Undertale / Tropes O to Z

You can view the main article here.

Tropes A here.

Tropes B here.

Tropes C-F here.

Tropes G-N here.

Tropes O-Z here

Note: "No Mercy" and "Genocide" are two names for the same, officially unnamed, route of the game.

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Trope O

• Obsolete Occupation: You meet Papyrus, a skeleton whose dream is to become the Leader of the Royal Guard. In one of the endings he indeed becomes the Leader, after the previous leader and the queen decide to disband the whole royal guard, making his position pointless. He doesn't seem to catch on to this, so he is still happy.
• Obvious Rule Patch: In the demo, it was possible to gain EXP and increase your LV while still doing a pacifist run by wounding monsters, but then sparing them afterwards. In the full game, you can no longer earn EXP by simply wounding monsters, likely because EXP and LV have far different meanings which would go against the point of a pacifist run.
• Odd Organ Up Top: One of the receptionists in MTT Resort has a giant hand for a head.
• Official Couple: Undyne and Alphys in the true ending get together due in part to your encouragement to Alphys to confess her feelings and delivering Undyne's love letter to Alphys when she's too self-critical to do it on her own. They are the only members of the main cast who enter into or are in a romantic relationship in any ending: Asgore and Toriel have long-since broken up and are unlikely to get back together, Papyrus goes on a date with with the protagonist but admits he doesn't have any feels for them beyond friendship, and Sans and Toriel have Ship Tease together in a few endings but nothing more than that.
• Offscreen Moment of Awesome:
  • If you do a few multiple Neutral runs in a row while sparing Flowey, he will eventually tell you to stay away from Sans. Because Flowey had tried to complete a Genocide run of his own, but Sans barred his way, and "caused him more than his fair share of resets".
  • Alphys in a Genocide Run manages to evacuate the survivors of the Underground, revealing what she did in the True Lab since that's where they're hiding.
• Offscreen Teleportation:
  • Sans’s favorite method of travel, which he calls “shortcuts”.
  • Played for Laughs when Sans appears in Gyftrot’s area right outside the mysterious door in Snowdin Forest; when you first walk south into the area, Sans is standing around. As you walk past him towards the door, he reappears at the left edge of the screen just as the right edge of the screen moves past his previous spot. Talking to him will have Sans ask: “say... are you following me?” Moving back and forth in an attempt to have “both” Sanses on the screen at the same time (which is impossible) and then talking to him will have him wonder if you’re lost.
  • If you call Papyrus immediately after finishing the date with Undyne, the latter will join him in Snowdin despite having left her burning house in Waterfall seconds prior. Asked how did she get there so fast, Undyne simply responds: “I ran.”
• Off the Rails:
  • Upon discovering that Alphys has been secretly giving the player the answers to his quiz show, Mettaton retaliates by going "off script" and asking a question about who Alphys has a crush on.
  • More seriously, Mettaton breaks the scripted performance planned by Alphys where he would pretend to fight the player so that she could 'save' them from him. Instead, he locks her out of the room and actually tries to kill the protagonist and take their SOUL.
• Oh, Crap!:
  • Flowey when Toriel bats him away from you in the beginning with a well-placed Fireball.
  • Toriel's reaction when you ask how to exit the ruins twice, followed by her running to destroy the exit.
  • Toriel sports this expression in the semi-unlikely event that she accidentally kills you in her fight when her entire goal was to protect you.
  • Asgore's expression when he encounters you for the first time and realizes you're a human, even taking a few steps back.
  • Also Asgore when Flowey kills him just after having a Hope Spot where he believes that perhaps all of this could be resolved peacefully.
  • Flowey again after his boss battle, when he realizes that the human SOULS have broken free of his control and taken their SAVE files with them.
  • Just before the Final Boss of the Pacifist Route shows up, Alphys pulls this kind of expression upon hearing Papyrus mention a "tiny flower," clearly suspecting that Flowey is the flower she experimented on.
  • Flowey near the end of the Genocide Route, when he realizes Chara/The Fallen Child remembers exactly what happened when he was Asriel and screwed up the former's plans to erase humanity.
• Omega Ending: An extremely twisted example: If you complete the No Mercy route first by killing everyone in your path and selling your soul to the First Child to restore the world, the True Pacifist ending will be forever tainted by them taking over Frisk and possibly killing everyone. No, True Reset won't undo that.
• Ominous Fog: There's a heavy fog east of Snowdin that completely blocks visibility. Papyrus fights you the first time you walk through.
• Ominous Latin Chanting:
  • Wordless chanting appears for a few bars during Asgore's eponymous battle theme.
  • Wordless chanting is heard throughout "But the Earth Refused To Die", immediately before fighting Undyne the Undying.
• Ominous Visual Glitch: When you reach the end of the neutral route, Flowey crashes the game to desktop. The opening that normally plays will be slightly different upon booting up again, the faces of the human and monster in the opening crawl distorted, until it glitches out and launches back into the story immediately.
• One Degree of Separation: The main NPCs in different areas seem like they're separate from each other, but the end of the True Pacifist route reveals that they're connected much more closely than one might have guessed:
  • Asgore: Undyne's boss and mentor, Alphys' boss, Toriel's ex-husband, Flowey/Asriel's and Chara's father;
  • Mettaton: Napstablook's cousin, Alphys' friend, Papyrus is a fan;
  • Alphys: Asgore's subordinate, Mettaton's friend, Undyne's crush/lover, created Flowey, knows Sans via Gaster?;
  • Undyne: Asgore's subordinate and protegée, Alphys' crush/lover, Papyrus's best friend, also knows Sans;
  • Papyrus: Sans's brother, Undyne's best friend, unknowingly communicating with Flowey;
  • Sans: Papyrus's brother, knows Undyne, Toriel's friend, knows Alphys via Gaster?;
  • Toriel: Asgore's ex-wife, Flowey/Asriel's and Chara's mother, Sans's friend;
  • Flowey/Asriel: Son of Asgore and Toriel, adopted sibling of the Fallen Child (as Asriel), created by Alphys, secretly manipulating Papyrus, has apparently fought Sans before (as Flowey).
• One-Hit Kill: This is Justified in a No Mercy run; it becomes very difficult for monsters to fight when their opponent has significant hatred, causing most bosses to have their defense stats decreased when you face them.
• One I Prepared Earlier: Briefly discussed at the end of the "Cooking With a Killer Robot" segment. After going through all the trouble to obtain an MTT-Brand Always-Convenient Human Soul Substitute™ so that Mettaton won't use your soul in his cooking show, he tells you to forget about it since he already made the cake in question ahead of time.
• O.O.C. Is Serious Business: If you kill Papyrus in a Neutral run, Undyne will realize that you did something to him because he didn't show up at a meeting. By her own admission, despite how much of a goofball Papyrus is, he has never, ever, missed a meeting.
• Optional Boss:
  • In the Switch version, the Dog Shrine added in the PlayStation versions is replaced with one. Near the end of the game, you can unlock a hidden red and blue door behind the Dog Shrine with the analog sticks. This leads to a hidden room containing Mad Mew Mew, a life-size statue of the titular protagonist of the "Mew Mew Kissy Cutie" series possessed by Mad Dummy.
  • If you walk around the room in front of the Mysterious Door in Snowdin for two minutes straight, you'll encounter Glyde. As it's a tiny area that you'll quickly realize has nothing for you to interact with but some mushrooms that make a squeak sound when you click on them, you're incredibly unlikely to ever encounter them by accident. Unlike regular monsters and bosses, encountering and killing them isn't necessary for a No Mercy run.
  • Even harder to find is So Sorry. To find them, you need to access a hidden area in Hotland on October 10, 8pm, that the game gives no indication even exists. On that day and time only, you will initiate an encounter with them by interacting with the art club sign in the hidden room. Like Glyde, they are a boss fight that isn't necessary to trigger and defeat in order to proceed with a No Mercy run, and in fact can't be found during those runs — the area to access the hidden room is sealed off by Alphys. Glitching your way there anyway reveals an empty room with a sign that says "Art club is cancelled!"
• Our Ghosts Are Different: Ghosts in Undertale are just another type of monster, although they have all of the typical abilities of ghosts in fiction. Souls are something else entirely (see below).
• Our Mermaids Are Different: Played with. Aaron is explicitly stated to be a seahorse (as indicated by the "Check" option), but his appearance is of a muscular, horse-headed merfolk, instead of the typical seahorse.
• Our Monsters Are Different: "Monsters" are beings made of magic. They start at Funny Animals and end somewhere short of Lovecraft Lite abominations, with the occasional thing you can't really classify (like a tsundere airplane) in between. However, they're all more or less friendly if you get to know them.
• Our Monsters Are Weird: The principal characters are all fairly "normal" — skeletons, a creature from the black lagoon, goat-people… the enemy monsters and background characters, however, include disembodied heads, beings of pure flame, tsundere planes, people with flashing polygons for faces…
• Our Souls Are Different: While both humans and monsters possess SOULs, human ones are much stronger and can linger indefinitely after death thanks to the power of determination (which monsters can't naturally produce in large enough amounts; artificially giving them determination makes them melt instead). Monsters can obtain human SOULs to become more powerful, and gaining at least seven of them can make one godlike, which led humans to fear and wage war with monsterkind. Asriel is a special case in that he was revived as Flowey during Alphys' experiments with determination. Because he was reborn without a SOUL, he couldn't feel empathy, which eventually turned him into the game's Big Bad.
• Outside-the-Box Tactic: It's possible to completely avoid having to fight Muffet. The main way would be to buy an item from her Spider Bake Sale when you encounter it in Hotland, except the treats cost literally as much money as you can possibly carry. However, the Ruins has a branch of the sale that sells treats much cheaper.
  • If you buy a spider treat in the Ruins, then eat it during the fight with Muffet, she'll let you go right away. She'll initially believe you stole it from the Hotland bake sale, but a spider will come in and tell her you really did buy it from the Ruins.
  • If you actually go through the effort to grind the GOLD required to buy a treat from Hotland, you won't even get into the FIGHT mechanic; Muffet will immediately mention your generosity, and let you go without a fight.
    Muffet: However... ...that can't be true, can it? Ahuhuhu... You donated so much money to us. All the spiders want you to know how thankful they are! We're in your debt, dearie~
• The Outsider Befriends the Best: In the pacifist ending, Frisk can be adopted by Toriel, who in the past was the Queen of the Underground. They also become a friend of King Asgore. And this is not the first time that Asgore and Toriel become friends with and adopt a human child.
• Overly Long Gag:
  • A pack of instant noodles almost takes as long to cook in-game as it does in real life. And is about as nutritious. If you're in "Serious Mode" (i.e. in certain boss battles or on a Genocide Run), you eat it raw and it heals up to 90 HP instantly.
  • Petting the Lesser Dog, and watching it stretch itself to impossible lengths. This can go on for a very long time.
  • Ringing the bell on Doggo's station. The poor guy can be tormented by it for good amount of unique responses.
  • If you buy a Hot Dog...? from Sans with a full inventory, he'll put it on your head. Talk to him again, and he'll put another there for free. And then another, and another, until he decides that 29 is enough because he can't reach any higher. If you walk around, they'll come crashing down. Then you can get more by speaking to him.
• Overly Narrow Superlative: During a phone call with Papyrus while in Hotland, he states that Mettaton is his "favorite sexy rectangle".

