Filch takes Harry and Hermione to McGonagall's office, where Harry expects the worst. She believes that the two of them told "some cock-and-bull story about a dragon" to Malfoy, "trying to get him out of bed and into trouble." She thinks Neville Longbottom believed the story, too; she found him wandering around the castle as he is trying to warn Harry of Malfoy's trap, but she chastises him for being out and about at night in such dangerous times. She assigns detentions to the students and subtracts 150 points from Gryffindor House-costing the Gryffindors the lead that Harry had won for them in the last Quidditch game. Harry finds himself enormously unpopular the next day. One afternoon, a week before final examinations begin, Harry hears Professor Quirrell whimpering in protest and sees him hurrying out of a classroom, straightening his turban, looking very distraught. Harry assumes that the professor has just had an encounter with Snape, and has revealed the secret to getting past Fluffy and to the Sorcerer's Stone.
On the night when Harry, Hermione, Neville-and Malfoy, who has also received a punishment-are serving their detentions, Filch takes them to Hagrid, who tells them they must go into the Forbidden Forest to help him try and find a badly hurt unicorn who must be euthanized. (Hagrid has previously found a second unicorn already dead.) He warns the students to stick with him or his dog Fang, and to stay to a clear path. Hagrid, Harry, and Hermione go in one direction to look for the unicorn; Fang, Neville, and Malfoy go another. In the forest, Hagrid and his companions meet the centaurs Ronan and Bane. They, however, do not offer any information about any strange goings-on in the Forest. Later, they see red sparks shooting up from another part of the woods-the pre-arranged signal that the other search party is in trouble. Hagrid leaves Harry and Hermione, promising to return quickly. When he does, he is angry: Neville had sent up his sparks in a panic when Malfoy sneaked up on him to frighten him. The team changes groups: Harry now accompanies Fang and Malfoy. They find the dead unicorn, and see a hooded figure crawl toward it and drink the blood from the wound in its side. Malfoy and Fang panic and run, but Harry is transfixed, and feels a terrible pain in the scar on his forehead. By the time he recovers, the hooded figure is gone, and another centaur, Firenze, is present. He lets Harry ride him, and tells Harry that unicorn blood bestows upon the one who drinks it immortality, but at a price: "You have slain something pure and defenseless to save yourself, and you will have but a half-life, a cursed life, from the moment the blood touches your lips." Harry realizes that the hooded figure drinking the unicorn's blood must have been Lord Voldemort, seeking a way to stay alive until he can return to his full strength. Later that night, back in Gryffindor Tower, Harry tells Ron and Hermione that he believes Snape wants the Stone, not for himself, but for Voldemort. As he gets into bed, he finds that his Invisibility Cloak has been returned. A note pinned to it reads, "Just in case."
Harry's encounter with the centaurs of the Forbidden Forest reveals yet another kind of prejudice: although Firenze helps Harry, Bane and Ronan seem appalled that Firenze would let a human ride on his back. Readers can compare this prejudice to the prejudice Malfoy demonstrates toward those who are not pure-bred wizards (those he calls "Mudbloods" in The Prisoner of Azkaban). Furthermore, the centaurs' focus on astrology, and Bane and Ronan's refusal to help Hagrid and Harry because of Mars' ominous appearance leads readers to revisit the question of destiny. Is fate something that is-as in this chapter, literally-"written in the stars," or is it something over which we can exercise influence and control? This theme recurs throughout the Harry Potter series; for instance, in The Prisoner of Azkaban, when Madame Trelawney sees "the Grim" and predicts Harry's death.