"Siren" is quite possibly the creepiest game ever, where out of 10 distinct characters you play as, only the little girl properly survives.
Sightjacking in general. Looking through the eyes of a Shibito gives you not only the ability to see them coming right towards you, but also allows you to hear them talking in their own language...
Even worse: while you're sightjacking, you cannot move. The one thing you do not want to see when you're looking through their eyes is yourself. Worse is that there's no real way to avoid doing so, either. When you're discovered you see a flash of their viewpoint with you on it. When you die, you get to watch your character's last moments from the perspective of the one that killed them.
The Shibito themselves. In their first form, they look like deranged people, with blood seeping from their eyes. Then they mutate. The first appearance of the dog shibito, spider shibito, winged shibito... Oh, and don't forget the evolved forms of the Onda Twins...
One advert for the game had to be pulled off Japanese TV for being too frightening. The scene in question being Tomoko who having turned into a Shibito ends up screaming for her parents with tears of blood on her face while banging on the window of the church.
The music itself is frightening at times, especially when you can hear voices in the music but can't quite make out what they're saying.
Only a few characters have access to guns. If you cannot sneak past, or choose to go the action route, you'll need to beat Shibito to death with an umbrella, or a crowbar - that is, if you even have access to a weapon...
Datatsushi. Not so much in the battle against it, but in the cutscene where you first see it alive, it's terrifying. Oh, and in that battle? It's invisible. You have to sightjack it to figure out where it is from the direction you're facing in its view.
Worse is that, as mentioned, this is the last thing you see in a full playthrough. You've gone through every stage, at least twice, and think you've defeated the siren... and then the last thing you hear is it's first appearance.
The location deteriorating and change as time goes on. Infamous mentions are the school gymnasium suddenly transforming from an barren and dark hall into a spider's nest specifically built for the Spider Shibito having made the gymnasium their home.
Whenever Harumi Yomoda, a ten year old girl, is caught by a Shibito. The screen suddenly pans to a terrified child cowering in fear as the Shibito ominously watches her, especially when the screen darkens until the Game Over screen is displayed. The fact that you don't get to see what happens to the poor girl once she is caught makes it all the more terrifying.
And finally, there is a glitch in Harumi's first level whenever she is caught by the Mother Shibito. While every playable and companion characters can be killed by being stabbed to death by a Shibito, all Shibito will never attack Harumi Yomoda...save for the Mother Shibito, who may suddenly swing her knife at a 10 year old girl without warning and with the appropriate sound effects. Though there is no blood and Harumi will still cower in terror, the sight of an adult attacking a child with a knife is enough to send a chill down your spine.
Shibito Naoko is just generally unsettling. First is the factor that she behaves very unusually for a Shibito by not attacking Tomoko, but rather following her around and constantly begging to be looked at, occasionally interrupted by her cackling madly; a behavior that very effectively plays on Nothing Is Scarier, as you cannot help but feel a creeping fear of she suddenly turning aggressive and start to attack. Then are her facial expressions and the way she shifts between them, combined with the blood on her face.
There's the boss "Otoshigo." It's mercifully difficult to find a still picture, but there's a video of it here. Be warned that it includes spoilers for the final battle against it.
The scene where Mother is first released and the following scenes of the Yamirei arriving. Seeing those terrifying white wriggling things is enough to frighten a lot of people even without Shu Mikami having been swallowed by tentacles during the same scene. On top of this, their defense against the light involves taking over the bodies of Shibito, presumably killing the Shiryo already possessing them.
The smiles on the faces of the Yamibito.
How about the Yamibito that have giant faces?
The fate of the real Ichiko Yagura, who had drowned years ago in the past after having fell overboard from a ferry she was on. Her corpse was found by an ancient being named Otoshigo, who then created a replica of the drowned girl known as a 'Simulacrum'. The Simulacrum Ichiko then wakes up in the present time, believing itself to be the real Ichiko that had been brought back on the ferry. As time passes on, Otoshigo gradually takes over Simulacrum Ichiko's mind, and eventually goes on a rampage, killing anyone who stands in its way in order to be reunited with Mother again. After having failed to meet Mother once more and having fell off the pylon tower, Simulacrum Ichiko eventually dies from its wounds, thus angering Otoshigo and prompting the ancient being to resurface and lay waste on whoever stands in its way.
In some of the archives, you can find a magazine about paranormal activities, dedicated to a strange creature living in the island no one has ever seen, and a video showing evidence of the existance of that creature (like Big Foot, or Nessie, to put it in context). Now, in a late mission in the game, you can Sightjack with the Shibitos... and something else. And that something is real, and is out there, lurking in the shadows.
The 'Making of' featurette on the disc reveals that all of the freaky movements you see from the Shibito were modeled from professional dancers. Their movements are creepy.
The game-exclusive Maggot Shibito. It's big, it's fast, and it can One-Hit Kill you. It's body is large and thin, looking like a maggot itself, with elongated limbs and legs. The face is horrifically bloated, too.
The redesigns for the Shibito Brains. They have more variety this time, including one that has four tentacles that allow it to hang, with the rest of the body limply lying there mid-air.
Bella's last level: after exiting a house filled with a family of shibito, you come across the path that allows you to leave the area... then Melissa Gale calls out for her daughter, and you think it's all good, right? Then Melissa comes forward to reveal herself bloodied, with a distorted voice, happy, and says that "I came to get you!" You then have to run around the place, desperately avoiding her grasp.
Melissa traps Bella in the yard by shutting the entrance door and winding the lock tight, meaning that Bella has to get it open again by unwinding the lock while Melissa is searching for her a few yards away. Seems easy enough...until the moment you start unwinding the lock, Melissa can hear Bella trying to escape.
Melissa: "Stop fiddling with that, honey!"
In Episode 6, Howard has to defeat Shibito Seigo Saiga. There's actually detail to Seigo's head, given how he committed suicide by shooting himself in the head with a shotgun; there's a great big hole in the back of his head.
Sam's last level in Episode 12. Melissa bursts through as a Maggot Shibito, demanding her daughter. For the most part, you have to run the hell away from her before reaching several safe spots that allow you to electrocute her through chucking antennae, making sure that you don't fall off while she's circling the place like a predator waiting for its prey. And after that, you have to shoot about a dozen Spider Shibito with a shotgun, taking into account the constant reloading and the fast recovery rate of the Spiders.
In Episode 7, as Seigo, you need to help Bella escape, you're sniping at first and feel very badass. Then you go down and need to find an item but, as explained before, it is vague to find exactly what you want, so if you open a certain closet you find a freakinggigantic facein it screaming at you. It is a Jump Scare, but a very good one.
The advertising. One of the trailers for the first game featured insanely laughing shibito, as well as some gruesome environments and Gross-Up Close-Up used to full effect.
Then a couple of years later, there was the print ad for Siren 2. A close up of a woman's face. With the eyelids sealed shut. That brought back uncomfortable memories of the trailer, as well as being terrifying.