Trope P

• "Pachelbel's Canon" Progression: Appears partway into "Fallen Down (Reprise)", specifically at the part that quotes the game's main theme.
• Pacifist Run: Not only possible, but the only way to get the Golden Ending is to not kill anyone during the game.
• Painting the Medium:
  • Sans and Papyrus talk in… well, Comic Sans and Papyrus. The character W.D. Gaster speaks in Wingdings.
  • When the Fallen Child destroys the world at the end of a No Mercy run, the game window repeatedly shakes, mirroring the animation for One Hit Killing a boss.
  • During the shows that Metatton forces the player through, the name of the game on the top right of the window changes if you aren't playing in full screen.
  • When you reach the end of the neutral route, Flowey crashes the game to desktop. If you die during the final boss, the game will also automatically crash to desktop.
• Permafusion: The Amalgamates are the result of a botched experiment that attempted to implant Monsters with Determination; this caused the monsters to literally melt together into monstrosities that are one of the few creatures in the underground that the player cannot kill in any way, shape or form; trying to hurt them only causes their lifebars to grow (With the exception of Snowdrake's Mother whose lifebar can be depleted when hurting her but her lifebar is completely restored in the next turn. While she "can" be killed in one hit with certain glitches or hacks to increase your attack power high enough, she does not give EXP as it is not possible to do normally in the True Pacifist Route).
• Permanently Missable Content:
  • Getting the bad ending forever taints any future Pacifist ending (barring the player editing the games files directly), locking the player out of the regular ending cutscenes for the Golden Ending.
  • Photoshop Flowey can only be fought once on any given save file prior to a True Reset. Flowey still appears after Asgore's death to finish off his SOUL, but decides against absorbing the human SOULs and taking over the game because he remembers what happened last time and knows you'll just defeat him again if he does.
  • After the battle with Toriel, you're locked out of the Ruins until the Playable Epilogue. The primary consequence of this as far as the rest of the game is concerned is the Ruins is the only place you can buy spider pastries cheap, which can make the encounter against Muffet significantly easier.
• Phlebotinum Overdose: One of Alphys' experiments in barrier-breaching research was to inject monsters with human Determination as a substitute for SOULs. This ended up being a very bad idea: apparently neat things happen if a monster gets the Determination threshold just right (such as Undyne the Undying and possibly Sans), but even just a tad too much will cause a monster's magical body to melt. You see this very thing happen to Undyne if you kill her, whose will not to die creates more Determination than her body can handle. The Amalgamates in the True Lab also came about this way when several melted monster bodies fused together.
• Planet of Steves: Temmie Village. Apparently, all Temmies have "Temmie" as a name. Except for Bob, that is.
• Platform Hell: Most encounters tend to play as Bullet Hell, but some require extensive platforming, especially the final boss of the No Mercy run. You're gonna have a bad time indeed.
• Playable Epilogue: In the Golden Ending, you can go back and talk to the NPCs around the world. Like in EarthBound, they will have unique dialogue to share; most of it isn't too important, but back-tracking to the very first room in the game gives you a bit of a surprise.
• Player Death Is Dramatic: During battle, the human's SOUL is visible when fighting monsters. If the human's HP hits 0, then all monsters on screen, along with the entire interface, vanish. Only the human's SOUL remains visible before cracking, and promptly shattering into pieces, signifying the human has died.
• Player Nudge:
  • If you fail the Behind the Black puzzle in the Ruins enough, the game will pop a text box telling you that there's a switch behind the first pillar.
  • If you kill Toriel, Flowey will snidely remark that "it's not as if you can go back and change fate". If you're playing the demo, his comment on the last page of the manual will also change, taunting that "you didn't even try to SPARE her".
  • If you've killed Toriel and reloaded a SAVE, then try to talk to her twice, the game will give you a hint on how to get past her without killing her^note "Can you show mercy without fighting or running away...?". You don't get the same hint if you're facing her for the first time.
  • Completing a Neutral path with kills at least twice (ignoring his earlier hint to finish the game without kills) will have Flowey suggest that it might have been better not to care about anyone at all, hinting at the No Mercy path. Even if you did kill some monsters, as long as you at least spared Flowey, he'll pretty much spell out the exact conditions for a True Pacifist ending.
  • A very subtle one in Waterfall: the area is long enough and has a low enough monster population that if you play the game like a normal RPG and kill every monster you encounter without deliberately grinding, you're likely to fully depopulate the area and get the "but nobody came" encounter. A curious player might then wonder what happens if you reset and do this everywhere...
  • Eventually, during Undyne's boss fight, she'll insist that you will never escape from her, hinting that you have to flee from her in order to get past her without killing her.
  • Completing a No Mercy run after you've already done one and sold your SOUL will have the Fallen Child suggest you take a different path, hinting at the True Pacifist path and the corrupted ending you get for completing it without a SOUL.
  • After the 1.01 update, the solution to the Waterfall piano puzzle is displayed on the wall after giving the statue an umbrella.
• Playing a Tree: During the ending credits, Burgerpants finally gets his chance to be on stage with Mettaton. Unfortunately, it's as a bush.
• Point of No Return:
  • After you leave the Ruins if you spared Toriel, the door locks behind you, preventing you from going back.
  • There appears to be one at the end of the game, with a Save Point appropriately named "The End" located right before the final boss, who's waiting in the door right next to it. But even so, the final boss asks that you finish any business before you're ready to fight him. You can still leave and go back to the rest of the underground even after saying you're ready by dying in the fight, the only consequence being that you skip his opening monologue. The actual point where you can't go back to the rest of the underground happens elsewhere, depending on your ending path.
  • On Neutral: Right after you defeat Asgore. Thanks to Flowey stealing the human souls, he locks you out of your save file until you beat his powered-up form and reach the ending. But afterwards, you can load your game to right before fighting Asgore, allowing you to explore again as well as try for the pacifist ending if you haven't killed anyone.
  • On Pacifist: By heading back and entering the True Lab. A power failure causes the entrance elevator to crash and prevents you from leaving until you restore power and finish the area. But then, when taking the exit elevator, another accident happens and carries you too far, depositing you at New Home. But now, a mysterious tangle of vines has sealed the lift doors shut, preventing you from going back until you reach the Playable Epilogue.
  • On No Mercy: Entering the Throne Room and meeting Asgore (though the preceding fight with Sans unique to this path can also be this from a thematic standpoint). The ending then plays out immediately, with no effort needed on your own part, and the game ends. Also, due to what happens in the ending, you can't reload and explore anywhere anymore; not without resetting the game, at least. However, the events from this ending will follow you through the reset and affect your file in certain ways, making it a pretty big point of no return.
  • The No Mercy run itself is this; specifically, the fight with Mettaton NEO. If you don't kill enough monsters in Hotland, he tells you you can't be all bad and lets you go, putting you on the track for a neutral run. Apart from just giving up and quitting, this is your last chance to end the No Mercy run, story-wise.
  • The biggest point of no return happens when you make your deal with the Fallen after the battle with Sans; from then on out, the True Pacifist Route's ending and future No Mercy Route endings will be permanently altered.
• Poison Mushroom: The "Bad Memory" item you can get by using the ITEM act against Memoryhead. It can't be dropped and consuming it deals 1 point of damage...unless you're at 3 or less HP, in which case it fully heals you.
• Poke the Poodle: Among the 'evil' things you can do is take multiple pieces of candy from a bowl that says "take one". Additionally, the game itself does this on the hidden Hard Mode:
  Narration: In this hellish world, you can only take 3 pieces of candy.
• Post-Final Boss: In the No Mercy route, Sans the skeleton is the effective Final Boss, as the next two bosses, Asgore and Flowey/Asriel, are killed via cutscene.
• Post-Final Level: New Home comes right after the Core, a big final area with strong enemies (or, in the True Pacifist route, the True Lab), and is a short trek through a few rooms before facing the various final bosses in each route.
• The Power of Hate:
  • Monsters are chiefly composed of magic, and are vulnerable to strong, negative emotions. If you play the game violently, you can overwhelm some bosses and kill them almost instantly. In one instance, the Mad Dummy will bond with its body due to the intense hatred it has for you (which opens it to attacks as it is now corporeal).
  • The Fallen Human implies at the end of a Genocide route that the player's sheer violence and bloodlust against the creatures of the Underground gave their body the power to reanimate. If the outcomes of any True Pacifist routes following your first Genocide are anything to go by, this certainly seems to be the case...
• The Power of Love:
  • Frisk is implied to be powered by a mix of love and determination during the battle with Asriel, allowing them to reduce damage from attacks, create healing items, and resurrect themself shortly before their deaths.
  • On the Nintendo Switch, you can encounter Mad Mew Mew, a Mew Mew doll inhabited by the ghost from the Mad Dummy. If you talk to her, she starts to fuse with her new body through love.
• Pre-Asskicking One-Liner: Both major bosses of the No Mercy route have them.
  • Undyne the Undying, after reforming after an attack that should have killed her through pure determination, smirks at you and tells you that "You're gonna have to try a little harder than THAT!" right before your battle with her starts.
  • The final boss, Sans has a Call-Back to the monolgue given by the final boss of Neutral runs. But while that speech is regretful and somber, his is pure bitterness for everything you've done so far.
    "it's a beautiful day outside. birds are singing, flowers are blooming... on days like these, kids like you.... Should be burning in hell."
• Pre-existing Encounters:
  • Most non-plot important enemies in the game are scripted to be encountered in specific areas, though natural random encounters exist alongside them. This guarantees you'll encounter most enemy types at least once because how you deal with them determines what kind of neutral ending you get or what the credits in the Golden Ending say.
  • One puzzle in the Ruins features a couple of Vegetoids half-buried in the ground that emerge to attack when examined.
• Pre-Insanity Reveal: The Ax-Crazy Flowey turns out to be Asriel Dreemurr, the son of Asgore and Toriel after losing his soul and thus sense of empathy, combined with realizing he has the ability to reset the world and thus none of his actions are permanent or matter.
• Prematurely Marked Grave: In New Home, there is a room with seven coffins marked with a heart in a different colour. These are the coffins of the previous humans, as monsters don't leave behind a solid body, and the hearts represent their souls. The closest coffin has a red soul like the player's, is empty, and has the the name entered at the beginning of the game. However, this turns out to be a subversion. The name you entered is not the player character's name, but instead the first human to fall, and the coffin was theirs. It was empty because their body was taken and buried elsewhere.
• Production Foreshadowing: On certain playthroughs, you can find a monster known as Clam Girl in Waterfall. When talked to, she'll tell you that her neighbor has a daughter named Suzy, and while the game never gives you the chance to meet her, Clam Girl believes the two of you would be good friends. After the game's 1.001 patch, she'll also cryptically suggest that Suzy may be the reason you've come to the underground in the first place, and after talking to her, you can find a hidden drawing in Sans's basement that shows three smiling people and the words "don't forget". Finally, in the Nintendo Switch port of the game, if you talk to Clam Girl during the True Pacifist epilogue, there's a chance that she'll tell you that even though you may not have met Suzy today, the day when you will is fast approaching. All of these appear to be allusions to Toby Fox's next game, Deltarune; its first chapter was released less than two months after Undertale's Switch port, its theme song is titled "Don't Forget", and it focuses on a party of three heroes, one of whom is a monster girl named Susie.
• Production Throwback:
  • A remix of MeGaLoVania (a song written by Toby Fox for his EarthBound Halloween Hack which later appeared in Homestuck) plays during the fight with Sans at the end of the No Mercy route.
  • One of this game's Final Bosses, Photoshop Flowey, has a similar game over sequence to the hack's Dr. Andonuts: laughing endlessly and forcing a reset.
  • The last set of minibosses in The Halloween Hack and this game have something very important in common: They're both called Amalgamates.
• Product Placement: In-universe example during Mettaton EX's boss fight. One thing you can do to increase ratings is eat MTT brand foods.
• Prophecy Twist: The prophecy of the Delta Rune foretells that an angel who has seen the surface will return, and the underground will go empty. Frisk fits the bill… but so do Asriel and Chara. In a Double Subversion, the prophecy comes true in both of the more extreme endings: either Asriel destroys the barrier in the Pacifist ending, or Chara makes the Underground go empty in the Genocide ending.
• Protagonist-Centered Morality:
  • Subverted big time. Spare the major encounters but kill common monsters on the way? You'll get called out on it.
  • Lampshaded by Alphys: "Watching someone on a screen really makes you root for them".
  • Played Straight in a way with Undyne. Undyne is the first major character shown to make an earnest effort to try and kill the Player Character^note Followed by Mettaton, who tries to take the character's SOUL to prevent Asgore from reigniting the war with humans, who is a child. She is not called out for this, and befriending her is required to get the Golden Ending.
• Pun-Based Creature: There's Snowdrake, a monster that looks like a drake (a male duck), with a snowflake-shaped plumage on his face. There's also Aaron the seahorse, which is a muscular, horse-headed merfolk.
• Punctuated! For! Emphasis!: When Undyne recognizes you as the same human that she tried to kill and ended up being saved by, after Papyrus introduces you as a friend.
  Undyne: Why don't. You two. Come in?
• Punny Name:
  • (Tu)Toriel
  • Alphys = Alpha / Physical
  • Undyne = Undying. Also, to a lesser extent, Undines.
  • Papyrus and Sans are a 'font family'. The Retgoned W.D. Gaster is also in on this.
  • Snowdin = snowed in. The inn there is called Snowed Inn. It's also a pun on Snowdon Mountain, a region in Wales.
• Purpose-Driven Immortality: If you die, your determination keeps you going (and restarts your adventure at the previous save point). If you die to Asriel, your determination tells death to get dunked on.
  Narration: But it refused.
• Puzzle Boss: On a pacifist/low kill run, all encounters essentially become this, with the 'puzzle' being working out how to end the encounter nonlethally.

