A kakute ring is a small, last ditch weapon favored by the ninja. It consists of a metal ring with one or more spikes, often coated in poison for greater effect. It is usually worn in the middle finger, with the spikes facing into the palm or outwards, to aid in punching.
Mine is made from a piece of aluminum crutch tubeing with an inner diameter of 7/8". I cut it to size and drilled a small hole for a shortened nail. I wrapped the ring with gutted paracord, to act as padding, secure the nail, and to make it fit my finger. =D At first, I doubted whether the paracord would hold the nail in place during a strike, but when I punched a tin can full force, I felt no movement from the nail.
It's unlikely this will kill anyone, but it sure as heck will hurt. A light blow to the head would most likely cause extreme bleeding, blinding an attacker long enough to get out your primary weapon, be it gun, knife, or whatever. The big advantages to the kakute ring are, (a) It's always drawn, so long as it's on you finger. As the saying goes, "The fastest draw is to have the gun in your hand when the trouble starts", and, (b) It's unexpected. Almost no one expects to start bleeding heavily after a fairly light punch. In low light, the ring might go unnoticed. Humans have a natural fear of the dark, and if it seems that you're bleeding badly just because the guy you're trying to mug hit you, you might want to run. I know I would. However, don't be stupid enough to assume that this alone will defuse the situation. If it does, great. If not, you have bought a few seconds to get your main weapon.
Disclaimer: I am not responsible for anything you may do. I do not claim to be a self defense professional, nor do I claim to have any self defense experience. My writing is intended to inform and educate. Only use this if you fear for you life.