The first sage said, "The Ghost Hone is lumpy, meteoric rock the size of a loaf of bread. It has a rough slot worn in it from blades being dragged edge-down, across it. If you do this to a blade it will ruin it for normal foes (-2 to hit) but allow you to hit things that could not otherwise be touched."
"No," said the second sage, "it is as you describe, but dragging a blade across its rough surface releases the soul of a creature that blade has formerly killed to, now, fight for you (summon undead of last creature slain)."
"No," said the third sage, "there may an item such as you describe, but the true Ghost Hone is a cursed jian that the three liches of the Southern Archipelago war over even now. If a spirit is struck with the blade it carves away the weakest part, each blow making it stronger.