Trope R

• Rainbow Speak:
  • Parodied when Sans warns you about Papyrus's blue attack — he comes up with a bizarre mnemonic about blue stop signs.
  • There's a gag where the dogs classify human smells as "GREEN rating", because dogs are red–green colorblind.
• Random Encounters: The protagonist shows a "!" speech bubble just before an encounter starts. Initiate the Genocide run, and the speech bubble changes to a smiley face similar to Flowey's and nobody shows up in the encounter.
• Random Event: The "fun" value in your save file is set to a number between 0 and 100 when you start a new game. Depending on what it is, you'll see an event corresponding to that number in a certain location, although values in the 60s have a lower chance of occurring.
• Rank Scales with Asskicking:
  • Asgore, the king of the Underground. After you befriend her, Undyne will let you know that Asgore is not to be messed with. She had only ever beat him in a sparring match once, and that was after he specifically taught her how to. And when you finally do fight him, he proves that he is anything but a pushover.
  • Muffet is the leader of the Spider Clans, and one of the more challenging bosses in the game, with her own SOUL mechanics.
  • Undyne is the captain of the Royal Guard, and she's as tough as she looks. In fact, the only way to actually beat her non-lethally is by running from her, since there's no way to spare her by AC Ting. She REALLY shows this on the Genocide Route when she transforms into Undyne the Undying.
  • Completely defied with Sans, who, despite his laziness and general slacking off while on duty, absoultely will kick your ass when you fight him at the end of the Genocide Route.
• Readings Are Off the Scale:
  • During your hangout/date with Papyrus, the final stretch of your "Friendship/Dating Power" bar just keeps going right through the bounding box until the screen fades to white.
  • After the January 2016 patch, Temmie no longer has ATK and DEF listed, instead reading "RATED TEM OUTTA TEM". On a related note, Mad Dummy's stats also include "DEF: YES"
• "The Reason You Suck" Speech:
  • Flowey loves these. Pretty much every single time he's on screen, he's mocking and/or gloating over the player, usually to rub in how much power he has over them rather than the other way around.
  • The Genocide Route's final boss fight is one giant one directed at the player, with Sans spending the entirety of the fight mocking and belittling the player and constantly reminding them of how much of a horrible person they are.
• Recurring Riff: There are a number of these, mostly having to do with character Leitmotifs. The most prominent riffs are the titular "Undertale" theme and the Ruins theme, which are featured in a bunch of other songs, especially towards the endgame. All the tracks that play during the Final Boss of the Pacifist Route in particular are a giant amalgamation of several recurring riffs and Leitmotifs throughout the game.
• Recycled Soundtrack: A number of songs in the game were composed for earlier projects, such as "Megalovania" (which was previously used in The Halloween Hack and Homestuck).
• Red Herring:
  • The game heavily misleads you as to how you're supposed to spare Toriel, to maximize the chance that you'll accidentally kill her, then reload so you can spare her properly.^note Specifically, the frog that tells you monsters can be spared at low HP is given the same amount of focus as the one that says you might need to use the Spare command even when a monster's name isn't yellow, and Toriel's dialogue when you do try to keep sparing her looks almost the same at first. The scene that results if you do is one of the first major Wham Lines, but it wouldn't have the same impact if you had been Railroaded into it, and the number of Lets Players that have done just that shows that it works.
  • A music-based one. The soundtrack has a song called "Song that Might Play When You Fight Sans". It doesn't. Instead, when you face down Sans at the end of a No Mercy Run... you get Megalovania.
  • One room in the True Lab has a row of refrigerators you can interact with. The fridge in the middle shakes intermittently, but has Flavor Text stating that it's empty. The area has monster encounters routinely come from unexpected places, priming the player to think the fridge will begin an encounter. Nothing happens after you interact with it. The monster encounter begins after interacting with the leftmost fridge, which doesn’t move at all and has the same flavor text as the shaking fridge.
• Red Oni, Blue Oni: Papyrus is the Red Oni, because he is overconfident and enthusiastic, wears a red cape, and dreams of being a royal guard. Sans is the Blue Oni, because he dresses in blue, is more likely to spend time chuckling at his brother's antics from the corner, and has a serious and philosophical side that he displays at times.
  • Two areas later, you have a similar dynamic between Mettaton (Red) and Alphys (Blue): while Mettaton is an overly dramatic character who focuses on entertaining, moves across multiple areas for different shows, and hides his true colors until he's entirely sure nobody will interfere, Alphys stays behind her computer lab most of the time, has trouble holding a simple conversation, and lacks confidence in herself because of her past failures as the royal scientist.
  • Another dynamic can be found between Flowey (Red) and the Human Child (Blue): both of them have a strong determination which allows them to rewind time to their last save (but the human's is stronger, leaving the flower as a simple spectator) if they died or want to repair something. However, while the Human Child is a quiet character who makes strong efforts to hide their powers to everybody, Flowey will taunt them with his memories of the events.
• Reduced to Dust: Here, monsters don't leave corpses when they die. Instead, their body disintegrates into dust.
• Released to Elsewhere: In Neutral endings where Papyrus is alive but all the other major characters ^note Toriel, Undyne, Mettaton (and, by extension, Alphys), and Asgore are dead, Papyrus says Sans has explained their absence to him by claiming that they have all "gone on vacation". Sans apparently didn't have the heart to him tell him that they were dead.
  • This is also the core of the secondary interpretation of the Delta Rune prophecy — that the "angel" will free the monsters by killing them all. Flowey flat-out suggests that he and the player do this if the Genocide Route is initiated.
• Removed from the Picture: A non-romantic variant. If you complete a Genocide Run and then attempt a True Pacifist Run, telling Toriel "I have places to be" results in the photo shown at the end of the final scene changing — Frisk is replaced with Chara, and the faces of your friends have been crossed out with red marker.
• Replay Value: The game has many paths and many easter eggs that occur if you replay it. At the same time, once you get the True Pacifist ending… Flowey will actively dissuade you from replaying it so as to not ruin the happy ending you have created for the characters for your own sick amusement. Of course, maybe you just want to go for True Pacifist again (which is kind of Heartwarming in a twisted way), but the message is probably targeted primarily at the many players who are at this point undoubtedly tempted to go for a Genocide run…
• The Reveal:
  • At the end of the Pacifist path, you find out that "Fallen Child" — the character you name at the beginning of the game — wasn't the protagonist (they have their own name, Frisk), but rather the "first child", Asriel's best friend.
  • Right before Undyne battles you, she tells you that your soul is needed so that King Asgore Dreemurr can become a god, revealing that the benevolent King Dreemurr and the monstrous Asgore who Toriel had warned you of are the same person.
  • When you reach the center of the CORE, Mettaton tells you that he actually has no desire to harm humans, and that he just wants to entertain. He and Alphys were working together the whole time and you were never in any real danger before you entered the core — Alphys wanted him to play the villain so that she could 'save' you from him.
• Rewarding Inactivity:
  • In some rare cases, it's better to just stand still to avoid enemies' bullet patterns. Blue attacks enforce this: they won't hurt you as long as you don't move when they are passing. Orange attacks invert it — they won't hurt you as long as you are moving as they pass.
  • In the CORE, a forcefield blocks your way to the exit, forcing you to take a longer path that requires you to solve puzzles (the Sage's path) or fight powerful monsters (the Warrior's path). However, if you just stand still in front of the forcefield, it will eventually turn off and allow you to proceed without having to do either one.
• Rewatch Bonus:
  • The goat-like monster and child seen within the intro may seem like random characters at first, until you realize they resemble Asriel and the Fallen Child, respectively.
  • The intro sequence almost gives away, to the observant, the fact that the child shown isn't the main character, but rather, the First Child: The main character's shirt bears two stripes, but the First Child's shirt has only one, as shown in the intro.
  • Also, take note of the area where the character in the opening scene falls. No flowers. What's the first thing you notice when you play the game? The bright, golden flowers your character fell upon.
  • The circled passage in Toriel's diary is a skeleton-related pun. She and Sans have been talking through the door of the ruins, including exchanging puns, for some time.
  • The cameras you can find here and there are probably Dr. Alphys' surveillance.
  • In Toriel's home, the three rooms on the right are a child's bedroom, Toriel's bedroom, and a third room under renovations. When you get to Asgore's home, the child's bedroom is available, but Toriel's is under renovations and Asgore's room is open instead.
  • The tune from the end of the program Papyrus was watching in his house is Mettaton's leitmotif.
  • At the randomized puzzle that Papyrus tries to make you do, the control panel is Mettaton◊.
  • If you read the lyrics to Mettaton's Opera before he starts talking about dumping you into the dungeon, the lyrics are his honest-to-God feelings on humanity, wanting to keep you away from Asgore because he doesn't want the monsters to break out and kill them.
  • Several, if not all, of the "dirty jokes" you tell Woshua are actually allusions to the backstory of Asriel and the Fallen Child, talking about kids who played in a flower garden and ate pie together as well as "a kid who slept in the soil".
  • In the Genocide route, Sans makes a seemingly nonsensical comment apologizing to an "old lady" about breaking a promise. It is only revealed if you spared Papyrus in another run that Toriel made Sans promise to protect any human who came from the ruins.
  • Napstablook mentions that their snail farm used to have a regular customer, but she moved away and now it's just some hairy guy that turns up every so often. An observant player may already have deduced that the regular customer who moved away was Toriel (who makes snail pies), but only in the light of The Reveal that Asgore is a large hairy monster and also Toriel's ex-husband does it become clear that he's been visiting the snail farm periodically because it reminds him of his wife.
  • Papyrus can mention getting toys from Santa. When first playing the game, it's easy to write that off as a joke about Papyrus's naivety, and it's only a long time later that the player has the option to discover a Santa Claus outfit in Asgore's closet, implying Papyrus's toys actually came from him.
  • If you already know the twist in Hotland that Alphys has been arranging all Mettaton's deathtraps so she can play the hero by "rescuing" you, it becomes obvious at several points that she's forgetting or flubbing her pre-scripted lines and Mettaton is covering for her. Further, it's apparent that once you reach the CORE, Mettaton's starting to actually work against her, since the directions she gives you at first are completely wrong, indicating the CORE is no longer configured to her expectations.^noteAlphys also says she has a map while a worker at the restaurant clearly said the CORE can be remapped, an eventuality she should be prepared for as the royal scientist. Also, she's surprised to find monsters there when she isn't expecting any, and the room with the lasers isn't dealt with as easily as previous ones — she has to resort to a Plan B to take care of them, and even then, she has to keep fighting to shut the lasers down.
• Rhyming with Itself: The check dialogue for the Dummy is 'A cotton heart and a button eye, You are the apple of my eye'
• Riddle for the Ages: The game has multiple questions that heavily hint towards an answer, but are never outright said. These questions may or may not be solved in the future, though given that Toby Fox is also active in the Homestuck fandom, it was likely deliberate to create Fanfic Fuel and the only true answer being "whatever you decide it is".
  • How long was it between the ancient War and the time the game takes place? Bratty and Catty say it's been millennia and the backstory in Waterfall mentions magicians, but the human shown in the opening looks like a stone-aged tribal. Furthermore, how many years have passed between 201X and when the game actually takes place? This is never even hinted at.
  • What were the Fallen Child's motives for killing themself? They hated humanity, that's outright stated, but did they kill themself for Asriel to take the six more SOULs to free the monsters, or did they just want to kill humans? Further, did they actually love their adopted family, or were they just a convenient stepping stone for them? Finally, were they even human to start with, or is their comment at the end of a second No Mercy run about being "the demon that comes when people call its name" meant literally?
  • Just who is Doctor Gaster in relation to the cast? Is he related to the Bone Bros somehow? What was the experiment that scattered him through space-time? Why does it seem like Sans is the only one who remembers him and what is the purpose of that machine?
  • Who were the six SOULs and what were their motivations to go to Mt. Ebott to begin with? All we know about them are the main trait of their specific SOUL and which items they used.
• Rimshot: Two of the jokes told by Sans when you first meet him are punctuated this way.
• Ripple-Effect-Proof Memory:
  • The protagonist has one, being able to recall what happened during previous playthroughs. Flowey has one as well. Repeating the opening sequence has him grow more and more angry at you. It also appears that killing him is ripple-proof… until you reach the end of the New Game Plus. He was just pretending to stay dead to taunt you.
  • In the full version, everyone seems to have it to some extent. Toriel, for instance, will remember whether the protagonist prefers butterscotch or cinnamon. Sans is… a weird case. If anything, his memory is weaker than everyone else's, but he supplements it with knowledge of the fact that there is a time loop in the first place, very good guesswork, and being Crazy-Prepared — and he manages to keep a memento from the true ending, after you do a true reset, which is supposed to wipe everything. This is also weaponized by the protagonist in the True Ending's "I Know You're in There Somewhere" Fight Boss Rush after Asriel erases their memories.
• Rousseau Was Right: According to Word of God, all of the monsters that the player encounters are basically good and only doing what they think is right, with the exception of Jerry. Even he seems to be mostly an unpleasant but not particularly destructive kind of evil.
• Route Boss: Depending on which ending you're going for, several boss fights play out quite differently.
  • In any Neutral route (any playthrough where you don't go to the True Lab), the last two bosses are Asgore, followed by Photoshop Flowey. It should be noted that you cannot fight the second one indefinitely; if you beat him in two playthroughs, he won't fight you again because he knows he'll lose, thus locking you out of the fight.
  • If you do go through the True Lab, the fights against Asgore and Photoshop Flowey are skipped and instead you fight the God of Hyperdeath. This is also the route that unlocks the Golden Ending.
  • If you're going for endings such as Annoying Dog as king or Genocide / No-Mercy run, you cannot skip Muffet.
  • The Genocide / No-Mercy route has by far the biggest changes, even if most of the individuals that fight you are the same.
    • Papyrus doesn't fight you, he just stands there. This is in sharp contrast to his Wake-Up Call Boss status in all other routes.
    • Mini-Boss Mad Dummy turns into Glad Dummy and also refuses to fight you.
    • Undyne is killed in one hit and turns into Undyne the Undying, who is much, much more difficult.
    • Mettaton ditches the show-ratings thing and the EX form, and instead shows up in the NEO form. He goes down in one hit, which is far different from his That One Boss EX fight.
    • This is the only route where you can fight Sans. He serves as the Final Boss. Asgore and Flowey are dealt with in cutscenes instead.
• RPGs Equal Combat: Hahaha, no. The game's tagline is "The Friendly RPG Where No One Has To Die", and one of the first things the game tries to hammer into your head is that you can progress without killing everything in your way; even though monsters will initiate combat with you, they're not necessarily hostile, and you don't have to fight back to get past them. If you try to play the game as that kind of RPG, expect some nasty surprises later on down the line.
• Ruins for Ruins' Sake: The aptly named Ruins, the first area in the game. It used to be where all the underground residents lived, but they expanded out over time, leaving only a few monsters scattered around.
• Rule of Seven: The monsters need the power of seven human souls in order to break the barrier keeping them underground. They already have six of them when the player character stumbles into their kingdom…
• Running Gag:
  • Save points based on mice someday being able to get at the food in the room. In the True Ending's Playable Epilogue, they all do — though the mouse that tried to eat Papyrus' spaghetti ended up giving up.
  • Variations of "Smells like x," where x is something related to the monster you're currently facing.
  • Mettaton's entries:
    Alphys: Oh no.
    Mettaton: OHHHH YES!
  • The antics involving the Annoying Dog. One time, it's preventing you from obtaining an artifact, while another time, it eats the bone that is meant to be for Papyrus's special attack…

