Note: This information was taken from TSR's original AD&D adventure, the Temple of Elemental Evil.
LENS OF TRANSFORMATION

This device is a sheet of cloudy crystal, 7 feet broad and tall and 3 inches thick. It was created by luz, who placed it in the dungeon for his own evil purposes.

The crystal sheet appears cloudy from one side, but clear from the other. No detect spell works within 20 feet of it, except for detect magic, which reveals that a powerful dweomer is upon the crystal sheet, and (if the lens is activated) that a lesser magic affects the area within 20 feet of it.

The lens is activated whenever any life force (living or undead) approaches within 3 feet of the clear side. The cloudy side of the lens then begins to brighten, its radiance producing a continual light effect within one round. Viewed from the cloudy side, the users (those on the clear side) appear as the purest opposites of their actual forms. A paladin would appear demonic or diabolic; a Chaotic Evil monster might appear as a paladin, or as a deva, planetar, or solar. Additional visual effects (flames and smoke, sky and fluffy clouds, etc.) are automatically produced as a background setting appropriate to the visual (false) form. Subtle feelings (fear, peace, etc.) are likewise projected to the viewers of the illusion, allowing no saving throw (though innate total immunity to ESP negates the emotional effects). The cloudy side of the lens quickly darkens when the user(s) back away from the clear side.

The lens will function as often as desired. It is protected by a powerful and permanent force field, and is impervious to harm by anything less than a godling. Even passwall and glassee are useless against its mighty dweomer. Once mounted, as it is herein, it cannot be removed by a power lesser than luz', except for a wish—and even that may cause the lens to shatter in the attempt (a 25% chance). A final dweomer lets luz know if the yens is destroyed, and who did it.