Adventurers will stick any appendage into anything if they think that it’s magical or they can get something out of it. So enter the Hat of Decapitation. It looks like a normal albeit magical hat. Put it on and zip. Make a Saving throw or die. Yeah, that’s really deadly to the first character but let’s face it player character can get pretty creative when it come to killing things.
So, once the characters do figure it, they’ll probably start trying to use it as a weapon. Good let them. Make a ranged attack roll at -5. If they hit then have the target make the save. Of course, they may also try to just put it on something’s head. And speaking of head. It’s a Hat of Decapitation. So it doesn’t do anything if you stick any other body part into it.