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This side quest becomes available after learning the Incendio spell. Travel to Transfiguration Courtyard in the Astronomy Wing then head inside the nearby doors to find Duncan Hobhouse to the left of the stairs. Speak with him to learn about his predicament. He’ll then ask you to go to the Hidden Herbology Corridor in his stead and bring back evidence he can pass off as his own.
Find and Enter the Hidden Herbology Corridor
Travel to the Library Annex then exit the building on the northside heading for the greenhouse. Along the wall north of the greenhouse, you’ll find some stairs leading down that you’ll want to take. Look across from the sundial to find some vines blocking a nearby hole in the wall. Use Incendio to burn them away revealing the door to the Hidden Herbology Corridor. Make your way inside.
Obtain Proof for Duncan
Follow along the corridor using Lumos when you reach the Devil’s Snare to clear the path. Keep an eye out for a doorway on the right side of the hallway leading up to a chest that you can jump too after you light the torches on the wall. As a reminder, light the torches as you come across them to make your life a little easier! There will be another chest at the end of the hallway being protected by Devil’s Snare. After collecting both chests, there will be a doorway at the end of the corridor on the southwest side. Head through it then follow the path to spot the Giant Venomous Tentacula. Approach it and take one of its leaves then exit the corridor to the greenhouses via a nearby ladder on the west side of the room.
Rewards
Return to Duncan to hand over the leaf (or keep it). Either way he’ll reward you with a Venomous Tentacula Robe!
