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"[...] 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?

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Related: How did Voldemort drink the regenerative potion without a body?, noting esp. the passage quoted in the Answer. – hardmath Sep 4 '14 at 15:11
up vote 24 down vote accepted

See what I have become?' the face said. 'Mere shadow and vapour... I have form only when I can share another's body... but there have always been those willing to let me into their hearts and minds... Unicorn blood has strengthened me, these past weeks... you saw faithful Quirrell drinking it for me in the Forest... and once I have the Elixir of Life, I will be able to create a body of my own... (Philosopher's Stone)

He needs Unicorn blood to strengthen himself. Without it, as per Quirrel's worries, he was even weaker:

'Master, you are not strong enough!'
'I have strength enough... for this...'

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Voldemort never drank unicorn blood through his face on the back of Quirrell's head. Instead, Harry sees Quirrell leaning over the body drinking the blood himself. During the graveyard scene in Goblet of Fire, Voldemort talks about how he still had the power to possess animals, but the animals all died shortly afterwards.

I sometimes inhabited animals — snakes, of course, being my preference — but I was little better off inside them than as pure spirit, for their bodies were ill adapted to perform magic… and my pos- session of them shortened their lives; none of them lasted long….

Goblet of Fire, chapter 33 (The Death Eaters)

It's likely that by drinking the unicorn blood, Quirrell was preserving his own life.

Keep in mind that contrary to the movie, it wasn't Harry's attacks that killed Quirrell. He died because Voldemort fled his body:

“The servant died when I left his body, and I was left as weak as ever I had been,”

Goblet of Fire, chapter 33 (The Death Eaters)

Firenze was right, he really was only an inch from death, and it was only a combination of unicorn blood and Voldemort's power that kept him alive.

Alternatively, unicorn blood was one of the ingredients in the potion that allowed Voldemort to take his weakened baby-like form before his body was fully restored. So it's possible that by drinking the blood, Quirrell was somehow allowing Voldemort to create a physical face.

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The first option seems very reasonable. – Andrew P. Sep 3 '14 at 15:54
This explains a lot about Quirrell slowly dying in HPMOR... – Domenic Sep 5 '14 at 10:45
This answer is 100% wrong. Voldemort explicitly says that Quirrell drank the Unicorn blood for V, not to strengthen himself. I provided a correct answer. – DVK-in-exile Nov 15 '14 at 18:33

The Horcrux kept Voldemort alive, in a ghost-like state where he wasn't able to do very much. When Quirell found him, he was able to have a body, but he was very much in a half-life, not able to do anything of significance. Eventually, that connection started to break down. The unicorn blood allowed Voldemort to continue to possess a body, in effect keeping him alive. Indeed, without the unicorn blood, he returned to the type of life he had previously, which wasn't really living life at all, as ended up happening eventually anyways.

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He posessed Quirrel's body longer than he was drinking the blood. – DVK-in-exile Nov 15 '14 at 19:39

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