Trope S

• Sacred Hospitality: Comes up if you visit Undyne's house. Much as she'd like to kill you where you stand, she can't, because you're a guest. Eventually defied, as the disastrous "cooking lesson" pushes her over the edge and she attacks you anyway, outright saying she no longer cares if you're her guest.
• Sadistic Choice: When reaching the end of a neutral run as a pacifist, you find out you must absorb a monster's soul to pass through the barrier. As a result, you are faced with a choice of either resorting to killing or being trapped in the Underground forever.
• Save-Game Limits: You only have one save file, and you must reset it to start a new game. This isn't because of engine limitations, but rather because it ties in with the game's theme; your choices are mostly permanent because you can't save to multiple files, and that makes events like Flowey replacing your save file with his own much more impactful. The game itself implies that a human SOUL is what creates the ability to save, thus explaining why you have one save slot and why Photoshop Flowey with six human souls in his possession has six slots. In addition, the implications of resetting the game because the single file is limiting are commented on by several characters, namely Flowey after a True Pacifist ending and Sans during a No Mercy run.
• Save Point: Appears as a yellow star-shaped icon. The manual describes it as a manifestation of your determination — something in the environment focuses the protagonist's will and enables them to record their progress. In the True Lab, one of the Amalgamations disguises itself in the overworld as a save point (hinted by the fact that it's blocking the way forward).
• Save Scumming:
  • Discussed. Characters in the game will react if you save and go back to previous events. Going back after killing Toriel and saving her reveals Flowey is able to do this as well, or was able to until you came along.
  • Your character actually considers telling Toriel they killed her last time if you use the "Talk" option on Toriel after reloading. You wisely think better of it on the basis it could make for an awkward conversation.
  • Used against you during the battle with Photoshop Flowey. If you dodge an attack from him, he'll immediately reload a save-state from before the attack (File 2) and try to hit you with it again. If you succeed in killing him, he'll start into his death-monologue… then reload a save-state from the start of the fight (File 3), bringing him back up to full health, and he saved once again (File 6) just to finish his speech while preventing you from running into the bullets.
• Schmuck Bait:
  • In Waterfall, the bridge seed puzzle's goal is to cross a body of water to progress, but you can also use the seeds to reach an island with a sign on it. The sign reads "Congratulations! You failed the puzzle!"
  • During the final battle in a Genocide Run, Sans will seemingly grant you one final chance to abandon the run by offering to spare you. If you try to take him up on this offer, however, he hits you with a One-Hit Kill attack, leading to a Game Over.
• Screw This, I'm Outta Here: While you can do this to any monster you're having difficulty befriending without affecting your Pacifist run, you have to do this to "defeat" Undyne peacefully — no amount of talking or stalling will allow you to spare her normally, you just have to flee the battle and keep running.
• Script Breaking: Prior to the January 2016 patch, it was possible, if you timed it right, to get onto the Golden Ending path despite having killed enemies^note Backtrack until you get the phone call from Undyne, starting the "Befriend Alphys" quest, then return to the core and start killing monsters. The game doesn't actually acknowledge this; if you skip the True Lab and fight Asgore, you don't get the "dirty hacker" ending because the game's logic checks your kill count before it checks whether you've befriended Alphys (so you get the same ending you would have gotten if you hadn't broken the script), and if you do enter the True Lab, the game doesn't check your kill count at all, since you usually have to have a kill count of zero to get there to begin with, so that's what it assumes you have.
• Sdrawkcab Name: Subtle. "Ebott" backwards is "Ttobe", which can be pronounced "Toby", as in Toby Fox.
• Seasonal Baggage: The four main regions are each loosely themed after the seasons. The Ruins are autumn, as there are red leaves all around and it's the first place the player character sees after a literal fall; Snowdin is winter, a snowy forest with Christmas references; Waterfall is spring, being a literal spring and having flowers play a part in the area's geography; and Hotland is summer, a hot region where you find that the schools are on summer vacation.
• Secret Test of Character: On a meta-level, Undertale as a whole seems to be this for the player. A major running theme is that your actions have lasting consequences even within a fictitious video game world. Even if you reset the game and try again, the game subtly (and not-so-subtly) "remembers" what you did the first time around — whether you went out of your way to spare a monster or whether you just killed it. The implication is that the first thing that the player decides to do reflects their true character, and if you go back and make a different choice after seeing the outcome, you're either trying to hide or fix your mistakes or just trying to see a different outcome for your own amusement.
• Seemingly Hopeless Boss Fight: The Final Boss of each run.
  • Neutral: In the fight with Photoshop Flowey, the player can only deal Scratch Damage, the human souls only being able to heal the player from time to time. Eventually, though, the souls render Photoshop Flowey vulnerable to damage.
  • Pacifist: Asriel Dreemurr completely paralyzes the player upon assuming his second form. It's only after a few rounds of futile struggling that the player gains the option to SAVE some of the souls Asriel had absorbed — and eventually Asriel himself.
  • Genocide: Sans's "special attack" is literally nothing. He tries to make his turn last forever, denying the player any chance to attack. If the player waits long enough, though, Sans will fall asleep, giving the player a chance to move the bullet board over to the "Fight" option and attack.
• Self-Deprecation: One save point in the Waterfall trash dump describes a very long process involving worthless garbage going down the falls into the abyss as filling you with determination. If you use that save point again, it just says, "Partaking in worthless garbage fills you with determination."
• Sequelitis: In-universe, with Alphys' online rant about Mew Mew Kissy Cutie 2:
  Alphys: Mew Mew Kissy Cutie 2 Is Neither Kissy Nor Cutie. It's Trash. 0 stars.
• Seven Heavenly Virtues: There are seven traits associated with the seven colors you encounter in battle, and each one has a rough analogue to, or shares a name with one of the virtues.
  • Patience is cyan, and shows up in battle on attacks you can only dodge by standing still.
  • Bravery is orange, and is most likely a counterpart to Humility. It shows up in battle on attacks you have to run through in some manner to dodge.
  • Integrity is blue, and is likely Chastity's equivalent. It shows up in the battle against Papyrus and the one against Sans as an effect on your SOUL, turning enemy attacks into a platformer segment.
  • Perseverance is purple, and is Diligence's counterpart. It shows up very rarely, only appearing in the battle against Muffet as a SOUL effect, restricting your ability to dodge by locking it to lines you can jump between.
  • Kindness is green, and shows up in battle as both healing effects, and as a SOUL mode in Undyne's battle, preventing you from moving, but allowing you to block incoming bullets.
  • Justice is yellow, is a counterpart to Charity, and appears in battle the only purely beneficial SOUL mode, allowing you to shoot down bullets in the fight against Mettaton EX.
  • The red soul has no stated trait, but probably is meant to represent Temperance. It's the default soul mode that allows you to move freely across the board, and is shared by both Frisk and the first fallen child.
• Shaped Like Itself:
  • Asgore names the first underground cavern the Monsters settled into as "HOME." When they left for another area, the place they eventually settled into was called "NEW HOME." Asgore's poor naming sense is duly acknowledged by historical texts.
  • Some of the flavour text when fighting Mettaton just reads "Smells like Mettaton."
• Shared Signature Move:
  • Toriel and Asgore can both summon fire from their hands that attacks the player from above. Makes sense given that they were once a couple.
  • Brothers Sans and Papyrus can both summon columns of bones to attack their opponents. But in particular they can perform the "Fabled Blue Attack". This inflicts a status effect on their opponent that limits their mobility, making them more susceptible to consecutive attacks. In his boss fight, Sans uses this ability to more relentless extremes, bombarding the player character with onslaughts of attacks including the aforementioned bone attacks and his "Gaster Blasters".
• Sheathe Your Sword: All battles can be won by "sparing" enemies instead of attacking them, though you may have to perform special actions first; a full Pacifist Run requires this. This is practically its own puzzle during the first Boss Battle: the game starts telling you that talking won't work.
• She Cleans Up Nicely: Undyne during the protagonist and Alphys' "date". When not fully-armored, she normally wears simple jeans and a tank top with her hair pulled back, but here puts on a stylish jacket, sweater, and lets her hair down. It becomes obvious that she is trying to look good for Alphys. Also applies to Alphys, who trades out her white lab coat for a black and white polka-dot dress during the date.
• Ship Sinking: Toby sunk pretty much every ship that Sans launched when he tweeted that he's too apathetic and lazy for any kind of committed relationship. To be fair, though, it's strongly suggested that he is starting to get better in that regard by the end of the Pacifist Route, and will presumably continue to do so if he lives a good life on the surface, so there may still be hope yet.
• Ship Tease: Despite how much Toriel hates Asgore in the Pacifist Ending, Alphys still ships them and asks you if you think they might get back together. While she seemingly wants to have nothing to do with him, they are working in close proximity at least in the epilogue.
• Shoo Out the Clowns:
  • An odd case of this happening to the items on the Neutral and Pacifist routes: during the final bosses, any item that had a comedic nature to it will have said comedic element removed (the Butterscotch Pie was shorted to ButtsPie, but in the final boss fights, it will be simply Pie; the instant noodles will be eaten dry and fast (they actually restore far more HP when eaten in this way); and the "Hot Dog?"/"Hot Cat" items will both be referred to as Hot Dogs and won't make an animal noise if you eat them). This also happens during the fight with Toriel. Also, you lose phone connection and can no longer contact anyone once you go into the Core and only get it back in the closing cutscene of the Neutral ending (or if you backtrack to fulfill the criteria for the Pacifist ending).
  • In a No Mercy run, much of the humor in the game is removed, either because you're killing the characters who provide it or because they've run off to avoid being killed. This extends to the items, where what happens during the most serious fights happens for the entire game. Some other moments are edited to make them less funny, like the Temmie shopkeeper conning you with an expensive Temmie Flake instead of letting you pay for college. There are a few jokes left, but they're either dark humor given the situation or made at the player's expense (including the final boss' jokes about about how frustrated you look after multiple deaths).
  • Also on No Mercy, you need to do this in Snowdin by killing Snowdrake; failure to kill him before you trigger "But nobody came" mode will result in a special save point message and end the No Mercy run. Most players doing a No Mercy run don't notice this, however, due to killing every monster they encounter on sight, including Snowdrake, who will inevitably appear as a random encounter.
  • During the story of Asriel when you go through New Home on the Neutral Route, a lot of the past monsters you've encountered will appear to sequently tell the tragic tale. Even though "Snowdrake" is one of the monsters that tell the story, it's possibly these are just different species altogether and not the comedians. However, two monsters that are noticeably absent are Aaron and Tsunderplane, both species that are rather silly and have goofy ways of speaking (Aaron with winking, Tsunderplane with Tsundere phrasing). It might have made the tale look a lot less serious.
• Shoot the Shaggy Dog:
  • Asriel did this rather than kill humans during the Fallen Child's Thanatos Gambit. He asserted control of his body rather than let the Fallen Child kill the humans, resulting in him being fatally injured by their attacks.
  • Asgore will die at the end of a Neutral Run either thanks to Flowey, Frisk, or to his own guilt about the children he has killed. Averted in a Pacifist Run, where Toriel, who hates Asgore, ironically saves him by interrupting the fight between you and him.
  • Toriel and Papyrus during a Genocide Run. One finds herself betrayed by the child she rescued, and the other is killed by a human he tries to spare, at the risk of losing his potential Royal Guard position.
  • The ending of a Soulless Pacifist run (a True Pacifist run completed after a Genocide Run) is unexpectedly dark. If you choose to leave Toriel, the photo of Frisk and their friends displayed after the end credits will replace Frisk with Chara and cross out the faces of the monsters, implying that Chara killed them at some point after Frisk left. On the other hand, if Frisk stays with Toriel, they become Chara, with the implication that they will kill their friends.
• Short-Distance Phone Call: If you call an onscreen NPC, you can see their mouth moving during their lines.
• Significant Anagram: The title of the game itself is an anagram for "Delta Rune", the name of the Arc Symbol of the game. Also, the odd spelling of Asgore's surname ("Dreemurr") was probably chosen deliberately to be an anagram of "murderer".
• Silliness Switch: Hard Mode. Among other things, several enemies that are normally encountered only in the Core are encountered in the Ruins, with the game lampshading their extremely premature appearance ("That doesn't seem correct."). Additionally, the flavor text for the Monster Candy pedestal states that you can only take 3 pieces of candy in this "hellish world", and the ending consists of the Annoying Dog announcing the end of Hard Mode after the Toriel fight, after which Toriel brushes off her defeat (even if you depleted her lifebar) to go bake another pie. At the end, a game logo with text announcing that a full Hard Mode is "coming soon...maybe...eh, don't count on it," and Flowey calls you a pathetic tryhard before being cut off by the Dog while attempting to question if you have anything better to do.
  • Inverted in certain fights where the game goes into a "serious" mode, which removes any quirky adjectives in a fight.
• Similar Item Confusion: A backstory snippet mentions how King Asgore got an accidental food poisoning when his children confused "cups of butter" in a pie recipe for him with "buttercups" (a mildly poisonous flower).
• A Simple Plan: Alphys has a plan: have Mettaton pretend to try and kill you and eventually “defeat” him, becoming a heroine and convincing you not to leave. Of course, Mettaton has his own agenda and actually tries to kill you, and the Player Character is a huge Determinator, so they wouldn't have stayed anyways.
• Sin Invites Possession: Taking the No Mercy path and deliberately killing everyone possible leads Chara, now a demonic entity, to possess you. This leads to a couple of cutscenes where they move you on their own, as well as the bad ending, where you have to agree to sell your soul to them if you want to play the game again. (Unless you manually reset it yourself.)
• Sliding Scale of Fourth Wall Hardness: An interesting feature of the game's writing is that the fourth wall itself gets broken or insulated based on the route.
  • During the Neutral Route, things feel more like a standard game with a bit of awareness in one or two characters. But it can be construed more as Leaning rather than breaking until the very end, with people aware there's something unique about the player character, but writing it off as just their Determination.
  • During the True Pacifist Route, all the previous fourth-wall-breaking actions, including ones in the Neutral route, are recontextualized with the realization that there's a third presence independent of the player character and the player themselves, which acts as the person that characters actually talk to. At the end, you are asked politely, but urgently, to leave the world as it is, treating it not as a game but a separate reality you had an influence in and can leave peacefully.
  • During the No Mercy route, the fourth wall is torn down handily, as well as the entire rest of the game as the pretense of this being a world you're looking into with a character finally, unambiguously addressing you personally rather than having a hunch that there's something beyond the avatar they're looking at.
• Sliding Scale of Gameplay and Story Integration: About as far on the integration side as it is technically possible for a game to be. Literally every interaction to be had with the game, even something as mundane as closing and re-opening it, has some kind of in-universe significance, and the ability to save and load progress is one of the most integral driving forces to the game's plot.
