Bone Arrow

The Skeleton Merchant rotates his stock with the moon, and these are missing from his inventory more nights than they are in it. That makes them impossible to treat as a standing supply. Check him every time you stumble across him underground and buy deep when the arrows are there, because the next meeting may land on a phase where he has nothing but torches and rope.

The draw is that they pass through one target before stopping. In the tight caverns where you actually run into him, that is worth more than a small damage bump, since anything chasing you down there arrives in a line. Against a single boss the pierce does nothing at all and ordinary ammo does the same job for less.