There’s a coffee shop in Bowness called “Cadence”. Go in and order a large black eye and specify that it needs to be made with the Prince of Darkness Roast. Your coffee will be served in short order, and will consist of four shots of espresso poured into a cup of the house’s darkest roast. The espresso will be oxidized, so the coffee will be the most bitter thing you’ve ever drank. If you fail to drink the whole cup, you will never be able to get the key.
If you do down the entire drink, you’ll find a small key blank at the bottom of the mug. Take it and leave. The blank will fit any lock in the city as if it were the appropriate key. However, the door will not open into the room it normally does. Instead, the room will be bloodstained and decayed, and a look out the window reveals a desolate apocalyptic landscape.
However, some of these desolate rooms contain secrets and artefacts of the years to come. Be warned though: if the door closes behind you, the key will turn to dust in your hands.