- "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 informing Harry Potter about the nature of unicorn blood[src]
The blood of a unicorn could be drunk in order to keep a person alive. However, the act of slaying a unicorn would cause the drinker to suffer a cursed life, though the specifics of what this actually entailed were not known well. It was probably non-tradeable, since the sale of this substance was controlled by the British Ministry of Magic, making it strictly forbidden. Firenze and Harry Potter both believed that only those who had nothing to lose and everything to gain would dare drink unicorn blood.
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 and inhabiting his body at the time, Quirrell drank the blood on Voldemort's behalf. This was 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. Prior to splitting up the group, Hagrid pointed out the unicorn blood on the ground.
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.
- Harry Potter and the Philosopher's Stone (First appearance)
- Harry Potter and the Philosopher's Stone (film)
- Harry Potter and the Philosopher's Stone (video game)
- Harry Potter and the Goblet of Fire (Mentioned only)
- LEGO Harry Potter: Years 1-4
- Wizarding World (Mentioned only)
- Harry Potter: The Creature Vault (Mentioned only)