This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Happy Appy
Two posts into the first story, we have the episode of the titular show that depicts the events of 9/11 two years before the attacks take place. It begins rather innocently with Happy teaching kids about the circle of life in frogs and plants. After a scene of Happy and the kids in the playground, the screen starts filling with smoke. They turn around to see two towers burning up in front of them. Then, it shows a child crushed by a piece of the plane. Happy and the other children just stand still and watch the crushed boy as he's screaming in pain, while other children try to lift the piece off. After this, the children that were with Happy ask him why people are falling out of the towers. Happy turns to the camera with a death smile and coldly says "That's natural, children", drags the children away, and leaves the crushed boy to die.
And the freaky predictions do not stop there. Throughout the show more terrible tragedies are predicted, from the 2011 earthquake and tsunami in Japan, to the Columbia disaster, the list goes on.
As Gerasim continues to watch Happy Appy, the show gets progressively more violent and dark, with Happy himself becoming a deranged Serial Killer whose victims are largely children, killing them in cruel and sadistic ways.
Happy occasionally encourages children to commit violent acts in real life, like arson and murder, all for the sake of "fun".
It's later revealed that later seasons of Happy were filmed using children who were kidnapped by Forenzik and his Followers, with an unknown number having been killed during production.
Early on, Forenzik taunts Gerasim by showing him the dismembered arm of a recent victim and burning his house down, Marble Hornets style.
The Reveal that there are multiple Forenziks, called The Followers, who aid the real Forenzik in his campaign of murder, torture, and filmmaking. Many of them are homicidally insane, and the remainder are revealed to have been forced into joining.
Which gets worse in the sequel, as their numbers appear to be growing. By the end of the pasta dozens have been killed by the police, and there's still plenty more left that managed to get away.
Forenzik showing off several horrifically mutilated bodies to Gerasim at the summer camp.
The snuff film scenes. All of them are grotesque and incredibly graphic scenes of people getting murdered and mutilated/used afterwards.