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Nightmare Fuel / Undertale

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It's hard to decide what's worse: the face he's making or the fact that he's not bluffing.
"Welcome to my special hell."
Lemon Bread

Despite being billed as a cute and quirky RPG, Undertale is incredibly scary at points, featuring some stuff that will downright traumatize a child… or an adult, for that matter.

WARNING: If you read these examples at night, you're gonna have a bad time.

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    Demo specific 
  • The manual changes throughout the game, depending on your choices. Some of the changes are... interesting, to say the least.
  • When getting the No Mercy ending, there is a small bonus on the ending screen.
    That was fun. Let's finish the job.
    • After a No Mercy ending, this same image also replaces every page of the manual except for the first and last.
    • 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 "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...

  • Flowey's funny faces turn out to be quite a bit more. Especially since he's easily able to mimic your friends.
  • Flowey's laugh is.… not a pleasant or comforting sound. It gets far worse when you fight him at the end of a neutral run.
  • If you backtrack a bit after certain points, you can catch Flowey quickly retreating underground. He's been stalking you the whole game. And following you all the way up to the end of the game. On a run where you spare Toriel, find him in the last Waterfall Echo Flower field then talk to the Echo Flower he was just next to. You hear what sounds like Toriel looking for you… but in reality, it's Flowey imitating her voice, then taunting you about how she'll just forget about you the moment she finds another human child. Finding out who Flowey really is adds another layer to this. It's Asriel projecting his own fears and bitterness at the loss of his parents to mock you. If you decide to kill Toriel instead, then Flowey will instead imitate Toriel sounding horrified, wondering where she is, followed by him saying "... But nobody came."
  • Flowey's backstory, and the sequence of events that resulted in such a deranged being. They make you think hard about why you want to defeat innocent creatures who didn't do anything to you. Worst of all? Flowey/Asriel was a child of about eight years old, or at least the Monster equivalent. It's as heartbreaking as it is terrifying.
    • 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. 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 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).
    • The simple fact that Flowey is Asriel adds more eeriness to his character. As we're shown in the Pacifist Route of the game, Asriel is nothing more but a sweet, well spoken, if very shy monster who was dragged into a situation beyond his control by the Fallen. Learning that he went from that to a violent monster all in a desperate attempt to reunite with the Fallen/Chara makes it more horrifying and saddening, especially when you remember that, as stated above, Asriel is eight years old. All of that trauma arguably took a toll on him, making him see the Underground as simple pieces that became repetitive, and involving the simple mantra he stuck by until Frisk showed up: Kill or be killed.
  • You know that low droning noise that plays after you've routed an area in a No Mercy playthrough? It's a slowed down Flowey's theme called "But Nobody Came", alternatively "Too Much" according to the internal filename.
  • His Establishing Character Moment, one of the first things that happens in the game? He tricks you into taking hits from his bullets, then laughs at you for being naive (complete with one of his patented nightmare faces and distorted font), then he tries to kill you, even yelling "Die." And that's only if you get hit by them — you can avoid them and annoy Flowey. While amusing at first, the music gets increasingly distorted and low-pitched and Flowey's anger becomes more apparent, with him even saying "bullets" before looking at his own text box and changing it back to "friendliness pellets". If you avoid them all, either because the situation seems creepy or you started the game over, he says, "You know what's going on, don't you? You just wanted to see me suffer", before shrinking the box your soul is in and attacking you.
  • If you do a Neutral run in which you kill him at the end and then reset, Flowey is completely absent where he would normally show up first. Not seeing him is even more creepy because it can make you wonder if he's decided to stalk you from the shadows from the start this time around.
  • Sparing Toriel when you have killed other monsters in the Ruins leads to some… interesting dialogue from our old pal Flowey.
    Flowey: Clever. Verrrryyy clever. You think you're really smart, don't you? In this world, it's kill or be killed. So you were able to play by your own rules. You spared the life of a single person. Froggit, Whimsun. Vegetoid, Loox. Migosp, Moldsmal. Think about those names. Do you think any of those monsters have families? Do you think any of them have friends? Each one of them could have been someone else's Toriel. Selfish brat. Somebody is dead because of you.
  • 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. The theme playing is frightening too, being a simple, slow piano loop of Flowey's theme.

    No Mercy/Genocide path 

  • In a Genocide run, all songs are slowed down or edited to some degree. If they don't sound sad and depressing, they instead fall squarely into Hell Is That Noise territory.
    • Bonetrousle slowed down 310% sounds more akin to something that should be in a Silent Hill game.
    • Waterfall becomes a menagerie of funereal bell sounds slowed down 666%.
    • If you thought the bleak soundtrack played when facing down Monster Kid sounded familiar, that's because it's "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 kicks in again further slowed down (to 250%) at the moment when Flowey 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.
    • The one song that plays only after completing a Genocide is an unnamed track that plays when encountering the Fallen Child at the end of the route. It's a terrifyingly minimalistic track, made up of horror-movie like trilling noise playing over the sound of a deep, slow heartbeat-like noise.
  • Normally if you check the mirror, the game's narration gives a cute description: "It's you!" But on a genocide run, this is the first sign that something's not quite right with the human you're in control of:
    "It's me, (name)."
  • Imagine a new player who (somehow) didn't know about the difference in a pacifist or genocide route. Now imagine that player decided to do some early grinding in the Ruins. Now imagine the shock when an encounter results in "But nobody came," and the music switched from the serene Ruins theme to Hell Is That Noise.note 
  • While checking out Toriel's kitchen, you may come across a cupboard full of gingerbread cookie cutters; but if you're particularly ruthless, this description is replaced with a startlingly red "Where are the knives." The fact that no other descriptions are changed makes this even creepier.
    • This comes up again near the end of the game: At the Fallen's former bedroom in Asgore's home, you might find a Worn Dagger that says it's appropriate for gardening. But by keeping true to the No Mercy path, you'll find a shiny Real Knife instead. By that point, you'll have no real use for it, but it feels great somehow, doesn't it...?
      • Even more chilling, somehow, is Chara's reaction when you equip the Real Knife: "About time." This is the same knife they use to murder their adoptive father, adoptive brother, and you.
      • The Real Knife is even creepier if you know the story behind it. In the demo version, there was a rumor among fans that there was a way to achieve such an item, prompting many to slaughter everything in the ruins hoping to find it. So in the final version, Toby included it near the end for players who did exactly that for the whole game. The Steam badge for it has the description "A symbol of the bond between player and character", which is chillingly accurate given the above.
  • Toriel's reaction to when you One-Hit Kill her. She'll ask if you really hate her that much… and then gives a rather unnerving smile, realizing she was never protecting you from the monsters outside of the ruins, but rather them from you. She laughs before she turns to dust. It's equal parts upsetting and unsettling to see Toriel die Laughing Mad and essentially cursing you in her last moments.
  • Flowey's response to you at the end of the Ruins:
    "Hahaha… You're not really human, are you? No. You're empty inside. Just like me."
