I haven't dived into Skyrim's creation kit yet (I'm not done playing around in the game itself yet), but I don't think there's a command/function that does exactly what you want to do.
OnHit paired with DropObject and a global variable might do it, but the OnHit portion would have to be placed on the enemies, not the weapon itself, which presents a whole new problem--You'd have to attach the script or add your code to every NPC/creature in the game, or try to run it as a global script (which may not work at all, I've never tried anything quite like that in Morrowind or Oblivion).
Now, if you can figure out how to "lock" the player in to a single dungeon (where they would have to get the weapon and work their way through), then adding the script to custom creatures/NPCs within the dungeon is much less daunting.
Actor side, you'd be looking at an OnHit event that sets the global with SetValue (or SetValueInt). Weapon side, you'd check for the global to be at what the OnHit set it to (via GetValue or GetValueInt), perform a DropObject, and then restore the global to its previous value. Because I haven't gotten into the creation kit side of Skyrim yet, I'm not sure on syntax and won't try to provide code that may or may not work, but the creation kit's wiki has the information you need for that.