Alarum Hoop

A small brass hoop carries a bell no larger than a barleycorn, its surface worn smooth as if rubbed by generations of nervous fingers. No one but the wearer has ever claimed to hear it, yet just before unseen danger strikes, the tiny bell seems to ring somewhere inside the skull.

The bell rings a half second before any blow the wearer cannot see, and the first such attack each round deals no damage to them. Nothing is spent and no action is taken; the ringing simply happens.