I can enter a lit room, leave it and turn the light off behind me, knowing that there is no one or no thing in the room, and not be able to turn my back out of fear that something in the room will attack me from behind. If I turn my back on the room, something probably magically manifests to attack me. I am pretty sure it actually happens, especially in all basements, even after an in-depth sweep of the room.
I am also terrified of the underside of my bed, again, only in the dark. It is perfectly fine if the light is on. However, once the light goes off, all kinds of things manifest underneath it to attack me. And probably murder me.
Making the list of scary things that might be underneath my bed are tommy-knockers, insane clowns, demonic children who were re-animated in an ancient native american burial ground waiting to cut my Achilles tendon with an old school razor.
I do not know exactly when this fear developed.