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, goldish 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

 * As Quirrell had been drinking the unicorn blood on Voldemort's behalf, it is uncertain if he or Voldemort would have been the one afflicted with the "cursed life" mentioned by Firenze, if not both of them. It is also possible this curse may have played a part in Quirrell's death after his abandonment by Voldemort at the end of the school year.
 * It is unknown if or how Voldemort was affected by his use of unicorn blood through Quirrell in the 1991–1992 school year and again in his Rudimentary Body Potion in 1994. It is even possible it may have played some part in leading to his eventual downfall in 1998.
 * It is unknown whether a person who drank unicorn blood without being the one to kill the unicorn, or even knowing what they were drinking, would be cursed, as Firenze stated that the drinker is cursed because they killed an innocent and pure creature.
 * It is unknown if Pettigrew is cursed as he had slayed unicorns to acquire the blood for his master, an act which Firenze described to be the affliction of the curse, as opposed to actually drinking it himself. Regardless, Pettigrew suffered a lack of respect from his peers over the next few years before he finally died unceremoniously by Voldemort's machinations.

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