  • The concept of gaining experience points and levels is deconstructed toward the end of the game, with Sans having the ability to read your stats and judge your actions accordingly.
  • If you reset the game, or even close and re-open it without saving, it becomes increasingly obvious that the reset isn't an 'alternate timeline' to the one you last played — it's still the exact same timeline, with the exact same world and the exact same characters, being restarted over and over and over again. Flowey has the ability to see through these timelines, commenting on your actions accordingly, and while he doesn't have the same level of awareness, this repetition is a very important part of Sans's character.
  • Infamously, if you bring the game's No Mercy route to completion, the game will end with the world being completely destroyed through a joint effort by you and the Fallen Child (or just the Fallen Child, if you refuse). This will replace the game with an empty black void, and the only way to play the game again is to give your soul to the Fallen Child. From this point onward, the game's Golden Ending will be permanently altered so that the Fallen Child takes control of your body at a critical point and implicitly kills all of the friends you've made, with the lasting message being 'you cannot hide from the consequences of your actions'.
• Sliding Scale of Idealism Versus Cynicism: Depending on how you play, this could be one of the most light-hearted and idealistic games of all time, or one of the most cynical and horrific games of all time.
• Slippy-Slidey Ice World: The Snowdin region, which is covered in snow and ice. It consists of a giant snowy forest and a cozy winter town. Yes, there's slippery ice as well, but it's used sparingly.
• The Social Darwinist: "Kill or be killed" is stated to be the guiding philosophy underground. Whether anyone actually believes that except for Flowey (and, optionally, the player) is another matter entirely.
• Solve the Soup Cans: Parodied with Sans's "puzzle," a word search on the ground. Literally nothing forces you to look at it, let alone complete it. (It's also technically unsolvable because of a typo.)
• So Proud of You:
  • Sans tells you this during a True Pacifist run while explaining what EXP and LOVE stand for — Execution Points and Level of Violence — because he knows that you had the capacity to kill monsters, but you didn't, finding a "better" path.
  • Sans tells you this if you abort a Genocide Run by sparing Papyrus.
• Sorting Algorithm of Evil:
  • Played straight with the random encounters, but averted with the bosses. They mix up the mechanics of dodging enough that the actual damage they do is almost irrelevant to how well you can handle each new system. Also, on a No Mercy Run, only two bosses pose a serious threat to you (one of them being the final "real" fight), which are surrounded by several other encounters that go down in one hit.
  • A fairly unique case in pacifist runs — each successive boss seems designed to successively test your pacifism. It seems fairly intuitive to refuse to use force on a kindly old lady and to try and spare an Ineffectual Sympathetic Villain who seems like a good-natured fellow to boot and will abort the battle before things get deadly. But can you maintain a Thou Shalt Not Kill policy in the face of a Determinator out for your blood, or a well-intentioned but still deadly Killer Robot? And from there: a Monster Lord who has sworn to destroy you against even his own better nature, or even a wholly evil demonic flower that's some sort of soul-devouring abomination? You get the chance to spare them all, even though some are much less likely than others to earn your sympathy.
• Soul Power: Enemies directly attack your SOUL. Monsters can also gain immense power by absorbing a human SOUL.
• Soul-Sucking Retail Job: MTT-Brand Burger Emporium, even down to the mandatory slogans. Management is incompetent in several respects and downright sadist in others, alternatingly micromanaging and operating entirely on whims. The leitmotif is the same pitched-down version of "Shop" you'd hear in other stores during a No Mercy run, no matter which end you go for.
• Soundtrack Dissonance:
  • The song that plays when the bird flies you over a small gap (called "Bird That Carries You Over A Disproportionately Small Gap") is overtly dramatic for the context it appears in.
  • "Unnecessary Tension", an ominous theme that plays only once, while you walk down a completely empty hallway in which nothing happens. And "Dating Tense", a dramatic and fast-paced song that plays during the middle of the Papyrus date when he considers events to be getting intense.
  • Visiting Grillby's during a No Mercy run after you've exhausted Snowdin's kill counter still results in "sans." playing, even though a) the town, and by extension Grillby's, has been evacuated, and b) the track for the town has been slowed down or replaced with the "But nobody came" BGM. This was probably intentional, however, as the track will stop playing once Papyrus has been killed; his death leaves such a significant impact on Sans that everything about him, even his theme song, completely disappears from the game.
• Source Music:
  • If you stay at Snowed Inn, the neighbors next door snore to the tune of "Determination" (the song heard on the Game Over screen).
  • During Muffet's boss battle, the flavor text between turns occasionally mentions the spiders clapping and dancing to the rhythm of her theme.
• Space Whale Aesop:
  • The Genocide route is still incredibly horrific and effectively deconstructs the concept of grinding in RPGs and killing characters you otherwise love in subsequent playthroughs just to see what happens, but the very last scene turns the message of the playthrough into "don't go murdering innocent people, otherwise you'll get possessed by a creature of pure evil and be forced to destroy the entire world whether you want to or not". Made worse by the fact that said creature possesses your character in every subsequent playthrough and prevents you from ever achieving the Golden Ending ever again, throwing a layer of Fantastic Aesop on top of that by implying 'you can't always use time travel to escape the consequences of your actions'.
  • For the pacifist run: try to resolve issues not with violence, but by talking them out, or else you'll leave an entire race to be trapped underground because there aren't enough people around for the villain to steal their souls and break the barrier keeping them down there after you talk him out of using that power to endlessly loop time because he's confused you for his adopted sibling.
• Spikes of Doom: Early on the story, one puzzle is a field of spike panels. Toriel holds your hand through the right path out of concern, but if you go back afterwards, you'll realize the panels are harmless.
• Splash of Color: A few areas, such as Toriel's and Asgore's bedrooms, are almost completely devoid of color — except for yellow flowers. The pacifist ending reveals that the yellow flowers have quite a bit of plot relevance. Monsters are also normally in black and white in battle with a few exceptions, such as Asgore's red trident and Sans's flashing blue and yellow left eye.
• Squick: In-universe; if you decide to use Junk Food from Bratty and Catty to heal during the Mettaton fight instead of MTT-Brand food from Burgerpants (the former is a lot cheaper and heals enough to be worthwhile on a Pacifist Run), you'll gross out the viewers and lose some ratings, which is a problem if you're trying to spare him.
• Squishy Wizard: Monsters are made out of magic, which allows them to use magic attacks, but makes them quite fragile compared to the more corporeal humans.
• Star-Crossed Lovers: In Mettaton's opera parody, he sings about a forbidden love between him, a monster, and Frisk, a human.
• Stat Overflow: Sleeping at Snowed Inn or MTT Hotel will grant you HP over your current max.
• Stats Dissonance:
  • The Final Boss of the Genocide Route is a One-Hit-Point Wonder who can dodge all the player character's attacks, and whose own attacks deal a measly one point of damage per frame they touch the PC.
  • The game is notable for nearly every boss's CHECK stats being an outright lie—both Toriel and Asgore are reported as having 80/80 ATK/DEF, but the data shows their stats to actually be 6/1 and 10/-30 respectively (yes, that is a negative sign). This has led to all kinds of Fan Wank about the characters in question being Willfully Weak.
• Status Line: There are two status lines, one at the top of the screen and one at the bottom. The former features Frisk's inventory and money, while the latter displays Frisk's HP bar and his/her Character Portrait.
• Stealth Pun:
  • The very first thing you do is get to Toriel's (get tutorials).
  • One of the dogs is named Dogaressa. "Dogaressa" is the feminine form of "doge" as in the Renaissance-era Venetian head of state.
  • Bratty and Catty are vendors who you can encounter just outside the MTT-hotel, off to one side in a little alleyway. This means that Catty is an alley cat... and Bratty is an alley gator. There even are another one on top of this, namely that they are both Valley Girls.
  • Near the end of the True Pacifist Route: "But it refused" can refer to both not giving up and the literal re-fusing of the SOUL.
  • During the True Pacifist final boss, determination is what keeps you alive whenever your HP reaches 0. In other words, determination is also de-termination.
  • Sans the skeleton loves to tell jokes and being a comedian. He's a Comic Sans.
  • Mettaton is a ghost possessing a robotic body Alphys built for him. He's, literally, a ghost in the machine.
    • Mettaton EX is a male-identifying robot with a Camp Gay personality and some feminine features. In other words, he's "android-gynous."
  • Unintentional, but Asgore does a Mercy Kill. He kills your mercy button.
• The Stinger:
  • Neutral (Demo): Flowey sends a message in the manual, berating you for any wrongdoings you did.
  • Pacifist (Demo): Sans and Papyrus have a discussion about the game's future. In the manual, Flowey tells you to not get cocky.
  • No Mercy (Demo): A certain someone invites you to "finish the job" once the full game comes out. Not only does this screen replace most of the manual's pages, but on his page Flowey is then depicted as faceless. Those are big hints towards what the No Mercy path entails.
  • Neutral: Flowey asks you to perform a pacifist run or to clear all sidequests to achieve the best ending. By viewing this stinger over and over, he'll refuse what he perceives as your attempts at befriending him, comment on how Papyrus made a fanclub for him and warn you to avoid Sans. He'll then argue that you're just coming to him to exhaust his dialogue and ignore you from that point on.
  • Pacifist: If you decided to live with Toriel, she brings pie to Frisk while they sleep. If not, you see a picture of the cast happy together. Then Flowey begs you to leave them happy and not play the game again.
  • No Mercy: The Fallen Child proposes you sell your soul to them if you want to restart the game. This corrupts the aformentioned stingers of the Pacifist path, as the Fallen will then possess Frisk and kill everyone.
• Strength Equals Worthiness: The first boss will allow you to leave the Ruins only if you prove you're strong enough to defeat her. Deconstructed in that if you take the boss's HP down to zero, she really does die, instead of just yielding the battle and recovering in the next cutscene as is typical for RPGs. You're fighting Toriel, who's seen many children die due to being too weak to survive and doesn't want to repeat this.
• Stuck in a Chimney: In the 5th Anniversary Alarm Clock, Asgore mentions getting stuck in a chimney while dressed as Santa Claus. He also describes someone drawing a face on his backside.
• Studiopolis: Hotland is this combined with Lethal Lava Land. It's the setting closest to the CORE, which powers the entire Underground. It's also where Mettaton, the Underground's sole celebrity, lives and films his various TV shows. While most of his fans mean you no harm, he's gunning for you so he can rip out your SOUL and cross the barrier… but while he's working on that, he'll also have you on his shows as an honored guest. Pretty much everything in Hotland is named and themed after Mettaton, and there are cameras everywhere.
• Stunned Silence: If you manage to lose the fight with the tutorial dummy by repeatedly missing your attacks, Toriel will be completely baffled, coupled with a Fascinating Eyebrow, before moving on like nothing happened.
• Stupidity Is the Only Option: Before you encounter Muffet, the path through her lair is littered with spiderwebs that slow your progress before arriving in an area covered in one big web. It's obvious that you're going to get stuck if you keep going, but there's no other way to proceed.
• Subverted Punchline: An Easter Egg Prank Call from Sans involves the typical "Is your refrigerator running?" gag, the twist being that he's not making a pun out of the word "running" as with the typical punchline of that joke.
  Sans: is your refrigerator running?
  [if you select "yes"]
  Sans: nice. i'll be over to deposit the brewskis.
  [if you select "no"]
  Sans: ok, i'll send over someone to fix it. thanks for letting me know. good communication is important.
• Suddenly Voiced: In the entire game are two instances where an actual voice is heard. The first is during a Genocide run after your second encounter with Flowey; instead of his usual creepy/maniacal laugh, the sound-bite used is a cartoony, high-pitched "That's a wonderful idea!". The second is during a neutral/pacifist run when you flip Mettaton's switch, which transforms him into Mettaton EX. After a flash of light, a sound-bite of him saying "Ohhh yesss!" in a deep, electronic voice is heard.
• Sunk Cost Fallacy: At the end of a "No Mercy" run, Sans points out that the whole reason you're doing what you're doing isn't out of any sort of moral desire, but simply because you can and therefore feel like you have to, imploring you that instead of working so hard for what is quite frankly no net gain, you should give up and do pretty much anything else.
• Surprisingly Creepy Moment:
  • On the surface, Undertale is, as another influential indie game put it, a nice game for cute children. But Flowey will soon give you a hint of the game's true nature. And as you go on, you'll find some very disturbing scenes, especially if you don't play nice…
  • The Pacifist run is generally a very sweet and enjoyable experience, if a bit tragic at times, but occasional scenes such as the True Lab and Sans telling you that if it hadn't been for Toriel, he'd have killed you on the spot serve to remind you of the kind of experience you could've had if you hadn't chosen to go that way.
  • If you start off on a Genocide Run, Sans's cryptic remark of "you're gonna have a bad time" and Stealth Hi/Bye will be the first indication that something's gone south.
• Surprisingly Realistic Outcome:
  • In typical RPG fashion, you might try to sell your unneeded items to the various shops in the world. Unfortunately for you, the only shops you can interact with are food stores that have no reason to buy stuff from customers or people just as eager to get junk out of their inventory as you are. To sell items, you specifically have to find an impulse buyer who's bad at managing their money.
    Does this look like a pawn shop? I don't know how it works where you come from... but... If I started spending money on old branches and used bandages, I'd be out of business in a jiffy!
• Suspiciously Specific Denial:
  • Toriel calls you to ask if you like butterscotch or cinnamon, then ask if you're okay with cinnamon/butterscotch (whichever you didn't choose), and finally to ask if you have any allergies. She denies she's up to anything.
  • Annoying Dog denying he ate one of Toriel's pies at the end of "Hard Mode".
  • On the Pacifist Route, after the pretend date with Alphys, Papyrus sends her home from training early and wants you to find her at her lab for no apparent reason except he feels you should.
• Suspicious Missed Messages: Papyrus is such an enthusiastic Keet that, if the player kills him, his friend Undyne infers that he's dead because he didn't answer his cell phone after the second ring.
• Suspicious Video-Game Generosity:
  • Snowdin Town has a tunnel that takes you across the town, which is useful for travelling between the area where Papyrus fights you and the shop and inn, which have a save point nearby.
  • After running away from Undyne for the second time, you fall into the garbage dump in Waterfall. The dump has a save point, followed by a room containing a cooler with two food items that heal 21 HP each. The cooler room has a fight with the Mad Dummy begin when you try to leave, and it can't be damaged by attacks unless you're on a No Mercy run. Neutral and Pacifist players will likely need to use food items in the fight to help them last long enough for Napstablook to show up and end the confrontation.
  • The room just before Mettaton's boss fight has a save point and an elevator which takes you back to the CORE's entrance, allowing the player to stock up on healing items at the MTT Resort without having to backtrack through most of the confusingly laid-out CORE.
  • In a No Mercy run, killing Mettaton NEO results in enough EXP to jump from LV 15 to 19. You will most likely need the resulting HP boost for the Final Boss.
• Sword and Sorcerer: The monster "Knight Knight" (a large hulking knight) tends to appear alongside the monster "Madjick" (a floating, grinning wizard).

