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If you head downstairs from the main staircases right of the Great Hall to the dungeons, you will find a room with a locked door with a sign that shows it is a storeroom. If you use the spell "avomora" on the door, the door will open up for you. Additionally, if you are not able to find the spell, I would suggest checking your second hotbar by right-clicking with your wand, then hit your drop key (normally set to q). If you need any further help, feel free to create a support ticket in-game by using the "/t create" command as well.