In an empty lot on the outskirts of Chicago appears a mysterious door. The empty lot is now worthless property, once owned by some company which went under and now exists in some kind of legal no-mans-land where it cannot be developed. The door is a rustic wooden door, old wood by the looks of it. It is sealed tight within a free standing door frame of similar vintage, the frame is firmly planted in the ground by unknown means and will not fall over or move.
Its only features are 4 inset rectangles with raised centers, like any number of old doors and a oxidized brass door knob. Chips of red paint are visible suggesting it was once painted a dark red color. Now the surface is the grey-brown of old wood. The door knob is only on one side with no matching mechanism on the other side of the door.
Most people ignore the door, barely even noticing it is there. In fact even those who one would think would notice it will just acknowledge it’s presence should it be pointed out but have no interest in it. Even with this a small number of people are drawn to the door, trying the knob and it is clearly locked tight.
When a person tries to open the door, inevitably some part of the door pricks them. Either they brush against the wood and get a splinter or some part of the knob or its fixture is sharp and cuts them. Within 24 hours the door will open and a creature will emerge, it will chase down the one who tried to open it and either take that person through the door never to be heard from again or kill everyone the person loves, no matter how far or well protected they are.
Act 1 - Discovery
Act 2 - Choice
Act 3 - Consequences
I want this to take a format of a fable, the kind that tell children to do as their parents tell them / not go into the woods. Basic idea of being overly curious can get you or your family killed.