Trope T

• Take That!:
  • Near the end of the Genocide route, Flowey reassures you that at least you're not as bad as the "sickos" who don't have the gall to play the route themselves, but would rather just watch someone do it; a not-so-subtle reference to the countless players who watched Let's Plays of the Genocide route instead of playing one themselves.
  • There's a late-game interaction between Toriel and another character where Toriel takes a friendly jab at people making explicit art of her because of her age, though since the Underground has internet, this manages to stay within the realm of the game; odds are that in-universe art of her like that is probably floating around.
• Talking Is a Free Action: Zig-Zagged during the chase sequences in the Undyne fight. If you have a significant lead over her when Papyrus calls, she'll continue to run after you, then stop at a predetermined point, even if the dialogue box is already up. Calling Undyne later in the same room has her reveal that she saw you were taking a phone call, and stopped to let you finish.
• Talking the Monster to Death: The standard way of disposing of enemies (and bosses) in a Pacifist route. Reversed in the No Mercy route where Papyrus tries to do this to you.
• A Taste of Their Own Medicine:
  • On the Genocide Route, after bringing doom to the Underground several times over, Flowey can finally receive his comeuppance from the Fallen Child, killed just like he killed all the others.
  • The Player Character themself can be killed by the Fallen Child, the killer you created, at the end of the Genocide Route, again, just like you killed everyone else.
• Telephone Song: The "Wrong Number Song", which plays during a random event where a mysterious caller dials you in Snowdin, only to realize they've got the wrong number.
• That Came Out Wrong: In Alphys' potential "date" sequence, Alphys' frantic attempts to explain herself to Undyne only make what she's doing sound even weirder.
  Undyne: Are you two... On a date?
  Alphys: UHHH, YES! I mean, UHHH NO! I mean, we were, but... I mean, actually we were only romantically roleplaying as you!
• That One Boss: In-Universe, the final boss of the Genocide run tries to invoke this as one final effort to make the player abandon a Genocide run.
• Theme-and-Variations Soundtrack: Heavily. Several tunes of the game end up being sped-up or slowed-down reprisals.
  • "Once Upon a Time" and "Memory" show up in a number of other tracks.
  • Napstablook's house theme called "Pathetic House" actually is a much, much slower remix of "Ghost Fight", their fight theme.
  • "Ghost Fight" itself is later remixed into the Mad Dummy's fight theme, "Dummy!", Muffet's fight theme, "Spider Dance", and Mad Mew Mew’s fight theme.
  • Asgore's theme, "ASGORE", has a part from Toriel's boss theme, "Heartache", as well as the Game Over theme, "Determination".
  • Papyrus's boss theme "Bonetrousle" is a reprisal of "Nyeh Heh Heh!", with some added kick to it.
  • Undyne's boss theme during the Pacifist and Neutral routes, "Spear of Justice", is a remix of the theme of Waterfall sped up. Which, itself, is a remix of the theme of the Ruins.
• Theme Music Power-Up: Some characters use a more upbeat remix of their Leitmotif for their boss theme, such as Papyrus. But the Genocide run's final boss is the biggest of them all, as their normally relaxing tone is replaced with Megalovania. Prepare to have one hell of a bad time.
• Theme Song Reveal: In Finale, where about halfway through it starts playing a modified version of His Theme, hinting at Flowey's true self.
• There Are No Therapists: Many major characters can be reasonably theorized to be mentally ill or otherwise dealing with serious emotional issues, especially Alphys, Sans, and the Dreemurrs, but nobody ever mentions therapy. Granted, the game does take place over one or two days, so it's possible it's offscreen or the characters in question aren't taking the effort to seek help.
• There Is No Kill Like Overkill:
  • If you've slain as many monsters as possible, the first boss battle is over very quickly. At a time when the most damage you've done up to that point is around 30, your attack will deal about 22,000. The cheap shot described under I Surrender, Suckers also does obscene damage.
  • At the end of the No Mercy route, the Fallen Child attacks the game world itself for an endless amount of damage, represented by your screen filling up with nines.
  • If you kill Flowey while on the Neutral path, he turns into an ordinary flower instead of outright disappearing. If you kill him during the No Mercy path, however, he gets hit so many times, not only does it kill Flowey, it also destroys the flower completely.
• They Wasted a Perfectly Good Sandwich: Sans treats you to a burger or fries at Grillby's and talks to you about Papyrus and a suspicious flower that whispers things to Papyrus. After that tense moment, you and Sans leave the counter without eating. An NPC remarks that the food is probably cold by now.
• This Cannot Be!:
  • Mettaton gives a decidedly half-assed version of this speech during your third encounter with him:
    Mettaton: HOW CAN THIS BE, YOU WERE STRONGER THAN I THOUGHT, ETC. WHATEVER.
  • Flowey appears to be aware of how common this is and repeatedly subverts it by saying it just before he laughs in your face, calls you and your friends idiots, and comes back seemingly stronger than ever.
• This Is Gonna Suck: As put by the intro to the final boss fight on the No Mercy route: "You feel like you're going to have a bad time." You will, due to said boss being the hardest in the game.
• Throne Room Throwdown: The final fight with Asgore, the king of all monsters, takes place either in his throne room, or in a corridor slightly behind it, depending on your path. In the pacifist run, this is also where you enter the battle with Asriel, though due to him being a god, he quickly takes you to hyperspace instead.
• Too Awesome to Use: The slice of cinnamon butterscotch pie Toriel gives you is specified to replenish all HP. When you're at a point where the cheapest foods will replenish most health, it'd be a shame to just waste this. Invoked, considering it has a special effect on Asgore at the other end of the game.
• To Win Without Fighting: Refusing to kill monsters is necessary to obtain the Pacifist route (and the Golden Ending).
• Trade Snark: A couple of NPCs in Hotland will share their favorite Mettaton Moment™ with the player.
• A Tragedy of Impulsiveness: The king of all monsters, in a fit of rage after the death of his son, declared war on humanity. Later, when he calmed down and his better nature gained control, he realized that he couldn't take back his decree because it finally brought hope back to his miserable subjects. It may or may not have helped the other monsters, but it drove the king to misery and self-loathing after his wife left him in horror and disgust and he became responsible for the deaths of six human children.
• Tragic Bigot: With very few exceptions (most notably, Toriel, Mettaton, and, ironically, Asgore), prejudice against humans is very ingrained into monster culture, which is one of the reasons why almost everyone who is able to identify you as human tries to kill you. Bratty and Catty talk about how excited they are about Asgore wiping humankind out as if it's a movie they're looking forward to, and you need to go through a lot of hell and high water in order to befriend Undyne because of how strongly she holds her grudge. Considering monsterkind's history with humans, and the fact that monsters have been secluded for so long that they barely know anything factual about them anymore (Undyne thinks anime is supposed to be records of human history, for example), it's very difficult to blame them. It helps that, because of how naturally empathetic they are, the power of your friendship alone is all it takes for them to get over this prejudice during the Pacifist run, and they live in harmony with humans forevermore.
• Trailers Always Spoil: The trailer for the Playstation 4 port discreetly shows off Photoshop Flowey's head and tentacle.
• Train Problem: During his quiz show, Mettaton throws an "easy one" at you, to trip you up. Likely put there to make you notice (if you hadn't already) that Alphys spells out the letter of each answer to the quiz show with her hands.
• Trauma Inn: Snowed Inn and the MTT Resort heal your HP beyond max if you stay in a room.
• Treacherous Checkpoint:
  • The True Lab contains an ordinary-looking save point which, when examined, transforms into a shape-shifting Amalgamate.
  • The Final Boss can also "save" and "reload", and he uses this power against you in battle like the cheater he is.
• Triumphant Reprise:
  • The Core theme is an energetic, upbeat version of the Hotland music.
  • While the soul attack sections of Photoshop Flowey's boss fight are set to distorted versions of Flowey's theme, "Finale", which plays when the player can finally retaliate against him, is a much more hopeful remix.
  • Similarly, "Hopes and Dreams" and "SAVE the World" is a soaring remix of the melancholy "Undertale", with Flowey's theme incorporated as well. And though "Hopes and Dreams" is pretty triumphant-sounding on its own, "SAVE the World" cranks this up higher.
  • Asriel's theme from the end of the Pacifist run is this to the song the music box plays in Waterfall.
• Truce Trickery: The True Final Boss of the Genocide Route tries to offer mercy to the Player Character partway through their fight. If you agree to spare him, he kills them with an inescapable Cycle of Hurting and asks you not to try again.
  Game Over screen: geeettttttt dunked on!!!
• Tsundere:
  • Examining a cactus will result in the observation, "Ah, the cactus. Truly the most tsundere of plants." Coming back at the end of the game and this changes to "It's not like this cactus was waiting for you to come back or anything..."
  • The Tsunderplane enemy in Hotland is a tsundere airplane. Its in-battle actions are reminiscent of the typical tsundere behaviors in anime (like turning up its nose or "accidentally" bumping you with its wing).
• Two Rights Make a Wrong: If you befriended Papyrus, then you'll recieve a call from him in Waterfall asking what you're wearing. You're given the chance to tell the truth or lie, and you're free to switch to a different armor piece after answering him. As it turns out, he's asking on behalf of Undyne, who is trying to hunt you down and kill you. He'll call you back shortly after a chase scene with her to inform you that he tried to pass her misleading information... however, no matter what you did, his guess as to how best to mislead her combines with your actions to make his information unintentionally completely accurate.

