Disturbing.

Old but gold:

But what makes things scary? I say it is the build, not just horrific details. It makes YOU, the viewer, more horrified if you can feel as the protagonists are feeling, putting yourself into their shoes. Even better, if the monster or entity is a creature we can relate with, our mind reels with the implications.

Such as how Plague says it:

Also, a completely HUMAN monster terrifies us, possibly more than a monster from space or a demon straight from another plane of existence. Humanity is considered in a mostly positive/neutral role, and the idea of a calculating, sentient monster, just like you or me in species, scares us hard.

True horror, to end this ramble, is not jump-scares and slithering monsters in ventilation shafts, belching acrid mucus and swishing a long serpentine tail about, although those things are excellent horror-fodder for good reason. As we are both predators and prey, we have a very attuned sense of fight or flight, pair that to our ability to reason and you can get our instincts going helter-skelter trying to keep up in a very good scary story, or make us make a grimace as the monster runs out onto the screen, our instincts preparing our bodies to kill or be killed, even if it is only a movie, YouTube video or some such.

For example, take the SCP Foundation:

That sets the mood for this glorious terror-show. It makes you wonder, it makes you think about it, some articles on their site have either sent me spinning in my head or nearly trembling in fear, and believe me when I tell you, I am a fairly difficult man to scare.

Now, when you read, you can also get a good scare. Stephen King is a master of horror not only because he talks about children cults, the evils of a small American rural town or monsters who prowl after Roland Deschain's Ka-Tet, but because he can do it in a way that seems plausible, and that can needle into your brain and make you recoil. (Not to say the man has had perfect works, From a Buick 8 was sort of a crap-heap, to me, until we got to the monstrous otherworldly stuff. Under The Dome was okay, but I was expecting much more weird, and it only delivered true weird after much too much backstory, but again such is opinion. Also, I like the first three Dark Tower books, but the last three, saving the midquel Through the Keyhole, were kind of a let-down to me.)