Unicorn blood

"[...] 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 explains to Harry Potter why the slaying of a unicorn is a crime

Unicorn blood is a thick, silvery substance that runs within a unicorn's veins. The blood of a unicorn can be drunk in order to keep a person alive. However, the act of slaying a unicorn will cause the drinker to suffer a cursed life though the specifics of what this actually entails are not known well. It is probably a Non-Tradeable Item, since the sale of this substance is controlled by the Ministry of Magic, making it strictly forbidden.

1992
In 1992, Lord Voldemort used unicorn blood to sustain his life until he could steal the Philosopher's Stone in order to regain his true body. As he was possessing Quirinus Quirrell at the time, Quirrell drank the blood on Voldemort's behalf, because Quirrell's body was dying as it is from sharing it with Voldemort's fragmented soul. Late in the 1991–1992 school year, when Harry Potter, Hermione Granger, Neville Longbottom, and Draco Malfoy were caught out of bounds, they were punished with detention in the Forbidden Forest. At the edge of the forest, they joined Rubeus Hagrid, the Hogwarts gamekeeper, who told them that a unicorn had been injured and that they were to search for it. After some initial difficulty with Malfoy's behaviour, Harry, Malfoy, and Hagrid's hound Fang found the slain unicorn; however, before they could report their discovery to Hagrid, a hooded figure swept down upon them, causing Malfoy and Fang to flee. Harry was paralysed by fright and the pain in his scar and could not run away. As Harry collapsed to the ground, he was saved by the centaur Firenze.

1994
In 1994, Peter Pettigrew used unicorn blood along with Nagini's venom to create a potion which would create a temporary body for Voldemort, until he could regain his true body. It is unknown if producing such a potion and proceeding to use it (orally or otherwise) creates a similar curse upon the user's life. In Voldemort's case, his re-usage of unicorn blood (having indirectly consumed some two years prior) may have had a cumulative effect on the curse's potency. Voldemort's questionable degree of humanity, however, may have diluted, or even negated entirely, the efficacy of the curse.

Behind the scenes

 * A sign above the Hog's Head forbade the sale of unicorn's blood, and warned customers not to ask about it.

Appearances

 * Harry Potter and the Philosopher's Stone
 * Harry Potter and the Philosopher's Stone (film)
 * Harry Potter and the Philosopher's Stone (video game)
 * Harry Potter and the Goblet of Fire
 * LEGO Harry Potter: Years 1-4
 * Pottermore
 * Harry Potter: The Creature Vault

Notes and references
Sang de licorne Sangue di unicorno