The Clock Tower courtyard in Hogwarts Legacy has a hidden unicorn door that must only be opened with a specific spell. Here's all you need to know about it and what's inside once you open the locked corridor.
In Hogwarts Legacy, how do you open the unicorn door?
The Arresto Momentum spell is required for you to open the Clock Tower Courtyard unicorn door. This ability is unlocked when you complete Madam Kogawa's Assignment 2, which involves you chomping balloons near Keenbridge Tower with your broomstick. However, the teacher quest doesn't appear until the middle half of the game, so you may have to wait for a while to solve the Hogwarts Legacy puzzle.
Enter the Clock Tower Courtyard and look up at the swinging pendulum. Use Arresto Momentum to freeze the pendulum in front of the unicorn symbol in this order: Unicorn, Owl, Twin Dragons, and Scarab. The door in the room will automatically be unlocked to reveal a chest and a hidden staircase.
The stairs leading up will take you to three other Clock Tower locked doors, each with its own symbol. Return to the Clock Tower Courtyard and activate the Arresto Momentum spell once more to freeze the pendulum in front of their matching symbols.
On Pro Game Guides, you can read How to Use Petrificus Totalus's Stealth Kill in Hogwarts Legacy.