Thanks, everyone, for the suggestions and advice. I ran the scenario last night, and it was a hit. Everyone had fun, and each Player said it was an interesting encounter. They seemed to honestly enjoy the scene.
Here’s what I finally decided on:
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The chamber:
Immediately noticeable in the chamber is a huge mirror filling the far wall, 30’ across the room. The mirror shows not only your own reflection, but the beautifully colorful wall on the other three sides of the chamber. The wall is covered with a huge fresco of many people in elaborate robes, sitting on rows of benches.
The reflection shows the chamber, as a whole, is 50’ wide and 50’ tall, with two other corridors leading out, one to the left and one to the right.
The mirror seems to slightly magnify the light in the room, extending flame or magic light enough to barely show the extent of the chamber. If a character steps into, or leans into the room to see the other two exits, they will discover that they only exist in the reflection.
To clearly view the fresco, the viewer and light source must be inside the chamber.
The whole of the fresco scene suggests the figures are spectators at an event. The figures are all human-like, but each has a subtle hint of “something else”. A Knowledge Planes check, DC 20 can identify the “something else” as various outsider touches (good, evil, lawful, chaotic, elemental, etc.). They all seem to stare into the mirror with a variety of expressions, from fearful to humored, concerned to entertained. The craftsmanship is masterful, and the surface is in perfect condition.
If detect magic is used, the whole of this chamber radiates a lingering (dim) magical aura of indeterminate school, but the mirror radiates an overwhelming conjuration, transmutation, and illusion aura. There is no alignment aura here.
The mirror is impervious to harm, physical, magical, or other. If struck with an instrument intended to break it, a crystal ping will be heard.
One round after all characters are in the chamber:
The room fills with bright light and the exits vanish.
After one more round:
The physical presence of the mirror disolves. The reflection materializes as real, making a 60’ long room with exact duplicates of yourselves standing in their previously reflected places. The duplicates spring to their own life and attack.
Roll initiative.
The duplicate characters have all the same stats and equipment as the real characters do at the moment of the mirror disappearing. Any spells in effect on the real character are also in effect on the duplicate character. The duplicates think they are the real characters and that the real characters are starting the fight. The duplicates have all the same knowledge and memories as the real characters, and they will fight as the real characters would.
Notes on the environment:
The chamber is made of solid stone, of infinite depth.
Magical teleportation (or similar escape attempts) will fail to take the targets out of the room, but will let the caster choose to relocate within the chamber. Even planeshifting or gating will fail to give exit, but merely allows movement within the chamber.
Summoning into the room works as normal, and once a spell’s duration ends, any summoned creatures disappear as normal.
When a character, real or duplicate, is killed, both real and duplicate will disappear from the chamber. The single character will then appear in an identical room, facing the corridor they entered through, with a black, non-reflective wall behind them, instead of a mirror. The character can leave immediately or wait for the other characters to be released from the battle.
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