History NightmareFuel / Undertale

** The Clamgirl NPC who sometimes appears based on your Fun Value is a mild version of this when she talks about [[BecauseDestinySaysSo fate]], but in the UsefulNotes/NintendoSwitch version, she is implied to be a Gaster Follower. Depending on your Fun Value, her usual dialogue in the Pacifist Ending about not worrying about not finding "Suzy" can be replaced with this:

*** Not to mention it's telling that most people who saw this agree that the image of Flowey's rotting corpse with an eyeless SlasherSmile frozen on his face forever is somehow a lot more horrifying that meeting him in the flesh.

* When [[http://www.twitch.tv/razorsharpfang/ Twitch Plays Undertale]] finally beat the game, the stream locked up and froze at [[https://i.gyazo.com/4a2629b4233d4796d5b36eddddd27ea5.png this screen]].

* ''Undertale'' takes a very basic game mechanic, the ability to save/reload your progress, and makes it the focal point of what sounds like a CosmicHorrorStory. With the ability to restart from any point, it seems like your actions have no consequences. You can do anything and get away with it, whether it's helping everyone overcome their problems or killing them all over and over again. People eventually start to become predictable after multiple rewinds, where you learn every single one of their responses to everything you do for them. Then, when you've exhausted every single thing you can do with your power, it feels like there's nothing left for you. Or maybe you don't have this power, but you're at least aware that the world keeps resetting around you for whatever reason. You make a new friend with someone one moment, then see everyone around you die horrible deaths because of them in another. Eventually, knowing that your life and the lives of everyone you care about are in someone else's hands, you just give up trying anymore and find yourself putting on a fake smile just to get through the day. This is the lot Flowey and Sans are in after god knows how many resets. Keep all this in mind the next time you're playing a game like ''{{VideoGame/Fallout}}'' or ''Franchise/MassEffect'' and you decide to save just before doing something purely out of curiosity.

** Speaking of this… Flowey claims to have exhausted every possible option he could think of when it came to killing and sparing before the player showed up. This means that while he's done his own equivalent of a Pacifist run, he's also done pretty much every possible Neutral equivalent you could think of, including some ''very'' dusty ones, and has killed everybody in the Underground multiple times. ''[[SelfMadeOrphan Including his own parents]]''. Even if we take the comment that Sans caused him "his fair share of resets" to mean that he never properly finished whatever his version of a Genocide Route would be, it still means that Flowey at least attempted it a few times and [[NearVillainVictory would've gotten away with it had Sans not stepped in]]. Both heartbreaking and terrifying, indeed (it also counts as a bit of a CrowningMomentOfAwesome for Sans since we know how determined he is to save the world when he feels that no one will survive, especially when his brother is in the crossfire).

** Speaking of this… Flowey claims to have exhausted every possible option he could think of when it came to killing and sparing before the player showed up. This means that while he's done his own equivalent of a Pacifist run, he's also done pretty much every possible Neutral equivalent you could think of, including some ''very'' dusty ones, and has killed everybody in the Underground multiple times. ''[[SelfMadeOrphan Including his own parents]]''. Even if we take the comment that Sans caused him "his fair share of resets" to mean that he never properly finished whatever his version of a Genocide Route would be, it still means that Flowey at least attempted it a few times and [[NearVillainVictory would've gotten away with it had Sans not stepped in]]. Both heartbreaking and terrifying, indeed (it also counts as a bit of a CMOA for Sans since we know how determined he is to save the world when he feels that no one will survive, especially when his brother is in the crossfire).

* Your first time wiping out all the random encounters in one location can be pretty unnerving. You'll still get another encounter, only this time it's completely empty, with nothing but the message "But nobody came." Cue the creepy music. To really hammer in the point, these empty encounters trigger much more often than regular encounters anywhere else once you trigger the first one, although it might not be completely intentional.

** The music that begins to play during that screen doesn't appear to be in the game's files at all. In fact, it's actually the intro to the battle theme, 'Anticipation', slowed down by 400%. Still extremely creepy, though.
* If you do kill Toriel in the demo, the song [[http://undertale.wikia.com/wiki/Empty_House "Empty House"]] plays. It's… unnerving as hell if you go back into Toriel's home to see it completely desolate. It was even included in the official game. If you do turn off the lamp in the guest room, it makes it more disturbing.
-->No one will use this anymore...

