"That is because it is a monstrous thing, to slay a unicorn," said Firenze. "Only one who has nothing to lose, and everything to gain, would commit such a crime. The blood of a unicorn will keep you alive, even if you are an inch from death, but at a terrible price. You have slain something pure and defenseless to save yourself, and you will have but a half-life, a cursed life, from the moment the blood touches your lips."

Harry Potter and the Philosopher's Stone - Chapter 15 : The Forbidden Forest
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