This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Mermaid Swamp
Being in the water is a lot less fun when you are kept in a water tank because of the fixations of a Big, Screwed-Up Family
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.
Even moreso, The Reveal. 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. 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.
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 posseses 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 depending of the action you can push him 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). If you push him away, he will chase Rin with an axe. If Rin decides to dodge the attack, he falls through the floor and ends up with the axe in his own chest. He comes back to his senses and thinks having an axe in him is not a dignified way to go, so he makes Rin pull the axe out and then he goes out to the swamp to drown himself.
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, and Yuuta gets shot to prevent him from telling what happened, leaving poor Yuka to A Fate Worse Than Death.
The fact that the old man Tsuchida, in the Good Ending, it is revealed that part of him felt bad for what his family had done to the "mermaids" and wanted to free them from their suffering, while part of him was 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 the way to return home. If Rin's friends died and she has crossed the Heroic B.S.O.D., well...depending of the ending, he keeps her locked until she reaches Sanity Slippage, shoots Yuuta to death or becomes a bystander, watching as Rin slowly loses herself and drives herself to suicide.