    • He then goes on to talk about how the two of them, together, are going to kill everyone and everything in the world, and to cap it all off, he says this line. And we do mean "says".
      Flowey: Hmm... that's a wonderful idea.
  • 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.
  • As you kill everything that moves, every puzzle you find is already solved because people are fleeing as far as possible from you. You have become the real, blood-thirsty thing that goes bump in the night.
    • Also, checking one of the switches in Snowdin reveals that it's being held down with vines. Flowey is apparently making everything easier for you.
  • In a Pacifist run, Sans asks you to play along with Papyrus's attempts to outwit and capture you to make him happy. In a No Mercy route, his speech changes a bit.
    Sans: well, i'll be straight-forward with you. my brother'd really like to see a human... so, y'know, it'd really help me out... if you kept pretending to be one.
  • 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.
  • It's a small detail, but the few comments we get on the child's appearance throughout the genocide route consistently give the idea there's something very wrong with their appearance. While Sans' request of the child to keep pretending to be a human is probably a jab, the scene where Papyrus is able to guess them to be a human in any other route while Sans jokingly notes the rock's presence instead is inverted, with Papyrus now only mentioning the rock and seemingly not even noticing the child standing mere feet away from him before Sans points it out. By the time you reach Asgore, he asks them what the kind of monster they are — this would definitely seem like a guilt-tripping question under most circumstances, but from how he offers you tea immediately afterwards, it's clear that Asgore is genuinely unable to identify what kind of creature the child is. This is the only acknowledgement of it we're really given, and the child's in-game sprite remains the same even to the very end. Exactly what the hell are these characters seeing that we can't?
    • One of the only real hints we're given to how the human might appear to monsters in the genocide route is Papyrus noting the way they "SHAMBLE ABOUT FROM PLACE TO PLACE" covered in dust during the confrontation with him at the end of Snowdin. Keeping in mind that dust is what monsters turn into once they die, the description paints the image of what almost sounds like the monster equivalent to a blood-covered zombie.
    • The fact we also never get to see the Fallen Child's "creepy face" that Asriel mentions not only in one of the tapes in the True Lab, but as they approach Flowey in Frisk's body towards the end of the Genocide route is also rather unnerving.
  • In earlier builds, during the scene where Undyne is searching for you, Papyrus's sprite might show up as a glitch even though he's supposed to be dead by this point in the run, and just stays there the entire time during the dialogue-free cutscene even as Undyne fades out of view. It's definitely… creepy, to say the least…
    • Given the dark themes of the Genocide route, some could interpret this glitch as Papyrus' ghost manifesting and trying to warn Undyne, but couldn't reach out to her.
  • Any phrase that comes from the Fallen Child can be quite disturbing. Especially when they talk in entirely red sentences like the "where are the knives" example above. Notably, SAVE points change to them telling you how many monsters are left to kill in each area. After the work is done, what would be a fluffy comment about feeling inspired by something in the room becomes a blunt, crimson-hued "Determination".
  • Once the fight with Sans is initiated, he precedes his epic Curb-Stomp Battle with a variation on Asgore's wistful comments about the day.
    Sans: it's a beautiful day outside. birds are singing, flowers are blooming... on days like these, kids like you...
  • While fighting Sans, the dialogue itself will contain an ominous message: "You felt your sins crawling on your back." It's not just creepy because of what it's saying, it's also creepy because of the fact that what it's saying is actually too true: Sans is Breaking the Fourth Wall!
  • While getting dunked on is hilarious in a masochistic way for most players, note that before this happens, Sans tricks you into sparing him, only to, out of freaking nowhere, trap you in the "bone prison" to finish you off, which is hailed by a sudden "SPROING!" sound and, depending on how much HP you have, the "SOUL breaking" sound immediately afterwards. All while Sans maintains his winking, smug grin, as if mocking you for thinking he'd forgive you after everything you've done so far. It can be quite startling if this happens to you with no prior knowledge of the scene.
  • There's something deeply disturbing about the cutscene after defeating Sans. What appears to be blood comes out of him, something no other monster is ever shown with due to monsters being explicitly stated to be made of dust. On top of that, besides a brief-yet-ominous warning of what's to come, he seems utterly detached from reality in his final moments; instead of commenting on his death or you being well over the Moral Event Horizon at this point, he just casually says he's going to Grillby's, even though the place — along with most of the underground — is empty at this point. And then he asks his brother if he wants to come along for food, even though you killed Papyrus some hours ago.
    Sans: so... guess that's it, huh? just... don't say I didn't warn you. (stands up) welp. i'm going to grillby's. (slowly shambles off the screen) papyrus, do you want anything? (dies offscreen)
  • 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.
    • Further enforcing this is that once a No Mercy run is triggered, unless you get back onto the Neutral run, almost all area music throughout the game will be slowed down significantly to change the tone of the game entirely, if not replaced altogether with ambient noise or unnervingly short and barren music. And once you kill enough enemies to trigger, "But nobody came," its own music will overlap the areas. That music? Flowey's theme slowed down to a complete and utter groaning crawl like something out of a survival horror game.
    • One way to interpret "But nobody came" encounters is that the Fallen Child has gotten so bloodthirsty that they will attack even empty space in an attempt to satisfy their murderous desires. That said, it's possible to trigger this message in a run that is already solidly Neutral (and can thus never become a No Mercy run), though the music returns to normal more quickly.
  • There's also an optional moment of completely unnecessary cruelty in Snowdin. If you decide to take up the Snowman's request to take a chunk of him all around the Underground, you end up taking a far bigger chunk then you need to, making him look miserable and slightly mutilated. You can keep taking more and more despite the snowman's pleas to stop until you've ripped it apart and killed it, and then just eat the snow later to regain HP. Its description afterwards reads "A useless pile of snow."
  • The towns are completely empty on the No Mercy route, with notes from shopkeepers expressing fear over their life. The few people that the protagonist can talk to are either completely hostile (and rightfully so) or blissfully ignorant (such as Monster Kid, who the protagonist later attempts to murder, with the Check text saying "Looks like free EXP.").
  • In a way, your own actions create nightmare fuel. You're like if Pyramid Head or Michael Myers looked like a small, innocent child that promptly started systematically murdering everyone in their vicinity. The entire tone of the game changes from a quirky, happy RPG with some dark elements into, arguably, a horror game — but instead of running from some abomination, you're the monster making everyone else run.
  • During the first fight with Undyne, once you've cut her down with a slash from shoulder to hip, she starts lamenting her failure to protect her friends, and as she speaks, the upper half of her torso starts to slip down, bit by bit, across the knife slash. It's extremely unsettling to see.
  • If you CHECK either one of the two guards, the description for them doesn't show stats. Instead, it says that the protagonist is mocking them for trying to stand in their way. Beyond just being very dark, it also suggests the protagonist already knows the guards are lovers, and takes even more pleasure in cutting them down over it.
    "I see two lovers staring over the edge of the cauldron of hell. Do they both wish for death? That means their love will end in hell. I couldn't stop laughing."
