What entity of horror would terrify you the most?

It's not so much an entity but a phenomenon like people who are all of a sudden not themselves because something slowly takes them over. A good example of this is Resident Evil, in one of the games there was a ship's captain who was obviously a zombie knocking to get out of a door. He says over and over again in a slightly changed voice, "mayday mayday" and something about a ship going down but his voice isn't right, you can tell by how slow he gets it out and his voice seeming offish. Almost as if that's the last words he spoke before he turned into a zombie. That was creepy and then he eventually bursts through the door where you need to fight him after you open something.

In other cases like the games Little Nightmares 1 and 2 is populated by humans who have been distorted into monsters but they go about their last actions often mimicking something human. Or there's this game called Soma, total spoiler here but I don't think anyone would play it even though they should. However we find out the main character is a copy of the original character even though he thinks the entire time it's really him.

I think it's because it reminds me that we're not unique as a species, of mortality and that identity is never fixed. That something can drastically change our original form into something new. It's kind of terrifying to me. Like Soma, to find out you're not even the original you, like the zombie from resident evil who is cursed to utter his final last words while his brain is actively not there or Little Nightmares, where adults are so distorted into monsters and they still perform their last human acts even though they are def. warped.

Vampires are the most terrifying because they are the most likely monster to actually exist in the world.

I remember scenes I saw about an intelligent shark that loves human meat. That was scary even though I didn't watch the whole movie.

werewolves

Their ability to sprint fast scares me. They also got those long claws and fangs like a wolf. It always annoys me when their victims wait for the werewolf to fully transform before they start running. I get that it's a movie but it still annoys me because I want the victim to survive.

an entity with a voice but no physically evident form.

Demons and Entities did when I was a kid.
Now not much does.

The Grim Reaper..stood at the end of my bed.

Revenants/zombies

A demogorgon

Ghosts

Demon

Shadow people

evil monsters