates this monotony.

It's weird to be a Mirror Man.

They are only partially aware of what has happened to them, and will not naturally fight or coordinate to the full extent of their abilities. Mirror People are all copies, so they all think exactly alike, but they are not linked by a greater hive mind, or are capable of explicitly interacting with each other. Because of this, their reactions to problems is often quite similar, only changing depending on their situation. Some examples of this:

If we took all eight of the Mud Malcomson clones and stacked them on top of each other in the same exact space, and then swung at them, they would all block with their dominant hand, in the same spot, in the same way. Or they might all duck, or jump back to the same exact spot. 

If an intruder was detected in an area some Mirror People were guarding, and the original would have ran for the alarm instead of fighting, they would all do that. They wouldn't say "ah, some of us should run for the alarm, and some of us should fight the intruder." The strangeness of their condition makes it so they simply are unable to strategically or creatively think about themselves that way.

In order for mirror people to be able to use their abilities (moving/attacking through each other, etc.) they must be specifically instructed to do so (you, stand here with a shield, the rest of you shoot crossbow bolts through him, or everyone stand in this one spot, but you hold up a shield, you attack with a dagger down low, you use a spear up high etc. etc.) Having such instructions helps, but the longer combat goes on for a mirror man or woman, the harder it is to plan for every possible circumstance. Therefore, the longer a fight or other situation is drawn out, the likelier the Mirror People will just wind up stacking onto themselves fighting one person the exact some way.


Same strengths, same weaknesses. 
Troop diversity is often a good thing. An army consisting of one person has all of that person's strengths, but also all of that persons' weaknesses. Sgt. Mud Malcomson is (to his shame) terrified of rats, so all of his mirror clones are going to panic when lots of rats are around. Long standing injuries, verbal/emotional quirks, etc. Learning about these can allow you to easily manipulate Mirror Men.  and this  is why a smart wizard will never rely on just Mirror People to guard his stuff. Having a more traditional sentient creature to guide a few Mirror People around is a much better strategy.

Conclusion

Learning about what happened to Sgt. Mud Malcomson proves challenging. The local magic archive has been purged of all literature about Mirror People, by order of the Magus Vizier. Savy magic users in the party may know a bit about the process. The best bet would be breaking into/infiltrating the wizards tower, or killing some Sgt. Malcomson clones and observing the tower to see what happens.  Non-violent options may included gaining an audience with the Magus Vizier or bribing the few servants/advisers they keep. Purloin Pete is true to his word and pays well for info on the Mirror People, and may decide to keep the information to abuse it (thieves in XIV start carrying bags of rats), or he made decide to pay the group to just smash the mirror. If they already have, he'll pay them extra. Purloin Pete may have other jobs for the group, but beware the ire of a Magus-Vizier.

I hope any of this makes sense, Mirror People are a tricky and persistent enemy that can allow for players to further master a dungeon in a creative way.

Originally I was going to name them "Speculum" the Latin word for mirror. Thank god I googled it first.