Trope U

• Uncommon Time:
  • The tune "Pathetic House", which plays in Napstablook's house, is in 5/4.
  • "Here We Are" is also in this time. "Amalgam" ups the ante by doing away with a key entirely. Both of these are heard when exploring the True Lab, expressing just how unsettling that part of the game is.
• Unending End Card: Completing the True Pacifist run will treat you to an end card consisting of the game's title and "THE END" while "Memory" plays. If you cleared a Genocide run first, the "THE END" text will be blood red and a slowed-down version of "Anticipation" will play instead.
• Unexpected Art Upgrade Moment: Used to horrifying effect when, up until this point in the game, the art style has stayed in a low-color, comparatively low-detail style like late-1980s/early 1990's video games. It isn't until after Flowey steals the human souls and crashes your game, requiring a restart, that he reappears as the gargantuan Photoshop Flowey, with photorealistic full-color spiny plant tendrils, metallic H.R.-Giger-like machinery and pipes, fleshy face, eye, and mouth parts, and a vintage 1970's television set which shows a psychotic version of his normal face. Also included are photorealistic weaponry that he uses against you, including flamethrowers, fly swarms and Venus Flytraps, literal finger guns, and even nuclear bombs.
• Unexpected Shmup Level: Alphys can turn your soul yellow, turning it upside down and allowing it to shoot projectiles. This turns Mettaton fights into miniature shoot-em-ups.
• Unreliable Narrator: The pre-sealing details given by the monsters clashes with the opening narration. The intro claims that the war between humans and monsters was an even-sided war with reasons lost to time that ended with humans victorious after a long battle, whereas the version given in Waterfall paints it as more of a massacre. The Waterfall story also says the barrier was erected by seven mages, but only one mage is seen making the barrier in the intro. Gerson claims the Delta Rune predates written history and symbolizes the monsters' escape, but the intro shows the monsters wearing it prior to the sealing.
• Unscrewed Salt Shaker: Sans tampers with the ketchup bottle to prank you if you accept his offer of ketchup for your food at Grillby's. He's a good enough sport to give you his lunch after yours gets drenched in ketchup thanks to his prank. If you don't accept the initial offer, he just chugs the bottle of ketchup all in one go instead.
• Unstable Equilibrium: As your LV increases, your attack stat increases and enemies' defenses passively decrease. Thus, as you gain XP and increase LV, you get stronger and stronger until you steamroll almost everything in your path and nothing can stand up to you.
• Unwinnable by Design: In the demo, selecting anything other than "cheer" or "flirt" after Napstablook cries themself a hat will prevent you from ending the encounter nonviolently. (Yes, even "spare".) You can't even flee afterwards. Fortunately, this doesn't lock you out of the Pacifist Run ending as Napstablook is incorporeal, so they don't count as a kill if you reduce their HP to 0. The full version changes this so that the fight is dragged out much longer, but you will still spare them eventually if you try.
• Unwinnable Joke Game: The word search that Sans lays out in hopes of stopping you (yet can not only be walked past, but still does nothing to stop you if you read it). Every word can be found... except for the gibberish word that looks like the top row of letters but is actually a letter off.
• Urban Fantasy: The intro gives the year as 201X (although the ending shows the game actually takes place some time later), and despite what one might expect from the setting, things such as cell phones and blogging are quite commonplace in the Underground.

Trope V

• Vague Age:
  • Everyone. No, seriously, everyone; with the exception of one very minor character (Burgerpants, who outright says that he is 19), there isn't a single person in the entire game whose age is ever specified. The protagonist, Monster Kid, the Fallen Human, and Asriel are all clearly children but are never given anything more specific, Toriel and Asgore are on the later end of being middle-aged (though their species don't age normally, so they could be thousands of years old), and literally every other major character could be anywhere from their teens to their thirties and their roles and personalities would still work. And that's not even touching on the fact that at least certain types of monsters age differently than humans.
  • Muffet in particular is the cause for much confusion, as she's described as a "li'l baby spider monster" by her creator, but looks to be in her mid-to-late teens at the youngest, and that fact she lives alone, has a job, and calls the protagonist "dearie" implies that she's least 18. In addition, she lacks the striped shirt that explicitly child-aged characters have. Muffet's designer has gone on record as saying that "li'l baby" was meant to describe her cuteness and not her age, however, so her true age is anyone's guess — although as mentioned, she skews young adult.
  • Papyrus is a somewhat lighter In-Universe case. Like Muffet above, he lacks a striped shirt like other major child characters, as well as having a job, being in training to be a royal guard, living alone with his brother, and being very tall with adult proportions. It's also possible for him to become the ruler of the underground in one of the neutral endings, indicating he's meant to be seen as an adult. Talking to Monster Kid after fighting Papyrus, however, has them question if "that weird skeleton" is an adult or a kid, likely due to his more optimistic, childish nature.
• Variable Mix: Several of the game's songs are sped up, slowed down, warped, or otherwise manipulated, sometimes to such a dramatic degree that they sound like entirely different songs. "Ghost Battle", "Dummy!", "Pathetic House", and "Spider Dance" all contain variations on the same tune. "The Choice" is a section of "Undertale" slowed down to 666%, and "But Nobody Came", the theme that plays once you've cleared out all the monsters in a particular area, is a similarly very slow version of "Your Best Friend".
• Version-Exclusive Content: The non-PC ports of the game add the Dog Shrine to the game, accessed from the sink in Papyrus and Sans's house. The PS4 and Vita versions allow you to donate to the shrine in order to upgrade it, serving mostly as an area to easily achieve Sony's mandated game trophies. In the Switch version, the shrine leads to an Optional Boss which features a unique mechanic requiring the use of the two joysticks. The Xbox version makes it a casino-like area where coins can be bought and then donated, which Sans comments upon. The Optional Boss from the Switch version is present there but cannot be fought.
• The Very Definitely Final Dungeon: In the No Mercy and Neutral routes, the Core is the final area that has random monster encounters before the Final Bosses. In the True Pacifist route, the True Lab is this instead.
• The Victim Must Be Confused: Defied. When the protagonist first meets the two royal guards loyal to Undyne, Undyne will let them know that she had told the guards beforehand that she'd likely have been brainwashed if she ever became friends with a human if they contact her after befriending her. This is why she doesn't bother coming to talk them into letting you through.
• Video Game Caring Potential:
  • To be caring, there's a lot more to it than just being "kind." In order to get the best ending, you have to work your ass off sparing monsters, being nice to everyone, and so on. This takes a lot of work, and one of the points that Flowey makes during the Pacifist run is that you may not be able to keep this up for long.
  • There are two specific instances of this trope that have nothing to do with achieving the Pacifist ending and in fact don't benefit you in any way. One is the snowman near Snowdin who asks you to take a piece of himself with you so he can see the world. Said piece of snow then takes up a valuable slot in your inventory for the entire game (though you can eat it to regain HP), and carrying it to the end of the game simply gets you a remark from Sans that you "made a snowman very happy". The second is giving an umbrella to — of all things — an inanimate statue caught in the rain, which activates a music box playing the Undertale theme, and will net you some lines of dialogue if you call Undyne next to it later on, as well as giving you a hint to a musical puzzle as a small reward.
• Video Game Cruelty Potential:
  • Discussed and deconstructed to hell and back. One of the major themes of the game is that being "mean" in videogames when you have the option to be kind is nothing short of despicable, and that if you like being cruel instead of kind, you are a bad person. In the finale, if you've been nothing but cruel, the game labels you as a remorseless murderer who forgoes any chance of sparing those who get in your way. Considering you've been murdering monsters who can easily be convinced to stop or are defending themselves, this can easily ring true.
  • One of the cruelest things you can do is reset the game after you get the true ending. Flowey tells you outright that doing so means undoing all the success and happiness you worked so hard to give everyone, and that choosing to do so would make you even worse than he is.
  • One can also do a special version of the No Mercy run where you "pretend" Pacifist until the last second when you can spare them and instead insta-kill them. Most special enemies, bosses, and mini-bosses have special dialogue for these events where they utterly break down.
    • Alternatively, you could do a reverse Annoying Dog neutral run by killing random encounters in each area until you get "But nobody came", then sparing all the bosses. (You won't get a special ending for doing this, though; just the Exiled Queen ending.)
  • On the flipside, it is possible to spare many enemies by beating them within an inch of their lives. But if you try this on the bosses, you'll be in for an unpleasant surprise.
  • Even in a No Mercy run, some enemies might still willingly spare you, hoping you are still redeemable. Papyrus being a particularly notable example, though his response if you do spare him after all this time is bordering on Video Game Caring Potential: "YOU DIDN'T DO A VIOLENCE!!!"
  • You're given the option to say "What a loser" instead of "let's be friends" while sparing Papyrus. In addition, you can actually say negative answers during the "date" with Papyrus. This doesn't negatively impact the scenes due to Papyrus Comically Missing the Point each time. You also have the option during a conversation with Sans to say that Papyrus is "not cool" rather than "cool", which will offend him but won't actually change the outcome of whatever route you're on.
  • If you encounter the secret encounter So Sorry, and start FIGHTing him, he will try to bribe you not to kill him. You can extort up to 318G out of him in this fashion by beginning to SPARE him after eight hits (at LV 1).
  • Even in a Pacifist run, it's possible to still be an asshole even when you're not killing anyone; in addition to saying the mean things to and about other characters mentioned above, you can make the Snowman or the Monster Kid permanently hate you even while getting the Golden Ending:
    • If you eat the Snowman Piece in front of him, he'll refuse to give you another one and then tell you when you talk to him in the Playable Epilogue that, even if you have other characters fooled, he knows what kind of person you truly are.
    • When the Monster Kid falls off a bridge while Undyne approaches you both, the "best" option is to save them from falling, at which point they'll defend you from Undyne, who retreats, and decide that the two of you can still be friends even though you're a human. However, you're not required to save them to achieve the pacifist ending, and can run away and leave them to be rescued by Undyne rather than doing it yourself. This causes them to grow to hate you, and if you interact with them in the Playable Epilogue, they'll tell you not to talk to them.
• Video Game Cruelty Punishment:
  • "Killing" Napstablook will earn you a negative experience point. Subverted in that this doesn't affect the game in any way. In this game, experience points are something entirely different from "EXP".
  • This is the only way to describe the No Mercy Run final boss, aka, the hardest boss in the entire game. You only face this rule-breaking Marathon Boss if you kill every single enemy possible, and he'll say anything he can to make you feel bad about it.
  • Relatively minor compared to everything else in No Mercy, but it's still impactful on gameplay: aborting a No Mercy playthrough up to Hotland causes Alphys to not meet you and give your cellphone the Dimensional Box upgrades, as she's actively opposing you by setting up refuge for whatever monsters are left. Even if you abort No Mercy by killing Mettaton NEO without exhausting the local monster population, you still cannot get the upgrade.
  • If you do go through the entire No Mercy path, the Fallen Child will erase the game itself. Pester them about it and they'll call you somebody who believes themself to be above consequences. You can then start over by selling your soul to the Fallen Child, and boy, that doesn't end well at all.
• Villain Protagonist: The main character on the No Mercy route. On a thematic level, the game discusses the nature of playing this way in video games, and the idea that the player is doing so just to see "what would happen". Going through with a Genocide route play-through also deconstructs the role of the player, with the game destroying itself because you killed everyone in the entire game. The Fallen Child even reacts with abject disgust if you ask for the game world back in exchange for your SOUL, wondering if you believe yourself to be above repercussions for your despicable actions. As it turns out, you most certainly are not — doing so results in the game permanently tainting all further playthroughs to ensure that even in the best ending, everyone will die.
• Violence Is Not an Option: Downplayed in the Pacifist Ending. The Final Boss is immune to physical attacks, so attempting to fight him the traditional way is never going to end well for the player. However, as this is a Pacifist Run, ending encounters without killing anyone isn't exactly a new concept.
• Violence is the Only Option: For a game that promotes pacifism and nonlethal resolutions, your first fight against Asgore makes it clear that you have to take him down the hard way. If you've been playing pacifist up until now, prepare to get schooled, since you're never really allowed any chance to practice how to fight beforehand without it changing your run to Neutral.^note Unless you encounter So Sorry and do what is described in Video Game Cruelty Potential.
• Visual Pun:
  • Muffet's bake-sale table has eight legs, just like a spider.
  • What looks like an asterisk on the Tem Shop sign could be an "I" crossed out, so it would read "Item Shop".
  • When your SOUL turns blue, it can no longer fly. Being blue gives you a heavy heart.
  • If you take too long in Mettaton's maze section, Alphys will help you by hacking the firewall. That is, a literal wall of fire that is approaching you.
  • Photoshop Flowey's hand-cannons have literal green thumbs.
  • In the Nintendo Switch port teaser trailer, Sans and Papyrus are seen standing on a Switch gamepad. Sans is standing on the left side next to a blue Joy-Con, while Papyrus is on the right next to the red Joy-Con, matching their clothing.
• Voice Grunting:
  • Even though the game does not have any genuine voice acting, every major character makes a different sound when their words are dispensed on the screen. The ability to recognize them by this voice alone is essential for a couple of twists. The only two characters who have genuine voiced dialogue are Flowey and Mettaton, but only one sentence each.
  • Sans’ sound actually rises in pitch whenever he quotes Toriel, symbolizing him imitating her.

