Quests: 3 · Act: Act III · The Final Repository
Unlocks: Act III, the Headmaster's office
1. It's All Gobbledegook
Act III opens with the goblin plot moving from rumour to something the school can no longer ignore. Professor Matilda Weasley becomes central from here.
The middle quest of the three, "The Headmistress Speaks", is easy to blink past — it's over almost as soon as it starts. What it does matter for is where it sends you next: to the Map Chamber, to meet Professor Niamh Fitzgerald's portrait, which is what sets up the plan that becomes "The Polyjuice Plot".
2. The Polyjuice Plot
The Polyjuice Plot is the act's standout: an infiltration wearing someone else's face, built around timing and staying in character rather than combat.
It is a change of pace from the trials, and one of the few quests where fighting your way out is the wrong answer.
Professor Fig is who actually supplies the means: found up in the Astronomy Wing, he hands over the Polyjuice Potion needed to pass as Headmaster Black himself. The house-elf Scrope is the other half of the plan, letting slip the password to the Headmaster's office once he's fooled — which is the whole reason the disguise has to hold.
Take the conversations slowly. The quest is more forgiving than it looks, but rushing dialogue is the quickest way to blow your cover.