Hear-ring
Magic item.
A simple piece of jewelry worn on the ear. While worn, wearers can communicate with one another no matter the distance. Frequently crafted as a pair, sometimes as a larger set.
Why this is cool. How many do you let your party find, just one? Who has the other ones (the bad guys perhaps)? Or do they find a pair? Perhaps they are part of a much larger set and left by the bad guys so they can listen in on the party’s plans!
From Game Master's Guide