Basement Demon

A basement demon is a common yet vague term for the "Demons" or monsters that hunt you in the basement or a dark area or room after you turned off the lights in that area. Basically, it is a fear of something hunting you in the dark which makes people run for their lives after switching of the lights.
Related: Nyctophobia

"Bro I had a near escape from the basement demon yesterday"

"me too"
"what's a basement demon?"
"Did you spawn in the world as an adult or do you seriously not know what it means"