Quest-item thing worn around the neck

That you cast contact other planes on, etc. I'm trying to be a little vague here. It does work in combat, you don't need to do anything specific to use it, and it does work correctly. Its function is only useful in certain situations, and you can fight forever and it won't trigger unless the circumstances are right. As a hint, things have to be reasonably dire for the wearer. If that doesn't make any sense, consider another piece of gear that you may have heard of that can automatically recall the wearer a couple times if they're near death. This is vaguely like that.