    • Even more, this is actually a quote from "Kitchen" by Banana Yoshimoto, a book that talks a lot about about death and grief. Make of that what you will.
  • During the walk through Asgore's home, Flowey talks to the Fallen Child in place of the usual monsters, and talks about how they're the only one who understands them even after all the time that's passed since he knew them as Asriel. However, after pointing out how guys like them wouldn't hesitate to kill each other if they got in each other's way, Flowey comes to a horrifying realization: he can be a target for you. He begins shaking in fear and starts pleading with them to back out of the plan despite being on board with it for most of the run. That's right — you're so twisted that even Flowey is terrified of you despite being his only 'friend.'
    • And then after killing Sans, it turns out that Flowey came crying to Asgore to warn him of you — after the Fallen Child delivers a fatal blow on Asgore, he quickly comes in to destroy his SOUL, then insists (while visibly terrified) that he didn't betray them and that he's still useful. His voice and face then changes to Asriel's as he begs them not to kill him, only to get horribly mutilated afterwards if you don't quit. Even if you don't know the full extent of his backstory, it's difficult not to feel terrible at such a brutal murder.
    • Look at the battle interface when you engage Asgore. The Mercy button is gone. Unlike in the Neutral path where Asgore shatters it, he doesn't even get a chance to do it here. You've crossed the Moral Event Horizon and will never have mercy again.
    • Also, as Flowey begins growing terrified of the Fallen Child, the somewhat sad, ambient music is replaced with the slowed-down version of 'Anticipation' mentioned above. It fits the moment of him realizing that even he isn't safe from being killed and slowly growing scared perfectly.
  • 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. Do the latter, and the Fallen Child will ask if you thought you were in controlnote , 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 Floweynote ). 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.
    • What's even better is if you're playing in windowed mode, said window shakes uncontrollably during the Fallen's jumpscare. And when the slash-attack happens, the window shakes just like an attacked monster, implying Chara just killed YOU.
      • The shaking of the window could also imply that the attack was damaging the game itself.
      • Even better: If you're playing in full-screen, the game will exit full-screen to do said special effect.
  • Attempting to reload the game after finishing the No Mercy path simply results in a black screen, along with a sound effect of blowing wind. You did, after all, allow the world to be destroyed. After ten minutes pass, The Fallen Child reappears and, after pointing out that this whole thing was your fault, offers to restore the world to the way it was… if you'll sell them Frisk's SOUL. Doing so will taint any future playthroughs, as the Fallen Child possesses Frisk — even doing a True Pacifist run will result in the fallen human taking over at the very end.
    • 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.
    • Considering the fact that the world is already completely erased when Chara asks for "your SOUL", it can be taken to mean that they don't mean Frisk's… but yours.
  • After finishing a subsequent No Mercy/Genocide ending after completing it once previously, the Fallen Child will once again appear, with altered and truly disturbing dialogue, indicating the player's "perverted sentimentality" in continuing to create the world only to destroy it over and over again.
    • This time, when offered the choice of destroying the world, turning it down has the Fallen Child mention this is the strange sentimentality they had been talking about. With blank eye sockets, they then remind you that as you have already sold your SOUL, "YOU MADE YOUR CHOICE LONG AGO". They then rush the screen in an identical jumpscare as the previous Genocide ending.
  • The sheer despair in the "fight" against Asgore. It only takes a second in-between the moment he gently offers you tea and when he's mortally wounded on his knees with the most massive expression of "oh, shit" that pixels can convey. Then, as Flowey moves in for the kill, you can see visible terror on his face. Watching one of the Underground's mightiest and most heroic figures get cut down and swept aside like nothing is beyond terrifying.
    Asgore: Now, now. There's no need to fight. Why not settle this... Over a nice cup of tea? [slash, as he collapses to his knees and his eyes erupt with pain] Why... You... [Flowey's unavoidable pellet attack surrounds him as he looks on in terror, before it kills him]
  • Note also that within the game's canon, a human cannot absorb a human SOUL just as a monster cannot absorb a monster SOUL. Yet one cannot legitimately undo the bad ending without agreeing to trade in their own SOUL to the Fallen.
  • If you check the coffins, Chara says, "(It's as comfortable as it looks.)" Brr...

    The True Lab 

  • To start, entering this area is a Point of No Return, due to the elevator suddenly going haywire and crashing. You are not informed of this in advance. Want to get out? You'll have to bear witness to the many horrors of this area first.
    • The elevator going haywire can be creepy enough on its own, considering the suddenness of it.
  • It should be stressed that the True Lab offers a double dose of Mood Whiplash. First, in contrast to the bright interior and quirky music of the surface floors, the True Lab is dark, dreary, and comes with eerie music. Second, this area comes right after a Romantic Comedy dating scene where you're just trying to get Alphys and Undyne together and nothing in that scene hints at any sort of dark twist up ahead.
  • The Amalgamates are particularly disturbing. Especially as you go through the area and learn their origins.
    • The song that plays when you fight them is also disturbing.
    • Seeing the first Amalgamates and recognizing them as similar to Flowey is unsettling enough on its own.
    • Several of the enemies stand out, particularly Lemon Bread, who is more sentient and aware of their plight than the others. They even give the quote used at the top of the page.
    • However, Snowdrake's mother is particularly notable. Her attacks barely even summon a handful of bullets which float listlessly, the music sounds utterly broken, and she can barely hold her shape together, while other amalgams were at least somewhat recognizable. You learn later that "she" is actually 17 people melted together into one.
      • Worse yet: She feebly calls out, "Snowy." She's just sane enough to be calling out for her son. Also counts as a Tear Jerker once you realize that.
      • Even the Narrator seems to be slipping in this fight. Rather than their usual witty and somewhat sarcastic dialogue, they just start repeating, "It's So Cold." Either they're just scared witless by the Lab, or the place is seriously messing with their head in a way unlike any other place in the game.
    • The Dog amalgamate, Endogeny, doesn't have a face, but a single black hole that takes up most of its head. Its battle music is rhythmic dog barking, which sounds like low, demonic tribal chanting and gives you the feeling of being hunted…
      • While the other Amalgamates enter the battle screen as soon as they reveal themselves, Endogeny gives the player a few moments of Controllable Helplessness as it slowly advances towards you while blocking the path.
      • What really seals the deal is if you had a childhood fear of dogs and you fight Endogeny. Everything about it reminds them of what scared them about dogs as a kid, including "bounding all over the place and scratching the walls," rushing toward you while barking and trying to lick you, etc. It's a surprising change of perspective from all earlier depictions of dogs in the game.
      • The fact that it's a giant dog that's scared and most likely in pain is pretty horrifying for dog lovers as well.
    • While most of the Amalgamates are a combination of previous enemies, with Act command options taken from each of their fights, the first one you find, "Memoryhead", is a trio of enemies that don't resemble anything (except maybe Giygas/Gyiyg/Geigue from 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.
