A bronze statue of a beast of some kind, or a person, stands on a stone pedestal, where the type of creature, or the name of the person, is written in carved letters.Anyone who touches the statue switches places with the beast or NPC it depicts (no Saving Throw); the beast or NPC becomes real, and the character becomes a statue, standing on the pedestal. The character's name now appears in carved letters there.
A beast will attack immediately (GM consults their favorite monster guide). An NPC might attack, or might try talking to the party, or something else. Who, exactly, this person is could be the jumping-off point for a whole new adventure.
Freeing the former party member will require a living (or possibly undead) creature to touch the statue. Slain or otherwise dead creatures will not activate the magic. Remove Petrification magic might work, too, at the GM's discretion.
The Lady of Elche by Following Hadrian and released under a Creative Commons Attribution-Share Alike 2.0 Generic License modified with Inkscape by David