This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Digital Devil Saga
In both games, you learn new abilities by eating your enemies for atma points. The second game reveals the real world is in ruins, stuck under a sun that always shines black, and the sunlight turns anyone not infected with the demon virus to stone!
The consequences of the Demon Virus. Endless hunger. Mindless rage. Losingyourself. And that's if you're lucky. When you gain extra atma points if you use Hunt skills to finish your enemies. Rather than killing them and then devouring them, you eat them alive. Once infected, you can never stop eating, or you risk the demon inside just going berserk, overruling whatever higher brain functions you may still have in working order beyond the maddening hunger, and wantonly attacking anything. In rare cases, the demon virus will go so far with you, it will literally overwrite everything in there. And then, there's nothing left in there, but a demon. Problem is, if the virus has gone this far, it's likely it's a strong demon with ideas and intelligence of its own, and it's gonna go find food and shelter - AKA your friends, family and home...
Anahata's Citadel area has a few sections where you must face a variety of busts with Glowing Eyes of Doom. Blue-eyed status are safe, but the red-eyed status will drop you into a basement if you stand in front of them, and yellow-eyed ones will change to blue- or red-eyed once as you approach them. There's one room where there's a red-eyed statue in front of you and nothing else! The basement itself is also pretty creepy - nothing is down there except corpses and a few preset demons, as well as the random encounters. Every now and then, the poison gas in there will disorient you. It gets even worse at the end of a New Game Plus as it's where the Demi-fiend hides out.
Meganada. Where do we begin? The fact that it had to be frozen to be kept under control; the fact that it has extendable bitey-boobs that try to bite Argilla's arm off and nom on the already severely-injured Roland; the fact that you have to face in on The Sun, and it's a That One Boss; or the fact it mindlessly says "INDRA" over and over.
Serph's Disney Death in 2 is essentially the prelude to EVERYTHING going south for the heroes, in particular the moment Sera screams in horrified despair. Remember: she's still hooked up to God, and the Big Guy Upstairs feels everything she does. Also remember that he's committed at this point to killing humanity with Cuvier Syndrome. He hated humanity before: now, he DESPISES us. Tearing control of the HAARP away from Cuvier while the thing isn't even connected to Him, He begins more or less disintegrating the entire planet. No thunder crashes or booming voices are necessary: it is clear beyond a shadow of a doubt how FURIOUS He is.