      • The start-of-turn text when fighting Memoryhead is "But nobody came." Remember, you only visit the True Lab on the Pacifist route, so you probably wouldn't know the significance of that text unless you did at least part of No Mercy on another playthrough (or cleared out an area of random encounters in a very dusty Neutral run, even if it never became a No Mercy run at all), in which case it goes from simply "creepy" to a brief but chilling reminder of your past actions.
      • One of the Memoryheads will sometimes say "Lorem ipsum docet." While on the surface is seems like a random reference to the lorem ipsum placeholder text, the full phrase could be roughly translated as "It teaches pain itself."
    • Reaper Bird's appearance. You walk past a series of mirrors, when something catches your attention. Suddenly, your alarm bubble gains a face, and then transforms into a monster. This is essentially a demon growing from your voice.
      • Its first "attack" is just a birdlike humanoid standing there. Then, a swarm of butterflies descend on its face, and apparently devours its head. You can clearly see it writhing in agony. Then its turn ends.
      • But it's okay because, in the next attack, the humanoid grows another head! And another. And another. And another... And it launches these heads at you.
  • Some of the journal entries.
    Entry Number 15: Seems like this research was a dead end... But at least we got a happy ending out of it...? I sent the SOULS and the vessels back to ASGORE, returned the vessel to his garden... And I called all the families and told them everyone's alive. I'll send everyone back tomorrow. :)
    • There are two "journal entry 17"s in-game (both of which are inaccessible in-game). One is written by a mysterious man (as explained below in "Non-specific"), but what about the one written by Alphys that connects the dots between entries 16 and 18?
      monsters' physical forms can't handle "determination" like humans' can. with too much determination, our bodies begin to break down. everyone's melted together...
  • The Reveal of Asriel and the Fallen Child's plan to escape: collect six human SOULs, and break the barrier. These are children planning murder, and the Fallen Child committed suicide to set it in motion.
    • As if the idea of the Fallen Child planning their own suicide and the murder of humans isn't bad enough, they kill themself by eating buttercups and poisoning themself. If you know the symptoms of buttercup poisoning (bloody diarrhea, excessive salivation, abdominal pain, severe blistering of the mouth, mucous membranes, and gastrointestinal tract), it's incredibly unpleasant and painful, and their death was most likely slow and agonizing. Even worse, imagine being Toriel and Asgore seeing their adopted child agonizing this way for days and being powerless to stop it.
    • The way it's done is rather disturbing, as well. You find a TV with some tapes next to it, and there's a note saying that these videos probably shouldn't be watched. The first one is a rather cute video of Toriel and Asgore telling each other puns… But then they slowly turn into Asriel and the Fallen Child's plan. The final one even has Asriel sounding scared and getting reluctant to go through with it, but obviously that didn't work out well.
  • The shaking fridge? So far everything odd we have seen shows up in threes, and when you enter the room with the fridges, what do you know, the third one is shaking every few seconds. That's creepy enough on its own. It's also a dead giveaway that there's something inside, right? Most people will check the first two for items, skip the third, and check the rest first — except, when you get near the last fridge, that one becomes the amalgamate. So… once you're done with that, you go back to investigate the third fridge… and it tells you "it's empty." And it's still shaking. What…???
    • The fridge room is even creepier if you examine it before clearing up the fog. You can still examine each one to make a guess at what it is by touch, and the only description that sounds remotely like a refrigerator is the one that is really a monster.
  • To get one of the keys, you have to approach a tub with the shower curtain pulled around it. Inside, you can see the shadow of… something, which looks like a single tentacle, moving back and forth like a metronome, making a creepy sound when it does. When you walk toward it, it starts moving faster and making more frantic sounds, but you can only move at a snail's pace, no matter what you do. Finally, when you get to it, and pull the curtains back… all that's there is the key, and the room is silent. Interact with the tub again, and it will say "Just an empty bathtub." It doesn't help that the wording makes it sound like you're just trying to reassure yourself that it's nothing…
  • The giant determination extraction machine in the room where you fight Lemon Bread. It's essentially a big, red, demon-like skull that looks like it could start doing something at any moment.
    • Does it look familiar? It should. That's a skeletal form of Flowey's final boss form from the neutral endings. This is further affirmed by the fact that the television found just beyond the extractor closely resembles the screen on top of Omega Flowey.
    • So, basically, Flowey either fused with or based his appearance on the determination extractor. Assuming he did this for a reason, one shudders to think: Just what was he planning to do to Frisk, let alone the rest of humanity, in that form?
  • The entire True Lab level feels like a Shout-Out to The Shining and similar horror movies; an innocent exterior hiding blackness and Eldritch Abominations within, speaking of great crimes and madness. There's even a monster in a bathtub. Some of the Amalgamates are like the twins: innocent victims. Others are malevolent and hostile.
  • If the player sleeps on an untucked bed, a phantom-like Amalgamate will appear, slowly reach out, pull the sheet over Frisk, pat them on the head, and disappear.
    • Unlike the others, this one has almost no features whatsoever. It appears to be a bean sprout shape with a single tentacle-like appendage for an arm.
  • The screens for the last two logs initially are depowered. After restoring power to them, passing by either screen causes it to show a smiley face. The same face you see in place of a '!' once you're deep into a No Mercy run and on Flowey's face.
  • As you make your way to the area where you get cornered by the Amalgamates, you may notice an elevator panel in one of the corners of the hall. However, it does nothing, the elevator having apparently lost power, so you move on, likely forgetting about it. When you come back to this spot after talking to Alphys, you get a call from an unfamiliar voice (the game informs you "(It's a voice you've never heard before.)" with extra-wide space between each character and none of the usual text "blipblipblip" sounds) who tells you that "everything is falling into place." When they bid farewell, the doors suddenly close and you get rocketed up to New Home. Yikes!
    • By the vines that hold the elevator after you get off, some speculate that it was Flowey who somehow managed to call you. Or it's Asriel himself in his God of Hyperdeath form calling you.
  • If you're playing the Xbox version of Undertale, and your controller disconnects while you're in the True Lab, the "No Controller Connected"-screen has some... additional text.
    No head is connected.
    No body is connected.
    No arms are connected.
    No legs are connected.
    No skin is connected
    No ears are connected.
    No face is connected


  • Asgore destroying the Mercy button when the fight with him begins. It is incredibly chilling, and gives off the impression that he's serious about killing you and isn't going to make peace with you or let you escape (not that you can run from boss fights aside from Toriel and Undyne, anyway). Knowing why he destroys the button is just as bad: He isn't doing it necessarily to kill you and claim your SOUL, he's doing it because he wants YOU to kill HIM. On subsequent runs where Flowey doesn't show up to kill him, if you attempt to spare him, he will have a brief heartbreaking conversation with you, before taking his own life in front of your very eyes.
  • If you spare him during your first playthrough, Asgore will realize that life may be worth living again... before Flowey shows up and kills him at his most vulnerable moment. After that, he takes the human souls for him, gives you his speech about how "it's killed or be killed" while his face is melting down, then the game closes the same way it started, accompanied with Flowey's Evil Laugh.
