Hmm, running with your idea...we can actually make this a simliar style to a DM Tribute. Basically, you get an item off doing a few chain quests. You get the item and equip it. You change into a Ninja. Whenever you walk/run by a ninja (NPC will be stealthed), then will check to see if you have the item with you. If you have the item, you're safe, if not, consider yourself dead. Ninjas will be neutral, so you can fight back if you do aggro them.