If you complete 'The Headmistress Speaks,' you'll eventually unlock 'The Polyjuice Plot,' in which your wizard or witch has to disguise as Phineas Nigellus Black. In Hogwarts Legacy, you must complete a level 23 test.
In the final part of the 'The Polyjuice Plot,' you'll need to meet Scrope and learn the Black family motto in order to complete the quest and restore your original appearance. Regardless of being disguised as Phineas Nigellus Black, you'll not have his knowledge and therefore don't know the Black family motto.
If you get stuck in this part, don't worry, because Scrope will not fail the quest if you don't give him a wrong answer. But, here's the correct answer you must give to Scrope in the Hogwarts Legacy quest.
In Hogwarts Legacy, what is the Black family's motto?
Then, in order to go past Scrope, you must choose the option "It's to do with purity of blood." This will prompt Scrope to tell your wizard or witch, disguised as Phineas Nigellus Black, that the Black family motto is Toujours Pur, which is French for "Always Pure." This is because the Black family was really interested in remaining pure blood, which in Harry Potter's universe implies a family lineage without muggles or squibs.
After 'The Polyjuice Plot' is completed, your wizard or witch will be able to go upstairs through the left side of the Great Hall and hide as the Polyjuice Potion wears off. Your character will be back to his original appearance and you'll be rewarded with 260 XP.
There are a number of other useful potion recipes you may obtain as you progress through the Hogwarts Legacy's story.