  • The final boss on the neutral path, dubbed Photoshop Floweynote , comes right the fuck out of nowhere and is jaw-dropping. From its beyond disturbing appearance to the music that plays when you're trying to survive its attacks, it is horrifying in every sense of the word. Never has the term Outside-Context Problem been so appropriate. But if you really want to see what it looks like... here.
    • Let's dive more into this. Before the fight even starts, the game closes itself. Upon opening the game up again, the intro glitches out and you find your save replaced with Flowey's, which claims that he is LV 9999. And this is at the point where you learn that LV means Level of Violence, and even on a No Mercy run you can't get above LV 20. And then there's its appearance. The game is made with sprites, which are standard 2D design, and the battle sprites are all monochrome. The final boss defies that. It is a photorealistic mashup of parts of things, mostly from plants and human faces. And then there are its attacks. They are... comical, yet terrifying. The final boss has some of the most ludicrous yet unnerving attacks a final boss could have. To put it lightly, it rains nuclear bombs down on you, then throws a few finger guns your way before deciding to crush you with ballerina shoes. These are not the things you'd expect from a monstrosity like this, but it's scary nonetheless. Oh, and it can save right before an attack and reload if you dodge.
      • The intro before it's hijacked has some subtle but eerie changes: The picture accompanying the first lines of text replaces the Toriel-like creature with a Loox, which of course was in the Demo. As for the text itself...
        "Long ago, two races ruled over Earth: HUMANS and MONSTERS. One day, they all disappeared without a trace."
      • The intro's image and text aren't the only things that glitch. The music gets stuck repeating a single split-second of the song, and it persists until the player opens Flowey's save file. Without actually putting in the video of it happening, it goes a little like this:
        Buuum buuum buuum buuum, buum buuum buuuum - d-d-d-d-d-d-d-d-d-d-d-d-d-d-d-d-d-d-d-d-d-d-d-d-d-d-d-d-d-d-d-d-
      • There is a reason behind the most comical attacks: the weapons you find in the game are ballerina shoes, fighting gloves, a book, a toy sword, an empty gun, and a frying pan. They used to belong to the previous six humans who died before you, and whose SOULs the last boss just absorbed. The weird attacks like the ballet shoes or the frying pan throwing fire are basically Flowey channeling the spirit of those previous SOULs, or forcing the SOULs to attack you.
    • 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.
    • His taunt if you lose to him is especially goddamn chilling:
      (in Asgore's voice) This is all a bad dream... (in his own voice as "Determination" stops) And you're NEVER waking up! ("Determination" starts back up, but is so sped-up and distorted as to sound more like "Psycho" Strings than anything else) HAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHA(the game closes)
      • Worse, in the PS4 and Switch versions, the game doesn't crash. It freezes, letting out a horrific droning noise, before the picture just conks out. Then, it cuts to black...and then it goes to the dialogue explained right below.
      • Load the game back up after that and the entire intro and load screen are skipped; instead, you're greeted with only a black screen with Flowey taunting that he's going to kill you again and again and again, before jumping you back to the beginning of the boss fight.
    • Occasionally, the boss' face screen shows not its own face, but instead a high-contrast black-and-white photo of a human screamingnote . What really stands out is the person's eyes, which are un-comedically bugging out of their face. Also, although the "human" looks like it's screaming most of the time, there are some frames where it appears to be smiling. It doesn't help that they closely resemble the infamous "Who are you running from?" faces from Game Boy Camera.
    • His last words if you decide to kill him, because you just proved his "kill or be killed" mindset:
    • Prior to the final fight, Flowey, having absorbed the six human SOULs, states that he can "feel them wriggling". It's such a small thing to say, but still.... It's utterly disturbing.
    • Digging around in the files will reveal a picture titled 'unnamed_6385' which appears to be early concept art for Photoshop Flowey, largely the same but with a few extra-disturbing additions: instead of the vines supporting his body having thorns on them, there's many, many eyes with the SOULs being trapped in glass cases supported by structures that have arms growing out of the ends.
  • The Neutral endings are bittersweet at best and heartbreaking at worst. If it's a Neutral run where you killed Undyne and/or Mettaton and Alphys doesn't rescue the remaining monsters from the walking apocalypse you are (which she does on both a Genocide Run and an aborted-Genocide Neutral Run), it is heavily implied she committed suicide in the True Lab. Outside of this, however, three endings take the cake:
    • The "aborted Genocide" ending, where Alphys is forced to take command after you kill nearly everybody: everyone is worried for their lives and Alphys makes her best to calm the situation. However, during the call, she cracks a bit. It is disturbing to hear her of all people threaten you so coldly.
    • The "king Mettaton" ending, gotten by killing only Toriel and Undyne, is Crapsaccharine World at its worst: Mettaton is an incompetent and narcissistic ruler who abuses his status to brainwash people with his TV show and wastes public money to glitter gold everywhere and build monuments to his own glory. If Papyrus is still alive, he mentions things don't go well for those who don't worship the new king, claiming that they "tend to kind of… disappear". What happens to them exactly?
    • The creepiest may be the ruler less ending (unless you only kill the possible rulers, as that leads to… another ending): you practically killed every monster but did not commit Genocide, you only hear a blowing wind behind Sans' voice, and he heavily implies the Underground has fallen into despair.
  • By the end of the No Mercy path, even Flowey is scared shitless of you, and begs for your mercy, to no avail: you brutally murder the flower. To cap it all off, the Fallen Child shows themself, congratulates you, and says to "erase this pointless world, and move on to the next." You can refuse, but it doesn't matter; they tell you that you never had an option, gives you one hell of a Nightmare Face, and erases reality, filling the entire screen with 9’s before the game closes. Yipe!
    • Flowey — or rather, Asriel — gives a deeply unsettling account of how he came to be the way he is. Particularly chilling is when he talks about waking up and crying because he couldn't feel his body. The worst part is what happened when he discovered both his mom and dad were unable to make him feel anything due to the loss of his soul. He attempted suicide before he realized what could potentially happen when a being without a soul dies and discovered his ability to use DETERMINATION to alter the timeline. After going through many cycles of saving everyone, he started killing them out of boredom, and has exhausted every possibility in the Underground.
    • The fact that unlike the enemies, guardians, Sans, or, hell, even you, Flowey doesn't turn to dust or fade away when he dies. Instead, the Fallen Child cleaves him out of existence, and you get to watch each and every swipe as the poor bastard's sprite is horrifically chopped down, eventually becoming nothing more than a pile of chopped remains on the ground before finally disappearing. Here's the scene if you're willing.
      • In line with this is a simple fact: Monsters and Humans have a soul. When humans die, their soul remains. When monsters die, they turn to dust and their soul quickly disappears. Flowey being torn up like this proves he doesn't have a soul that keeps him going. Which actually opens another question: If Flowey doesn't have a soul, can he really die? He thinks so, which, in a setting where that is not the case, is another form of nightmare fuel from a different angle. Alternatively, another possible outcome of Flowey’s death is the thought of what could potentially happen to a being without a soul when they die.
