D&D 3e (3.0/3.5) Rules
These mirrors have been around since the Age of Dreams, Although very few are left mostly just pieces of originals that have been broken. There are no known full mirrors left intact. The secret of creating these mirrors have been long lost.
The Mirror appears to be just like any mirror however they are actually Portals to a pocket dimension. No matter the size of the Mirror you may enter it. Once inside you are in a dark misty place the floor is visible but not walls or ceiling. To emerge you simply envision a mirror that you know (it has to be one you have seen and can easily envision). then it appears like you are looking from the inside then simply step out.
There is air inside and it is not known the exact inside dimensions if there are any Therefore it may be used for storage. All pieces of the same mirror enter the same pocket even though chances are remote another owns a piece of the same mirror.
References: Defenders of Magic Trilogy volume 1 Night of The Eye.
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