Poison Fingernails are used by Irina Spasky, the only person in the world known to have/use them but she is now dead.
How they are used
Poison Fingernails are tiny needles underneath the nail. They contain poison and can be extended with a flick of the finger. They are almost impossible to detect.
Poison fingernails have a short history. The invention of the poison ring was becoming obvious and Lucians have a tendency to be sneaky, so what they did, was at the Russian spy agency aka the KGB, some Lucian agents stuck needles in their fingers, put poison in it and made them retractable like cat claws.
How they work
Poison fingernails are lethal because now, a fist bump could kill you. If you were to be stabbed with a poison fingernail, then if you stayed calm, you would only have eight hours to live. The poison destroys everything it touches, but it works slowly. If you freaked out, you would have a lot less time to live and the Lucians would have another victory.