How to find and use the Dirty Key in Nioh 3

Finding out where to use the Dirty Key in Nioh 3 is surprisingly tough, though perhaps not as tough as getting it in the first place. Early on in the game, you'll come to a ruined village, and inside you'll find a fearsome Yamainu enemy. Though you can run straight past, if you decide to take on this wolf demon you'll get the Dirty Key as your reward.

Unfortunately, you get no indication of where to use this key, and you'll definitely want to, since it gets you a nice chunk of treasure including a Secret Skill Text.