When the story mentioned the deep and how it was applied in context to the story, I'm unsure whether you based this on Cthulhu mythos feels. It really does feel like reading one of the pieces in Necronomicon. The deep and the cosmic horror that it involves.
Maybe you came up with a different horror in mind but this one really fits cosmic horror quite well.
I'm very happy you noticed the connection! Hopefully I wasn't so spot on it seemed derivative. I like the concept of a force that is really too ancient and powerful to defeat. It's not just that it is more frightening, but that it can be seen as an representation of the most oppresive of human emotions. Kids may eventually convince themselves that there is no one under the bed, only to graduate to adolescence where they feel nobody likes them and no one ever will. Loneliness, depresssion, paranioa and related feelings are common and sometimes blossom into real diseases. The cures are often limited and the sensations are always there. You cannot get away by just not going into a certain room. You are the one who is haunted. So we don't quite vanquish the monster we cope.
That was deep and surprisingly comforting to not be alone with those thoughts. The cosmic horror genre has recurring themes of the protagonist being alone and knowing there is no one to believe them if they squeal. Cosmic horror is within and out there. Cosmic horror is existential horror.
My thoughts exactly!