If this is what I think it is, try drinking an invisibility potion, entering the Cursed Citadel, and drinking a potion that works like indecency. The witch in red has all the necessary potions.
We might be talking about different locations: I meant the Cursed Citadel—it's a small castle with living furniture and items. The Dark World also contains the Generals' Castle and the Three Sisters' Castle. So, if in doubt, use hints from everyone: the bartender, the fortune teller, the world map, and achievements.