The Troll Compass

This small device can be worn comfortably as a pendant. The arrow in the center of the device is made from copper and iron held together with two pieces of brass.
It is widely accnowledged that certain etheral entities repell, and are repelled by, iron. While copper is more neutral, even attracted to/by ethereal energy.
Therefore, the troll compass will always point towards any ethereal entity that is close enough for the compass to feel its presence. This can be extremely useful if the entitiy in question can be harmful or destructive.
Hence the name, Troll Compass.