The Man Upstairs is the main antagonist of an urban legend known as "The Babysitter And The Man Upstairs".
A teenage girl is babysitting one evening while the kid's parents are out. The children have been put to bed and the babysitter is in the living room watching TV. The phone rings, and she hears silence. She, at first dismisses the calls as a practical joke, but she soon becomes scared when later, the caller asks her if she "checked the children". When she asks who he is, he hangs up. Rather than checking on the children, the teenager decides to ignore the call. The stranger calls back several times, each time angrier.
Eventually the girl calls the police, who tell her if she can get the man on the line, they will trace the call. When he calls again, she manages to keep him talking, for a few minutes, she hangs up and the phone rings again but its the police, they tell her that the call is coming from inside the house and to get out immediately as they have already sent some officers over.
As she runs to the door, she sees a man with a bloody axe coming down the stairs who just killed the children.
Some versions have the children and/or babysitter killed by the man, who in some versions gets away or is simply arrested without killing the children. What is summarized above is simply an idealized version of the scenario. One particular type of variation that is quite interesting is that in which the man is in plain sight rather than hiding upstairs but is initially mistaken for a clown statue because he is standing so still. A more lighthearted version has the man actually being one of the children trying to scare the babysitter as part of a prank, which results in the police scolding him/her.