Thanks, Cameron! That's a fair question. I think encountering the bone creature on the ground level is a big factor. They likely can't fight it and live, and if they try to run out the front door without gear and a plan (it is both locked and protected by a minor geas trap), they are almost certainly toast. That was my intention anyway--but it might not pan out that way! If they get out too quickly, the manticore could show up as they leave and pressure the PCs to go back in to get the locket.