Ok, I need help again with some research. I've been looking but I can't find what I'm looking for. Granted I'm not the best research but more than just one person looking ups the odds of us finding something.

Ok so for a story my friends and I are working on, we plan to have a scene where this person who is usually immortal (she's the persona of life) gets stabbed. She immediately begins to age as her immortality spell wears off. However, the blade/dagger/sword she gets stabbed with is a very specific one that I've heard of before but can't remember the name of. That's where I need help. This weapon is from some mythology (Greek seems to be the most likely candidate since it's by far the one I've done the most research in) that if a person is even so much as cut by it, they'll die instantly. I need to know what weapon that is or at least the name of it.