Here are some possible reasons why you might be confused:
* You're thinking of a different game. "Fable: The Lost Chapters" is a popular game, but there are many other fantasy games that might have a "demon door" element.
* You are recalling a mod. Mods for "Fable: The Lost Chapters" can add new features, items, and quests. It's possible the "demon door" is part of a mod you've played.
* You're remembering a different element from the game. There are several doors and entrances in Rosa Cottage that have special significance. Perhaps you're thinking of one of these, such as the entrance to the secret room behind the bookshelf or the hidden path to the graveyard.
To help me understand your question better, please tell me:
* What version of "Fable: The Lost Chapters" are you playing? (The original, or a specific mod)
* What are you hoping to achieve by opening a demon door? What are you looking for?
With more information, I can give you a more accurate answer!