"A key with a gold tag that reads “Attic Window” on it."
— Inventory description
Attic Window Key (天窓のカギ tenmado no kagi ) is an item in Resident Evil 7 Teaser Demo: Beginning Hour.
The Attic Window Key is used to unlock the window in the attic which activates the True Ending. Alternatively by just having it in your inventory while infected, and then climbing the ladder to attempt to use the key will activate the Infected Ending.
The Attic Window Key is located in the basement hanging from the arm of a doll on a shelf.