Every one of those damn reels. Which are every single doing of Bughuul.
Let's see, families including young children being lifted slowly off the ground by their necks while they twitch and spasm until they're all still. This scene (and the projector that played it) came from the nightmares of writer C. Robert Cargill after he first watched the American version of The Ring.
And last but not least, the damned lawn mower. Not even the movie thought it was a good idea to show that particular bloodbath.
Additionally, there's also that last second Jump Scare, just because.
And we can only imagine what's on the newest tape... oh wait, Ashley was kind enough to leave some illustrations
Bughuul's face's appearance on the surface of the waters of the pool.
On the laptop's screen, his image moved.
The way he does it, just casually looking at Ellison before turning back.
Bughuul appearing in the backyard. You saw it in the trailers, you knew it was coming, but it's nonetheless one of the most terrifying parts in the movie.
The ghosts themselves are deemed as this.
The scene where each one of the ghosts follow him around the house, sometimes close enough to touch, yet he can't see them. They cause creaks and other noises, and it seems like he catches small glimpses of them when they turn a corner, but the simple fact that in his eyes, there's nothing there, puts a whole new level of scare into Nothing Is Scarier.
The drawings on the wall, particularly the one of Stephanie.
The music that plays when Ellison burns the reels and projector and during the credits. Listen closely, you'll hear what sounds like the distorted murmur of children in the background, and you can't quite separate it from the rest of the movie.
The implication that the horror may easily continue, because if Deputy So-and-So starts investigating the killings following Ellison's death, he'll start searching for the case and once again plastering Bughuul's face all over the wall.
Or the professor Ellison was talking with online will, when the gruesome murder of a famous author's family hits the news and reporters start tracking down whether the research he'd been doing on his next book was connected to it.
Deputy So-and-So already knows what happened and has proven to be quite Genre Savvy. The rest of the authorities, on the other hand...
The overall sense of unease in the film, because there is a lot of movie shots were Ellison is alone in the night, with half the screen showing a darkened corridor. The audience keeps expecting something to suddenly crawl out of the shadows, and turning on a light or waking up his wife wouldn't have helped Ellison at all.