Finding a use for the old key in Clair Obscur: Expedition 33 is a bit unusual. Just like the weird pictos, you can acquire this mysterious quest item at the expedition festival before you leave Lumiere, but since you get it before you even arrive on the main game's map, it's impossible to know which door it's used to unlock.
The description doesn't give much away either; it's simply an old key used to unlock an old door. While I'll point you in the right direction below, note first that you won't be able to use the old key until a fair way into act two of the game, so you might want to return here later once you've progressed to that point.
