"[...] it is a monstrous thing, to slay a unicorn. 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 defenceless to save yourself, and you will have but a half-life, a cursed life, from the moment the blood touches your lips."
- Firenze, Harry Potter and the Philosopher's Stone
In the first Harry Potter book/movie, we found Voldemort drinking Unicorn blood. Why did he need Unicorn blood to stay alive? Maybe, unicorn blood works on an undead soul, but wasn't he already protected by his Horcruxes in terms of being kept alive?