High Hall Key is a miscellaneous Story Item in Act Three.
A small iron key.
Properties
Miscellaneous Items
Rarity: Story Item
Weight: 0.01 kg / 0.02 lb
Price: 1 gp 
UID
S_END_HighHallBlockingDoorKeyUUID
1ba12cad-591b-4b0e-9ca0-046f1bf5160b
Where to find
Looted from the corpse of Merug 'Curly' Dugald in the Upper City Sewers.