This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Xenogears
Though a Teen rated game, it seemed to warrant a Mature ratingnote It actually did get the Japanese equivalent, a D rating; it gave plenty of warning to people who caught the reference to the movie Soylent Green (which would likely be lost on the T rating demographic), nevertheless, one scene in particular occurred in a green factory with a conveyor belt that had dead mutated people dropped on it that then went through giant grinders resembling sink garbage disposals.
The first scene where we see the mysterious red mech in action , which will later be revealed as being as Weltall-Id, to say that the sheer power and Hopeless Boss Fight at said time is terrifying is an understatement. Specifically, the people mutating into the grotesque Wels and becoming parts for Deus. And it's no small number of people either; everyone who doesn't have Plot Armor (mostNPCs) mutate. It's also implied they don't get better in the game's ending, and are killed along with Deus.
Anything to do with the Wels, especially on the second disc, where people start turning into them while "Omen" plays in the background.
When Elly and Fei realize what they ate in Solaris...
Miang can reincarnate in any woman the moment she dies. As a theoretical, say you're in a town with only two women. She's one of them. You're the other. She gets stabbed and dies. Guess what's going to happen to you?