** [[https://www.youtube.com/watch?v=IXlDDlEJP8c Bonetrousle]] slowed down 310% sounds more akin to something that should be in a ''Franchise/SilentHill'' game.
** [[https://www.youtube.com/watch?v=qgBc5vwIJm0 Waterfall]] becomes a menagerie of funeral bell sounds slowed down [[NumberOfTheBeast 666]]%.
** If you thought the bleak soundtrack played when facing down Monster Kid sounded familiar, that's because it's [[https://www.youtube.com/watch?v=uGqvpMQQ48o "Anticipation"]] slowed down by 200%, sometimes called "In My Way". It sounded so different that fans were unaware that it was a pre-existing track at first. The song [[https://www.youtube.com/watch?v=A_tN5YPMQpc&app=desktop kicks in again]] ''further slowed down'' (to 250%) at the moment when Flowey [[OhCrap begins to shake in fear after realizing he's in your way]] in New Home (and continuing until you either go back enough or are confronted by Sans) and during the post-Genocide True Pacifist ending.

*** That's not the scariest part. The scariest part is [[https://i.imgur.com/1eG93L2.png what you'll find when you produce a spectrograph of a certain part of the "music."]] [[note]]The image on the right is taken from [=KyuuKyuu=]-kun, an NPC in Yume Nikki.[[/note]]

* Since you won't be befriending any of the major character on this route as you will be killing all of them, only Flowey will be waiting for you when you boot the game to the save file screen throughout the whole playthrough.

** While most of the Amalgamates are a combination of previous enemies, with Act command options taken from each of their fights, [[https://i.imgur.com/jcH7tRP.gif the first one you find, "Memoryhead", is a trio of enemies]] that don't resemble anything (except maybe Giygas/Gyiyg/Geigue from ''VideoGame/{{EarthBound}}'') and the ACT commands are nonsensically taken from the non-battle menu. Their graphics glitch out during the fight, they speak in glitches and modem sounds, and their attack is made of creepy grinning faces. When you use the CALL option, you hear voices from the phone saying "Come join the fun". Some fans believe these aren't amalgamations of normal monsters like the others, but are actually fragments of Dr. Gaster.

* Once you're firmly on this route, there's only one character waiting for you when you boot the game to the save file screen: Flowey.
* Once you're on the No Mercy path, the exclamation mark that pops over the protagonist when encountering monsters changes to a Happy-faced Emoticon ":)". That's likely the Fallen Child cracking a grin under Frisk's stoic appearance. The Fallen looks the same as them, but is always wearing a big "innocent" smile on their adorable blushing face.

--->monsters' physical forms can't handle "determination" like humans' can. with too much determination, [[BodyHorror our bodies begin to break down]]. [[EldritchAbomination everyone's melted together...]]

** To elaborate; both normally heartwarming endings to the True Pacifist run become extremely dark after a Genocide run. If you choose to stay with Toriel, then after she leaves Frisk pie in their bedroom, when she leaves, they suddenly jolt awake with the Fallen's frighteningly happy face and glowing red eyes before the screen cuts to black and they let out a laugh that despite being a lower pitched version of Omega Flowey's, is more terrifying than it thanks to the SurpriseCreepy factor of the scene. If you said you had 'places to go', the group photo at the end has the Fallen in Frisk's place, with every other lovable character's face crossed out in red. And of course, both ending screens have a blood-red 'The End' and the aforementioned slowed down version of 'Anticipation' playing in place of the usual music box melody.

(''[[SurpriseCreepy transforms into a]] [[JumpScare Gaster follower-esque sprite]]'')\\

!As per wiki policy, and also due to the [[SpoileredRotten sheer multitude of spoilers]], EVERY SINGLE SPOILER on this page will be unmarked. [[Administrivia/YouHaveBeenWarned Proceed with caution!]]

'''SPOILERS ARE UNMARKED, READ WITH CAUTION!'''\\

** When you damage the boss, it reacts with an inappropriately comedic "Oh!~", but once you damage it enough, this sound is replaced with groaning, and eventually becomes a pained, inhuman screech.

