The Crucio curse is one of the unforgivable curses you may learn in Hogwarts Legacy by completing the In the Shadow of the Study quest. It is not a traditional spell taught by instructors in class, although you may learn it through outside activities.
The Crucio spell in Hogwarts Legacy includes all the steps in the Shadow of the Study.
The Crucio spell is available in side quests, and you will learn it alongside Sebastian. He argues with Ominis, who flees after this exchange. When you speak with Sebastian, he states that he wants to enter a secret Scriptorium created by Salazar Slytherin. Due to his family history, he wants to avoid anything to do with the Dark Arts.
In Hogwarts Legacy, there are no differences between the houses.
After you hear the exchange, proceed through other story quests and wait for Sebastian to respond to you, and Ominis offers to assist. Eventually, Sebastian informs you that he has made some progress on the Scriptorium and is seeking your assistance in a quest called In the Shadow of the Study. He will lead you to a hallway where Ominis is waiting for you, and you must open the secret door with the Incendio spell.
The next door will not open until you solve the puzzle on the other side. The key to the puzzle will be there, but the way to open it is on the other side, to the left. The snake faces and bites you.
Repeat this step with the door on the right, and there will be a third door with a partially revealed key. You will need to memorize it and repeat it on the final snake pillar.
Once the pillars have been removed, you can proceed forward, where you'll see the words "Crucio" on the ground. When you can, speak with Sebastian, and then tell him that you'll continue to study the Crucio spell, and you can make the choice of having the spell performed on yourself or on Sebastian.
A brief cutscene will start playing out, and you may enter the next room. To proceed forward, find the Slytherin book beside the door, and then speak with Sebastian. The last part of the quest will have you leave the chamber, where you may complete the quest and learn Crucio.