The tale of the mermaid. A man kidnaps a mermaid because he has fallen in love with her and makes her live in a tank with swamp water. This ends very poorly for the mermaid, as the swamp water damages her health and she becomes ugly, before her body starts to disintegrate.
The Reveal about the mermaid. There is no actual mermaid; the mermaid's legend is based on events involving young women being kidnapped for precisely the same reason by the Tsuchidas, a Big, Screwed-Up Family with a fixation for girls underwater. They kidnap them and force them to live in water tanks, as they love the sight of seeing them underwater. Eventually this destroyed their health and they died, but the Tsuchida men loved them nonetheless, keeping their bloated bodies preserved in large tanks. The tale of the mermaid is an attempt of the Tsuchida women to keep outsiders away from their estate, so they won't end up as victims of the family's fixation.
All the Jump Scares when Rin is travelling across the old mansion, often involving close-ups of bulging eyes and Rin stating she feels she's being watched. Occasionally, a dark figure will flit across the room behind Rin. Perhaps the scariest part is that we never clearly see who or what is there, or if there's even anyone there at all.
Yuka's illness after she arrives into the house. Her whole body starts swelling up horribly. Couple this with some unnerving Scare Chords and you've got the player completely freaked out.
Whatever possesses both Yuuta and Seitaro near the end of the game. Yuuta starts acting perverted concerning the swollen Yuka, and eventually becomes violent; depending on the story path, he either kills himself or chases Rin with a knife, only stopping if you manage to destroy all the Ophelia paintings. Seitaro, meanwhile, tries to hug Rin, and you can either push him away or hug him back. Hugging him leads to a Non-Standard Game Over, in which Seitaro knocks her out and by the time Rin wakes up, he is drowning her to death in the bathtub.
The Twitter comic about Seitaro's point of view just makes this worse. During the time Seitaro was possessed, he's tortured mentally with thoughts about "dominating" Rin before she pulled back some sense into him. Even after Seitaro is back to normal and his friends are saved, he can never forget the utter helplessness, misery, and lust, and then he realizes that this is exactly how the Tsuchida men felt every second of their lives. No wonder he convinces Rin to spare the old man Tsuchida in the Good Ending.
Considering how creepy Yuuta was behaving towards Yuka, who was lying helpless in bed, it's just as well Rin came into the room when she did and confronted him. It's disturbing to think what could've happened if Rin hadn't intervened.
The bad ending "Yaobikuni". Rin is caught and locked up in the same room where the mermaids are, and after several days of increasing starvation, she resorts to eating pieces of their flesh. We're treated to a lovely scene of a delirious and starving Rin with bloodshot eyes, her mouth stained with blood, muttering nonsensically to herself. By the time old Tsuchida finally lets her out, she's gone utterly insane. Convinced his dark secret is safe, he tells her that because she has eaten mermaid flesh, she's become immortal. Rin then runs away through the forest, laughing maniacally, with all her friends dead and her sanity in pieces. Eeek.
The bad ending "Secrets". Seitaro dies, Rin commits suicide to be with him again by drowning herself in the swamp (all the while insisting she can see Seitaro in the water and doesn't want him to be alone), and Yuuta gets shot to prevent him from telling anyone what happened, leaving poor Yuka to A Fate Worse Than Deathnote and it’s heavily implied that she ends up being added to the collection of mermaids by the old man.
The fact that while part of old man Tsuchida felt bad for what his family had done to the "mermaids" and wanted to free them from their suffering, part of him was still attracted to them as well. It Runs in the Family.
Even more so, the fact that he actually wants Rin to save the "mermaids", as he is unable to do so. In the Good Ending, where her friends are still alive and she keeps her cool, he lets her and her friends go and even helps Yuuta find the way to return home. If Rin's friends died and she has undergone a Heroic BSoD, well… depending on the ending, he keeps her locked up until she reaches Sanity Slippage, shoots Yuuta to death, or becomes a bystander, watching as Rin slowly loses herself and drives herself to suicide.
Yuuta's death in "Yaobikuni" and "Forever Deep". As a result of Rin lighting the fireplace, Yuka died. Distraught over her death, Yuuta runs to the kitchen where he stabs himself with a kitchen knife. What makes this terrifying is the fact that Yuuta was possessed by one of the Tsuchida ghosts, meaning that Yuuta didn't commit suicide of his own free will. Even worse is Yuuta regaining consciousness enough to wonder why he's bleeding.
Seitaro's death in "Secrets". If Rin dodges him while he's possessed and chasing her with an axe, he falls through the weakened floorboards. When Rin goes to find him, he's landed on top of a table… and the axe has landed in his stomach. But that's not the worst part, because Seitaro is actually still alive. In a state of shock and agony, he begs Rin to remove the axe, then gets up and manages to walk to the edge of the swamp, bleeding all over the place and repeatedly insisting he can't die like this. He ends up drowning himself in the swamp as Rin watches in horror (in fairness to him, being stabbed in the stomach is nasty way to go). And the horrible cherry on top is that Seitaro wasn't even in control of his actions; if he lives, he tells Rin he was mostly conscious but had no control of his body. He only regains control after he smashes through the floor and gets an axe embedded in his stomach.
When Rin enters the secret room beneath the fireplace, she hears something approaching and realizes she has to get out of there. As she races towards the rope she used to climb down, she accidentally drops her lantern, leaving her in darkness. She cowers against the wall as a deformed, water-soaked creature crawls out of the shadows towards her and promptly faints. One of the creepiest parts is that we never actually see the creature in its entirety and it's gone without a trace when Rin wakes, so it's unclear if there was anything there at all.
