Song of Stars with Moon
The Firewherian Empire has left behind a legacy of a lot of trinkets. One of these is a keystone, which is a magical object that acts like a "key" to a magical "lock." The most successful and powerful wizards employ such security on their personal vaults because of the difficulty and expense of obtaining them.
Although not powerful or valuable in and of themselves, if you know of a key-and-lock pair that works together, you can sell them for a lot of coin or favor to a wizard or government who wishes to use them.