    • Also worth noting: you know how all the characters in the game have some sort of Voice Grunting that plays whenever their text shows up? The Fallen Child has nothing. Just complete silence as they tell you about their love of power.
    • 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 beginning of the finale of True Pacifist. Every leader of the monsters you met comes back to congratulate you on sparing Asgore. Everyone has a grand old time, until Alphys wonders how Papyrus knew to call everyone there. Papyrus’s response? "Let's just say... a tiny flower helped me." Cue Flowey bursting though the ground and wrapping everyone in his giant roots, squeezing their souls out and preparing to turn into Asriel. It turns out this was his plan all along, and the reason he gave you hints about how to reach the true ending.
  • Despite the ending of the True Pacifist being incredibly heartwarming, there's a somewhat creepy moment after you boot the game up again. Flowey appears before you and (albeit somewhat calmly) begs you to not play the game anymore, as you'll be ripping your friends away from their lives and their positive futures. Flowey says that you'll have to be extremely callous to do so, and then notes that even he couldn't do that. He then says that if you really want to restart, he wants you to wipe his memories too... And then says that you've probably heard this 100 times before, since he wouldn't remember if you had. Many people who want to do a No Mercy run stop here, realizing they simply can't go through with it.
  • In most of the endings where Sans is angry with you, he tells you not to come back. But for one of the endings where you kill just about every leader and enough monsters to make Sans disgusted with you without going No Mercy, he ends with some very chilling words—
    see ya.
  • Some of Sans's cell phone calls at the end of the neutral path can be chilling, depending on who you chose to SPARE/kill. Specifically...
    • In any call where Toriel is left alive, but Papyrus is killed, Sans winds up moving in with her in the Ruins. In his phone call, he mentions how she occasionally talks about wanting to see the protagonist again and how he doesn't have the heart to tell her that because she protected you, you survived to go on and kill his brother. When he talks about this, his pupils disappear and the background music stops. He caps this off with a final warning to you:
      • Leaving only Undyne alive results in arguably Sans' most threatening phone call. He reveals that Undyne has taken up leadership of the underground and has instituted a policy saying that any humans found in the underground are to be immediately killed. She's more determined than Asgore ever was to break down the barrier and wage war on humanity, and has vastly expanded the royal guard as a result. Oh, and she's vowed that the first thing she's going to do once the barrier is destroyed is to take her army and personally hunt you down and kill you. As if that wasn't bad enough, Sans decides to twist the knife a little more: he describes heading to the door of the Ruins, where he had previously traded bad jokes with an unknown woman (quite clearly Toriel, whom you murdered at the start of the game), only to be greeted with silence on the other side. Like the above example, the final lines are delivered with Sans's eerie, pupil-less Black Eyes of Evil expression and with the background music abruptly silenced.
        Sans: maybe she's not feeling well? heh. or maybe she's not feeling anything at all...? you'd better watch yourself, kid. things are lookin real bad for you.
    • Just something to note: In most of the above endings, Sans ends the call with some variation of "see you later"… except for the "Undyne takes over" ending. With everything you did — especially considering you murdered his brother along the way — one cannot help but wonder if that lazy skeleton is planning his own revenge.
  • When Twitch Plays Undertale finally beat the game, the stream locked up and froze at this screen.
  • If you check the Coffins in New Home, you'll notice one of them is marked with a Red Soul. Just like yours. When you check it:
    (It's a coffin.)
    (There's a name engraved on it.)
    ("[Player's Name]")
    ("It's empty.")
    • Of course, this becomes a lot less scary and more sad once you find out that the character you named at the beginning was the first Fallen Child and not you, but still.


  • The naming screen at the beginning is strangely ominous. Every letter shakes and the name you choose zooms in as the game asks you to confirm it. A creepy sound plays as the game fades into the beginning of the story. The reason for this is that the "fallen human" you're actually naming can end up becoming the true villain, a demon who comes when people call their name. Just like in every other game where you choose that name for The Hero who kills all the monsters.
    • Trying to name the fallen human after the other characters will cause a reaction. Some of them are rather eerie:
      • Flowey claims he "already CHOSE that name," like how he capitalizes SAVE.
      • Sans says "nope." He's quite aware who and what the person you're naming really is, unlike Papyrus.
      • Asriel goes "...".
      • "Gaster" resets the game back to the opening. The character in question is torn to pieces through the time-space continuum. Not that his state of existence can stop him from showing disapproval. The worst part? It shows just how powerful he is, even while erased. Think long and hard about what he could do to you if he fully restored himself. In the PS4 and Vita ports, attempting to put down "Gaster" crashes the game; since it's Gaster, it's likely that this "bug" is intentional.
      • Input "Chara" and the game calls it "the true name". Given just who you're naming, this makes a worrying amount of sense.note 
  • The Close on Title for Undertale happens right as you leave the ruins to Snowdin. How it comes up though sounds extremely ominous, and it can happen regardless of which route you do. This can get even scarier if you happen to kill Toriel (either accidentally or intentionally) and hear Flowey's reaction in regards to it - which it will vary.
  • When you first enter Snowdin Forest and the lead-up to meeting Sans. It's eerily quiet, and as you follow the path, there will be shadowy flickers behind you at points, and the silhouette of someone following you. Going back to a branch on the path, you'll find that it's broken. Then you reach the bridge and your character is frozen as the shadowy figure walks up to you from behind.
  • Any time Sans shows his more competent side counts as this.
    • When threatening you, Sans's pupils usually vanish. This actually manages to make him go from looking goofy to just plain scary thanks to Black Eyes of Evil.
    • Even if you don't pursue the No Mercy ending and incur Sans' wrath, the one time he lets his facade down around you is creepy as hell. If it wasn't for his promise, you'd be dead where you stand. He's not kidding, as you will learn in No Mercy. Doesn't help that his Voice Grunting and the backgroud music/noise disappear for that moment.
    • Doubling as a tearjerker, if you killed Papyrus (but you're not on a No Mercy route), he'll ask you if it's your responsibility to do the right thing if you have special powers. If you say yes, he'll ask you why you killed his brother. If you said no, he'll say this (without his voice grunting) instead:
      Sans: heh. well, that's your viewpoint. i won't judge you for it. ... You dirty brother killer.
  • Everything to do with W.D. Gaster. In game, he was a scientist who predated Dr. Alphys as royal scientist and created the Core. Now after some kind of horrible accident, he's an entity scattered throughout the very code of the game. Tampering with the code reveals hidden messages in Wingdings font, hidden rooms with creepy entities, and NPCs that have to be programmed back into the game to be spoken to.
    • One such cut NPC talks about Gaster, but mentions that they shouldn't gossip about someone who's listening.
