This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Devil Survivor 2
Devil Survivor 2 easily trumps Devil Survivor in sheer horror, mostly because as you play the game, more and more of the world keeps getting swallowed into NOTHINGNESS!. This even extends to the map on the intermission screen, where you get the hellish image of more and more of Japan (implied to be the only place that had anything protecting it in the first place) is just GONE, as if it never existed.
This game somehow got past the ratings board with a Teen rating even though it has even MORE onscreen deaths than its predecessor. With the same "chibi"art style as before. One of the first scenes you see in the game is an earthquake that causes a train accident. You get to see all the crushed, bloody corpses scattered around. This includes a full-screen picture (not sprites like the corpses from the train but fully drawn art) of the main character dead and bleeding from a head injury.
Nicaea, the website that shows you the death of any person you type in. And yes, "any person" can include your friends and family. And no, sorry. You don't get to see your own. But look, here, see your friends burn to ashes!
Fail to rescue one of your potential party members, and the predicted death comes true. By "fail" we don't mean "lose the mission in which you rescue the person", we mean "skip out on the battle altogether". YOU could have prevented your friend's death!
The Septentriones easily prove themselves to be malicious monsters. How? Dubhe, the first one, appears at the Hills building and explodes. The curious people surrounding it are reduced to ashes. Fail to rescue Joe or Otome and you're treated to the same spectacle. It's even worse with Otome because you see Bifrons giving her a friggin' Finishing Stomp and then burning her to death.
Devil Survivor 2 deserves some kind of reward for making one of the Neutral endings Nightmare Fuel.This particular ending, the "Kill Polaris" one, involves killing Polaris, obviously. But there are several problems with this. All the damage that was done by the Septentriones? Never fixed. All the world that was consumed by the void? Never restored. All that's left is Tokyo and a endless sea. A sea that is probably neither drinkable nor has anything to eat in it. Humanity will simply die a slow death from malnutrition. And where there is starvation and poverty, there's usually violence to go with it. And while there are no Jerk Ass supernatural beings to screw with humanity, there are no benevolent ones either to help humanity. (Yes, there are good supernatural beings in Shin Megami Tensei). The only hope there is that the next guy in line for Heaven's Throne, Er Rai, will show up and fix things. This, however, is extremely unlikely. Good night, everyone!
Think about that for a second. In universe, the world is being swallowed by an ever growing void. A sudden error screen gives the implication that the whole world has ceased to exist, down to the programming!!
To top it all off, 403 Forbidden is used to indicate that the domain's there, but it's not accepted any host requests. This is usually because the code is being rewritten. It's not just that the world is being dispelled; it's being completely reshaped.
A game over during the Triangulum Arc ends with the more familiar 404 Not Found error message, because the Triangulum aren't here to test the world, they're here to destroy it.
Keita's death in the Devil Survivor 2 anime. Rather than getting pushed down the stairs and breaking his neck like he can die in the game, here he gets pushed down the stairs...and gets graphically, bloodily impaled on metal spikes!
Alcor gets interrupted while pondering by Botis and Bifrons. Botis in particular is annoyed he's starting to like humans and may be likely to side with them. So he summons his blade and attempts to kill him. With a gesture, Alcor crushes Botis into a very, very small metal ball, reminding everyone that while he may look human, he most definitely isn't.
Mizar's positively monstruous growth rate.
The JP's guy you see die at the Tsuutenkaku as Merak fires its Circumpolarity. He remains just for a moment, frozen as a statue, and then he topples over...
The Nightmare Sequence gets worse if the player talks to the various characters and finds out the order of how some of them died. Hinako died first, Taking the Bullet to protect Airi. And it turns into a Senseless Sacrifice, because Airi was too shocked and distraught over Hinako's death, she was the second to die. Then you learn that Jungo tried to get to Airi and save her from Arcturus' next attack, but failed. Jungo saw her die and then he was next.
One of Fumi's Fate scenes involves her experimenting on Jungo, who experiences a Flowers for Algernon moment through whatever Fumi does to the Summoning App and Jungo by extension. Jungo suddenly becomes aware, alert and erudite... Upon which Fumi tells him she needs readings from his normal state, grabs his phone and resets the app. All whilst Jungo begs her to stay her hand before narrating the return to his usual state of mind as sinking beneath a cloudy sea. This is supposed to be funny, right?