"I can only identify one thing, and it will take a good deal of time," Nethezar answers. "I will get started with the mirror. I would think these documents would be useful," he nods at Kushnak.
Erdolliel returns to the other other room. Just as there was a pair of traps at the top of the stairs, the elf finds a second pit trap mirroring the one previously sprung. Near the odd contraption at the west end of the room, the elf finds a locked chest. Inside, she finds two expensive-looking gems. Finding nothing else, Erdolliel rejoins Kushnak in searching through the documents.
The documents detail various activities throughout the Free City. It becomes readily apparent that, whatever these things are, they used their shapechanging abilities to very successfully infiltrate every level of the Free City from merchants' guilds, to churches, to the very heart of the government itself. While it seems that these creatures didn't literally have control over the Free City, the puzzle pieces Erdolliel and Kushnak put together suggest a very sophisticated operation that, when leveraging all its resources, would have significant influence. (There is nothing, at least at this point, that strikes either as pertinent to the party.)
Nethezar finishes his spell. "A very odd device. I've not seen the like before. It has the power to, a single time per day, show in its reflection the true form of any being physically altering its shape. Though it has no power to see through illusion magic. I don't know what it may be worth, but given its rarity, it may be considerable."