    • Another one asks if you've ever thought about a world where you don't exist, but everything goes on fine without you, and says that the thought is terrifying to them. This is not only the case for Gaster, but for any cut NPC from any game.
    • Some of the NPCs themselves are pretty creepy. One looks like a grey Monster Kid, but with no pupils. Another is a talking head (possibly Gaster's head itself) being held by a weird humanoid (actually a recoloring of an existent NPC, but with fewer features). The worst of them all, however, is probably Follower 3, which looks like a giant, bizarre, disembodied head that's sticking out from the ground (although the appearance of its normal, recolored counterpart in Deltarune reveals that it's actually just a weirdly-shaped humanoid bird. Even so, that doesn't make its appearance any less creepy, especially since its lower jaw appears to be missing and its legs are seemingly melting into the ground.)
    • There are two versions for the journal entry seventeen (both of which are inaccessible in-game). While one was written by Alphys, one was also in fact written by Gaster. And it makes the implications of just what the hell happened to him even more terrifying...
    • As of patch 1.001, it's possible for Gaster's stuff to trigger randomly instead of going into the game files to re-add them. This already adds a layer of creepy that Gaster's somehow putting himself back into the game without any outside influence, but imagine playing through blind and running into the mysterious door in Waterfall, or the Gaster followers (complete with the dead silence) at the Hotland elevators. In fact, this may be even worse if you already knew about Gaster beforehand but didn't do it yourself, because you know for a fact that you didn't edit your file so Gaster appears, so you know he's not supposed to be there, it's almost as if he's starting to pull himself together irrelevant of what you do.
      • This information becomes even more frightening as Gaster is feared by his followers. Beware the man that speaks in hands, and the sole one that says he doesn't disappears immediately. Also, there is something that the game creator spoke of with Gaster: "You've all seen the happiest outcome. Neither of them could fix the machine, no matter how hard they tried. No one can." The first sentence could be saying that the machine not being fixed is the happiest outcome, and that might be because it being fixed leads to a worse one.
    • Even Gaster's theme is at least mild Nightmare Fuel. It's just ominous, desolate piano notes that keep repeating. But the song itself isn't so bad so much as the details about it are. Every other Dummied Out song is listed with the filename 'mus_st_x' with the x being replaced with whatever the song's name is, like 'mus_st_happytown' or 'mus_st_meatfactory'. Gaster's Theme is listed as 'mus_st_him'. Additionally, selecting any of the other songs in the sound test plays a one-second loop of the song and you can choose freely which song you want to listen to. Selecting Gaster's Theme, however, locks you into a loop of the 17-second-long theme, where you can scroll through the other songs and even select them but Gaster's Theme continues unimpeded. The game then forcibly kicks you out of the sound test room with the message "Thanks for the feedback! Be seeing you soon!". Keep in mind, the only thing that happened was that music was listened to and absolutely no feedback was given. And who's going to be seeing us soon?
    • If a Gaster follower is around, the cell phone cannot be used to call Papyrus and Undyne. The game just says that the phone will not turn on. One can interpret this as Gaster's presence causing some kind of interference by just being around. Another is that he or his followers are blocking calls on purpose. Regardless, it has eerie implications.
    • The Clamgirl NPC who sometimes appears based on your Fun Value is a mild version of this when she talks about fate, but in the Nintendo Switch 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:
      "But please don't despair. Because the time that you will meet her...
      (transforms into a Gaster follower-esque sprite) fast approaching."
    • When she turns into the grey sprite, her arms become more tentacle-like and her face is replaced with an eye. And after she says the above line, she disappears, just like some of the other followers. According to the game's files, this sprite's name is "clam_goner_b," hinting at a relation to "Goner Kid" (The grey version of Monster Kid whose sprite name is something like "mkidgoner.")
  • Undyne's first appearance as this creepy, silent figure in heavy armor that relentlessly hunts you down is pretty tense (she somehow leaves her eye for last when she fades away, too). Especially since she almost catches you completely defenseless twice (though, the mood is abated slightly the second time, as she accidentally grabs the Monster Kid instead.)
    • Beating Undyne causes her to literally melt.
      • There is a reason for this that comes from the True Pacifist route. Undyne was filled with determination, a human substance that causes the monster's form to come back to life for a short period of time, and then begin melting because its body compounds can't handle such a substance.
  • Of all things, Temmie is a potential source of nightmare fuel. You can sell things to their shopkeeper, some of which the shopkeeper says that she really wants. If you then repeatedly refuse to sell them to her, Temmie says "You will regret this." without any of her usual speaking noises or silly capitalisations and misspellings.
  • Hidden in the files, one fan found a room with an incredibly loud (seriously, turn down your volume) version of Flowey's theme with strange red pillars that slightly drip of blood that leaves you stuck forever in a text loop.
    • Another hidden room is this one where a strange figure speaks in Wingdings. A translation of what the figure is saying reveals...that what it was saying was redacted. As in the text literally spells out [redacted], brackets included.
    • If you have a fun value of 66, a hallway will appear in Waterfall between the room with Sans’ telescope and the room with the cheese. There’s also a 1/10 chance for a gray door to appear in the hallway, which leads to another room, making the chance of getting it 1/1000. It can also be accessed by changing the fun value in the game's code, although you will probably need to try multiple times for the door to appear. The room in question is completely silent and contains only a smiling, deformed skeleton entity with a striking resemblance to Uboa. Interacting with it will cause it to startle and fade away in thin air. This freaky entity is speculated to be the aforementioned W.D. Gaster, or at least what remains of him.
      • Trying to access the phone here will tell you it’s unresponsive.
    • A room called "TESTROOM" includes unused text of someone's ramblings. There's no explained reason why it was made, and the text itself is...unnerving, to say the least.
      Text: * La, la. Time to wake up and smell the pain.
      * Though....It's still a little shaky.
      * fhuefuehfuehfuheufhe
  • Killing a monster has them dissolving and turning into dust. At first, you might not think about it too much and just assume it's a standard RPG element like any other game when a monster is defeated. However, you quickly find out that monsters actually do turn into dust when they die and they don't leave behind a body. Just imagine a monster's loved one dying and their body turns into dust instantly, unable to be held ever again.
  • Imagine this — you've 100% completed the game and you feel like traversing through the files. While looking through the .ogg's, you find one called "abc_123_a.ogg". Expecting some kind of unused soundclip or just a strange name for an already existing one, you open it up... Instead, you're greeted with some ominously, creepy and disoriented Text-To-Speech voices from FLStudio, threatening you 'not' to spoil the game.
    Hello. He-he-he-he-have some respect and don't spoil the game.
    It's impossible to have mysteries nowadays
    because of nosy people like you.
    Please keep all of this between us.
    If you post it online, I won't make anymore secrets.
    No one will be impressed.
    It will be your faaaaaaaaaaaaaaaaaaaaaaaaauuuuuuuuuuuuuuuult.
    • And not to mention, this is the voice used for Gaster. So it's likely Gaster is aware of the game itself, and that he's watching YOU.
