Ruins of Adventure
A Harper pin gives its wearer a +5 bonus to saving throws vs. all enchantment/charm spells (and equivalent psionic powers). It also confers absolute immunity to all detection, mind- and alignment-reading magic and psionics; the wearer simply isn’t there. (It does not prevent magical detection linked to specific objects carried by a Harper, however, except itself; a locate object spell used to find a Harper pin would fail, but one used to find a specific item that a Harper is known to be carrying a particular crown, for instance, would find that item, but still not reveal the Harper carrying it.)
A Harper pin also absorbs all magic missile and lightning/electricity attacks into itself, protecting a bearer from all damage.
Harper pins are made by certain skilled (and) secretive) smiths in Evermeet, Mintarn, Waterdeep, Neverwinter, Everlund, Silverymoon, Evereska, High Horn, Shadowdale, Deepingdale, Lyrabar, and Starmantle. The smiths make and give pins only to specific senior Harpers known to them not just to anyone who shows up on their doorstep asking for one.
Harper pins are never attuned to a specific being or wearer. They are of silver treated to be everbright (never tarnish) and as hard as adamantine. They are AC 22, have 9 hit points, and suffer no damage from magical attacks. They gain a +5 bonus to all item saving throws.
Some Harper pins turn black in one round when worn by an evil-aligned being, and thereafter utter discordant jangling sounds, as if a metal-stringed harp was being savagely struck.