One short creepy story is 'The Tell - Tale Heart' by Edgar Allan Poe. It's about a narrator who is haunted by an old man's vulture - like eye and eventually kills him. He then hears the old man's heart still beating under the floorboards. Another is 'The Hitchhiker'. A man keeps seeing the same hitchhiker on a long drive, no matter how far he goes or how many times he tries to leave the hitchhiker behind. And 'The Monkey's Paw' where a family gets a magical paw that grants wishes but with horrible consequences for each wish they make.
There's a story where a player in Roblox was on a quest to find the key. In the game, the environment got progressively darker and more foreboding as they got closer to the key's location. The player started seeing glitches in the form of distorted faces. When they finally found the key, it opened a chest that released a horde of menacing creatures that chased the player throughout the virtual world.
The key could be found in various places. It could be in a dark corner of a basement, inside a broken clock, or even attached to the collar of a mysterious pet. The possibilities are endless in these kinds of stories.
Perhaps the key is attached to a doll's clothing or hidden in a drawer that's not easily noticed. You might have to inspect every nook and cranny of the dollhouse to discover it.