Item: Sword of Piercing Armor And Nothing Else
Long ago, the gods of the forge created a sword for a legendary hero that could phase through armor—any inorganic substance, in fact—to wound the flesh underneath. The hero used the sword to vanquish monsters and enemies, and today the sword resides in a place of honor in the Hall of Heroes in case it is ever needed again. However, this is not that sword.
In modern times, a clever mage obtained what was believed to be the formula to create another such blade. Upon gathering the magical ingredients and enacting the charms, he discovered that the formula merely acted to alert the forge-gods to a mortal attempting to duplicate their holy work, and the intended sword was cursed to do the opposite of its intention: the blade phases through organic material and can only damage inorganic materials such as metal and stone.