* The Fallen Child during the path's final scene is quite haunting, especially if you try to resist. Once they appear, they thank you for getting them there, then tell you they're going to destroy the world. You can either agree with them — in which case, they'll thank you for being a great partner before doing the deed — or try to stop them. [[SchmuckBait Do the latter]], and the Fallen Child will rhetorically ask '''''"SINCE WHEN WERE YOU THE ONE IN CONTROL?"'''''[[note]]or, in the Japanese version, flat-out '''declaring''' that you don't give them orders[[/note]], which in a way eerily answers itself — you haven't actually ''done'' anything since entering Asgore's throne room, as killing Asgore and Flowey was done without any input (besides advancing when you're asked not to kill Flowey[[note]]Asriel[[/note]]). At this point, their eyes widen, and then a jumpscare occurs where the background starts flashing black and red as they run towards you with their face melting, letting out a horrifying laugh all the while. A slash-attack graphic appears, the screen fills with 9's, and then the game finally shuts down.

* The Fallen Child during the path's final scene is quite haunting, especially if you try to resist. Once they appear, they thank you for getting them there, then tell you they're going to destroy the world. You can either agree with them — in which case, they'll thank you for being a great partner before doing the deed — or try to stop them. [[SchmuckBait Do the latter]], and the Fallen Child will ask if you thought ''you'' were in control[[note]]or, in the Japanese version, flat-out '''declaring''' that you don't give them orders[[/note]], which in a way is eerily true — you haven't actually ''done'' anything since entering Asgore's throne room, as killing Asgore and Flowey was done without any input (besides advancing when you're asked not to kill Flowey[[note]]Asriel[[/note]]). At this point, their eyes widen, and then a jumpscare occurs where the background starts flashing black and red as they run towards you with their face melting, letting out a horrifying laugh all the while. A slash-attack graphic appears, the screen fills with 9's, and then the game finally shuts down.

** To elaborate; both normally heartwarming endings to the True Pacifist run become extremely dark after a Genocide run. If you choose to stay with Toriel, then after she leaves Frisk pie in their bedroom, when she leaves, they suddenly jolt awake with the Fallen's frighteningly happy face and glowing red eyes before the screen cuts to black and they let out a laugh that puts Omega Flowey's to shame, and if you said you had 'places to go', the group photo at the end has the Fallen in Frisk's place, with every other lovable character's face crossed out in red. And of course, both ending screens have a blood-red 'The End' and the aforementioned slowed down version of 'Anticipation' playing in place of the usual music box melody.

** Once you return to the game, there will still be nothing left. After a while, the Fallen Child offers to bring everything back in exchange for your SOUL. The only thing you can do other than deleting your save files is to accept, which permanently taints your next playthroughs. What was once a heartwarming Pacifist ending turns into the worst possible outcome, as either the Fallen is shown possessing Frisk (accompanied with red eyes and a more terrifying version of Flowey's laugh) after Toriel brings them pie while they’re sleeping or gives you a picture of themself alongside the other characters... who have their faces crossed off.

* The conversation with Sans at Grillby's seems like a funny event at first, when he starts talking about Papyrus' quirks to enter in the Royal Guard, then time suddenly freezes around him and your character when he asks them if they heard of... a talking flower. Sans then starts talking about Echo Flowers, before mentioning one that gives his brother advices and premonitions. If you know Sans, you can only wonder if he legitimateely asks them the question or if he already knows and tries to see what they know or not. [[https://tobyfox.bandcamp.com/track/premonition The theme playing]] is frightening too, being a simple, slow piano loop of Flowey's theme.

** [[https://www.youtube.com/watch?v=qgBc5vwIJm0 Waterfall]] becomes a menagerie of funereal bell sounds slowed down [[NumberOfTheBeast 666]]%.

* During the cutscenes with Sans and Papyrus in Snowdin, you'll notice that the child moves ''without player input''; in other routes there are very, very few times when they do this (such as when Toriel escorts them by hand through the spike maze in the Ruins). This continues on in many further cutscenes along this route. One way to interpret this is that your murderous ways have awakened Chara and they're now taking more and more control of Frisk.