Is this a good idea for a spell in my book?

it can only be used on a crescent moon the shape of your middle fingernail, you speak "petals of ice, roots of blood" and a little ice shard appears on the person you want it to, it goes into their body, and when it finds a vein, it sprouts a flower of ice that starts freezing your veins and goes to your heart, then travels up to your brain and kills you, is that a good idea???