Skeleton Key

This unusual object has two distinct parts: a small metal handle and a thin blade about 4 inches long. The blade is made of a shiny blue material whose surface ripples and shifts like the surface of water. The skeleton key is an unusual item commonly found in the possession of doppelgangers and their ilk, who have an affinity with its shapeshifting properties. When inserted into a common door lock, the blade ripples and writhes into the precise shape of the key needed to open the lock, even if the lock is much bigger or much smaller than the blade. Any lock that can be unlocked with a physical key can also be unlocked with a skeleton key.
faint; (DC 17) transmutation, DC 3rd; Craft Wondrous Item, knock, creator must have the shapechanger subtype. Cost to create: 2,250 gp, 180 XP. Weight: 1 lb. Market Price: 4,500 gp.
Source: Dragon #359