Torrdok's Magnificent Forgery (Patreon)

Wondrous item, rare

This large, gilded mirror is 5 feet tall, 3 feet across, and weighs 20 pounds. While the mirror is against a wall or similar vertical surface, you can speak its first command word using an action to magically fasten the mirror to the wall. You can unfasten the mirror by speaking the command word again using another action.

While the mirror is fastened to a wall, its reflective surface becomes oily and moves ever so slightly. If the fastened mirror is facing a room that's no larger than 40 feet on a side for 24 hours, the reflection stiffens and becomes a beautiful painting of the room's reflection. The painting remains until the mirror is removed from the wall. While the painting remains, a creature can use an action to touch its surface and speak the mirror's second command word to be magically transported to a special demiplane identical to the room the mirror is facing, including any of its furniture and decorations. Doing so again from within the demiplane transports you back to the nearest unoccupied space to the mirror in the original room. Any furniture or decoration created by the demiplane can't be removed from it, and if a piece of furniture or decoration was magical in the original room, the copy of it is not. Further, any doors or windows found in the original room don't appear in the demiplane's version.

Anything brought into the demiplane is shunted back to the original room when the mirror is removed from the wall. While the painting remains, it is indistinguishable from a regular painting. A successful DC 15 Intelligence (Arcana) or Intelligence (Investigation) check can reveal the true nature of the painting, but not what command words are required to use it.

A trick of the light, a twist of perspective, and no small amount of magic.