"[...] 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, a unicorn blood works on undead soul, but wasn't he already protected by his Horcruxes in terms of being kept alive?