Crucio is one of the three Unforgivable Curses you can learn at Hogwarts Legacy. It is one of the most powerful spells in the game as it deals continuous damage to enemies and curses them, meaning they will lose more HP when hit. And while no teacher will teach you the Cruciatus Curse, your friend Sebastian will. You have to follow his quest line along with the main story and eventually you will be able to start the so-called In the Shadow of the Study quest.
During this quest, you will have the opportunity to learn Crucio from Sebastian. Of course, you can decline his offer, but keep in mind that using this unforgivable curse in the game will not have a negative impact on your character or the story. Instead, it will help you defeat more challenging enemies faster, along with the other spells available in Hogwarts Legacy. With that being said, let's see how to complete In the shadow of the study quest and learn Crucio.
In the shadow of the study there is a relationship quest related to Sebastian Sallow and you will be able to start it after completing the main story mission called The High Keep. In addition, the previous quests related to Sebastian must have been completed as well.
Once these prerequisites are met, you can contact Sebastian outside of the Slytherin common room and talk to him to start the quest. You will then follow your friend and meet Ominis to convince him to join you and gain access to the Slytherin Scriptorium.
To open the Slytherin gate, you need to light three nearby braziers. You can use Confrengo to do this. The first brazier is located in the corridor in front of the entrance to the Scriptorium. The other two are in the small entrance hall on the left, just before the stairs. You have to turn them on quickly; otherwise the door will not open.
Once this is done, go through the entrance. You will soon notice pieces of a broken sculpture on the floor and a locked door. Use Reparo to repair the image, then talk to Ominis. He will speak Parseltongue to open the door.
In this area you will have to go through three doors. However, they won't just open with Alohomora; you must solve three puzzles to progress. Near the first door, on the left, you will see a snake with two quadrants below it. You have to rotate them to unlock the passage, and if you don't complete it within a certain period of time, you will be attacked by the animal. You can try again, but failing will deplete a small part of your HP with each attempt.
At first glance, this puzzle may seem complicated. However, the solution is quite simple: the symbols on the statue must match the ones you see on each door. Look at them, go back to the lock and turn the dials until you get the right combination.
The other two Slytherin locks are further from their respective gateways, so you need to search for them. Fortunately, the area is relatively small, so you should find them just fine.
Once you get past the third door, you'll find yourself in an area with no exit other than a locked door with Crucio written on the floor. You'll find a note from Noctua Gaunt, Ominis' aunt, mentioning that the only way forward requires casting an Unforgivable Curse, Crucio. After talking to Sebastian and Ominis, you will have a chance to learn the Cruciatus Curse from Sebastian.
You can learn this curse and cast it on your friend or let him cast it on you. This will trigger the usual minigame for new spells, and once you complete the quest, the Cruciatus Curse will be added to your spell pool.
You can also choose not to learn it or do it later. You will have the chance to interact with Sebastian in Undercroft and he will be more than happy to teach you about this unforgivable curse.
If you need more help with Hogwarts Legacy, you can check out how to open eye chests, the best talents to unlock in the game, and all Gobstone locations.