    • Since Patch 1.001, the file just laughs at you. Considering what the fanbase has done and is doing, it's probably meant as a threat. Nothing Is Scarier indeed.
  • 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 Cosmic Horror Story. 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 Fallout or Mass Effect and you decide to save just before doing something purely out of curiosity.
  • EXP and LOVE. In the judgement hall, Sans reveals what these acronyms stand for. Execution Points are a method of quantifying the pain inflicted on others, while Level of Violence is someone's capacity to hurt others and how easy it is for them to distance themself so they will hurt less. In the underground, they have actual mathematical methods of measuring how much pain someone can deal out and how much of a sadist or sociopath they are. It makes sense seeing as how monsters are physically affected by things like murderous intent, but it does make one wonder how they discovered these things and evaluated them enough to put numbers and a ranking system on them. What exactly does one execution point represent as a unit of measurement, anyway?
    • Well, they did fight a war that pushed them to the edge of extinction. Then again, that's a whole different kind of Nightmare Fuel. Seriously, in this universe, the average human kid could wipe out a horde of monsters by themself. Trained soldiers probably hit LV 20 in a matter of a couple battles if they didn't go down by Death by a Thousand Cuts.
  • Imagine something that you can't kill because of its inexplicable control over time. A being that can't manifest itself in the world, but it can take control of a host vessel to use as a tool to influence your whole world. Something that will help solve all the problems your people face in one timeline, then murder you and all your friends in another, and finally restore them all and ensure none of you will ever remember it ever happened. Something that acts entirely on its own system of morality and treats genocide as a method of alleviating its curiosity or just boredom. Something that nobody can even see or touch, yet it's always present and knows everything that's happened to it, even after its host body has been destroyed countless times. Sans calls it the Anomaly. What is it? It's you.
  • The situation of the human souls — imagine being killed, and having your soul ripped from your body and placed in a jar for God knows how long, knowing your ultimate fate is to be used as a means to exterminate your own race and kill everyone you've ever loved — and most, if not all of the humans that fell were children — while Frisk seems rather okay with dying due to seemingly self-destructing their own soul, keep in mind these humans had no such ability.
  • Gaster's predicament is even worse in a Genocide run. Imagine: despite you being ripped out of the timeline, the Underground continues to progress normally without accidents. Then, suddenly, a child arrives, and starts killing everything, including your friends and loved ones. Best part? You can't do anything about it. You're forced to watch everyone being reduced to dust.
  • If you stop a No Mercy run at Papyrus, he explains that his "special attack", also mentioned on a Neutral run only to be stolen by the Annoying Dog, would've vaporized you. One, it's heavily implied that his special attack is a Gaster Blaster, i.e. one of those Wave-Motion Gun skulls wielded by Sans in his Final Boss confrontation with you. Two, that's right: Papyrus — who you know as the boisterous yet unconditionally kind skeleton who's impossible for you to offend, even if you off him on a No Mercy run — was willing to outright kill you if he somehow survived your unrelenting onslaught. In fact, fighting against a dead serious Papyrus would probably have been way worse than fighting against Sans. Sans has only 1 HP, his attack is also only 1; he's only such a dangerous boss because he refuses to play by the traditional rules. Contrast with Papyrus, who, as a result of being a Boss Fight, is one of the strongest characters in the game. Even Undyne mentions in the Pacifist run that the only reason he isn't a Royal Knight is that he is too nice to fight properly. He — out of all people — might have been the only monster both strong and dedicated enough to Curb-Stomp Battle Chara if he would fight seriously.
  • In a way, the way the protagonist's SOUL self-destructs after dying is this. Consider this. You are a Monster. If you're not part of the Royal Guard, magic is just how you communicate. A human wanders by, and is for some reason avoiding your magic. When you apparently get through, they die. You might think 'Well, at least this way the Monsters can be set free' to console yourself, but nope! The SOUL shatters immediately afterward, so you just killed a kid without meaning to and anyone they might have befriended up to that point will be heartbroken. If you're a Royal Guard, it might actually be worse: you're only after the human because their SOUL is necessary to break the barrier. When you kill them, their SOUL shatters moments later, so you just killed a kid for nothing. Even worse if you die to Asgore himself. Imagine him killing you and seeing himself so close to save all the monsters... Only for the SOUL to shatter and make this victory for him and his people hollow.
  • While the Undertale Art Book is mostly innocent, it has its doses of this. This is a sheet of early versions of Asgore's talksprites, as well as ones he would have used for a Dummied Out attack, as Toby describes. The thing is that the bottom row sprites just...come right out of nowhere, and certainly doesn't seem like the Asgore we know and love.
    • There are also unused sprites for Asriel in the Undertale Art Book that have a few nightmare faces. Asriel's mouth opens far too wide and he suddenly has a lot more teeth that are needle-sharp. His final form also used to be completely skeletal, with Flowey's creepy faces at various spots on his body.
  • The official 5th Anniversary Concert is a Moment of Awesome through and through, but the prelude to the Genocide Mode music is terrifying. After playing the upbeat Pacifist Mode credits music, the orchestra plays the game's cheery menu theme, but in the middle of it the music suddenly turns ominous and discordant. The live chat replay even shows viewers wondering if it's over, only to react with shock and horror when the music takes a turn for the dark. As if to symbolize Frisk/Chara's path of bloodshed up to Undyne, the orchestra also plays dreary Dark Reprise's of Heartache and Bonetrousle, presumably symbolizing the deaths of Toriel and Papyrus. And after playing Megalovania, the concert doesn't immediately wrap up; the stage turns dark and most musicians leave their seats as a droning static plays, presumably symbolizing the state of the universe after Chara destroys it. Only a few remaining musicians playing a mild reprise of the Menu theme (and Toby's piano playing after the credits) saves the concert from ending on a very dark note.
    • The "path" the concert takes is also quite unsettling. The orchestra first plays the Neutral mode songs, then the Pacifist songs, and then the Genocide songs, each punctuated by the menu music. This seems to indicate that the concert is symbolizing a "playthrough" in which a player does the Neutral and Pacifist runs, and then decides to override all that with a Genocide run.
      • This is also how the music is ordered in the official OST, with the prelude to Undyne’s Genocide battle theme coming immediately after the Pacifist Special Thanks music. The exception is the final track Good Night, which is preceded by Megalovania and doesn’t play in game. However, it certainly sounds like it would play at the end of a Pacifist run.
  • On October 30th, 2018, the official Undertale Twitter account was suddenly taken over by an unknown entity; this entity blacked out the icon and name, and began posting cryptic messages leading up to the surprise release of the game's official follow-up, Deltarune. The entity mentions that they have apparently been "working on" the project for a while; they also mention having wanted to meet the readers for a long time, and claim that they are "ON THE VERGE OF CONNECTION" and that they "LOOK FORWARD TO CREATING A NEW FUTURE WITH YOU". But the most chilling part is one pair of Tweets from the middle, which raise some very alarming possibilities of who exactly is speaking here:



But nobody came.