Trope W

• Waiting Puzzle: One option for getting past the force field in the CORE is to just stand in front of it for a few minutes. "I cannot fight. I cannot think. But with patience, I will make my way through".
• Wake-Up Call Boss:
  • Dogamy and Dogaressa are oftentimes the cause of the first death to even more skilled players, due to them being the first enemy that requires use of multiple ACTs to end the battle peacefully (Roll Around, Sniff, Pet), utilizing blue attacks in close proximity to regular attacks, and their axe attack, which covers most of the bullet board and requires precise movement to dodge.
  • Undyne also serves this role no matter which route you take. Pacifist players will have to fight a relatively difficult boss with low stats, and must realize that they can't spare her; they'll have to evade her long enough to get to Hotland and give her water when she collapses from the heat. Genocide players will kill her in one hit, like every other boss, only for her to come back as Undyne the Undying, with faster and stronger attacks that will prove a challenge to even a Genocide player, making her the second-most difficult fight after Sans.
• Walking Spoiler: There are enough characters who qualify as such to warrant their own character page.
  • Asriel Dreemurr is a key player in the Underground's history and is difficult to discuss without revealing he was resurrected as Flowey and is the final boss of the True Pacifist route.
  • The Fallen Child is another important character in the Underground's history, being the first human to fall into the Underground and Asriel's best friend. While most spoilers concerning them involve the Genocide route, important details about them are also revealed during the other two routes.
  • The six human souls factor into the plans of Asgore and Flowey, and are instrumental in defeating the Neutral route's final boss.
  • The Amalgamates' existence is a major contributing factor towards the present mental state of a major character. Additionally, their origins are connected to the creation of Flowey.
  • Knowledge of the player themself being a character spoils the more meta aspects of the True Pacifist and Genocide routes.
  • W.D. Gaster and his followers spoil the game's exploration of the in-universe perception of Dummied Out content.
• Wall of Text: If you die during the battle with Photoshop Flowey, you'll get the standard Game Over message followed by "HAHAHA" repeating over and over.
• Warning Mistaken for Threat: If you start the Genocide Run, there will be two moments when the character Sans the Skeleton tells us that if we don't turn back and become a better person, we will have a bad time, which seems like Sans is threatening us with a fight. And while Sans is the Final Boss of the route (and quite a hard one), once you kill him, he says "don't say i didn't warn you", confirming that Sans was actually warning us, and his fight wasn't the "bad time". If you don't reset the game and continue on to kill Asgore and Flowey, you will find out that the "bad time" was actually the Fallen Child erasing the world and ruining all future True Pacifist endings.
• Weaksauce Weakness: Monsters, all of them, are composed of mostly magic, and this makes them very sensitive to negative emotions. Thus, someone with a hateful heart and Killing Intent can disrupt a monster's soul enough to kill them in a single blow, even with something that isn't actually a weapon. However, the amount of intent it takes to get to that point is left ambiguous. The Fallen child was completely genocidal; it's possible a well-adjusted human wouldn't be capable of hating that much, if monsterkind's easy reintegration in the true ending is any implication.
• We Buy Anything: Parodied: despite the fact that every shop has a Sell option, selecting it just causes the shopkeeper to mock you, stating that they either don’t have any use for your items or that buying your stuff makes no sense from a business POV. The only shopkeeper that's willing to buy your items is a massively ditzy Cloud Cuckoo Lander.
  Snowdin Shopkeeper: Does this look like a pawn shop? I don't know how it works where you come from... but... If I started spending money on old branches and used bandages, I'd be out of business in a jiffy!
  Gerson: Ha! I'm tryin' to get RID of my junk, not get more of it!
  Burgerpants: (WHY IS THIS PERSON TRYING TO SELL ME SOMETHING THIS IS A HAMBURGER RESTAURANT I'M JUST TRYING TO SURVIVE)
• We Can Rule Together: On a No Mercy run, after clearing the Ruins, Flowey acts like the player character is an old acquaintance of his and asks for assistance in destroying the world.
  Flowey: THAT'S A WONDERFUL IDEA!
• Welcome to Corneria: Papyrus lampshades this when you call him at the Hotland dock:
  Papyrus: HEY, YOU SHOULD COME TO SNOWDIN AND VISIT ME! I'VE BEEN WORKING ON A FEW THINGS. A FEW SENTENCES TO STAND AROUND AND REPEAT.
• Well-Intentioned Extremist: The majority of monsters only want to kill you so that they can use your SOUL to escape the Underground.
• We Will Meet Again: If you kill just about every leader and enough monsters to make Sans disgusted with you without going No Mercy, he ends with two very chilling words: "see ya." Kill any more monsters than a certain limit, and he'll tell you to go to hell.
• Wham Line: There are enough of them over ther course of the game to have their own page.
• Wham Shot:
  • During the battle against Asgore in the Neutral route, the first thing he'll do is destroy the Mercy option, emphasizing even further that there is no other option other than FIGHT.
  • The final battle of the True Pacifist route begins with Flowey absorbing every single SOUL in the Underground except for the protagonist's, which he claims will allow him to reach his true form. As he's doing the absorbing and lights are flashing everywhere, you'd think Flowey would turn into a form even more horrifying than his Photoshop form — until the scene suddenly cuts to Asriel Dreemurr standing with his back facing toward you, solidifying The Reveal that Asriel is Flowey's true form.
  • At the end of that fight, you're given the ability to SAVE Asriel, which cuts to a flashback of the intro… then Asriel shows up, revealing that the child who fell in the intro is not the child you actually play as.
  • The final boss of the Genocide run bleeds (or appears to do so) when you land the killing blow, something no other monster in the game does.
  • Players who engage in a lot of random encounters either just to kill enemies for kicks or to level grind eventually get a rude awakening when the random encounter flow begins, only for no enemy to show up, along with the narration text reading "But nobody came."
  • For players that have previously done a very "dusty" Neutral run and fought boss monsters legitimately the first time around, then start a proper No Mercy run, seeing their attack do a One-Hit Kill to Toriel can come as a shock.
  • On No Mercy, players who killed Toriel and Papyrus and everyone else (except for Jerry) in between with relative ease get a second shock when they attack Undyne the Undying and see their attack only take off a small fraction of her lifebar, instead of instant-killing her like the other bosses and her first form.
  • Players will select FIGHT on the final boss of the No Mercy run for the first time… and he dodges. It comes as a shock as throughout the whole game, every attack lands.
• What Measure Is a Mook?:
  • Killing any enemies while saving Toriel will result in a massive Player Punch later on.invoked
    Flowey: So you were able to play by your own rules. You spared the life of a single person. [lists off each monster] Think about those names. Do you think any of those monsters have families? Do you think any of them have friends? Each one could have been someone else's Toriel. Selfish brat. Somebody is dead because of you.
  • The Genocide route features a bizarre inversion. To complete the No Mercy route, you have to kill a certain number of monsters in each area by the time you encounter the area's boss, as well as killing all the minibosses and bosses (and Snowdrake). ^note The only exception is Lesser Dog; while it's considered a miniboss, it's met in a random encounter. When it's encountered, sparing it aborts the Genocide run. However, if you somehow have not met it at all before running out of encounters, or make it to the town without killing it, the game won't force you to kill it, and you can see it as an NPC. The chances of NOT encountering the Lesser Dog at any point before reaching Snowdin proper are quite small, however, which means that this quirk is in all likelihood an oversight (especially considering that you do have to kill Snowdrake, a "true" random encounter, in order to progress the Genocide run into Waterfall). However, you can spare as many generic monsters as you want and still complete a No Mercy run as long as you meet your kill quota by the end of each area (and kill all the unique monsters (including Snowdrake◊, who you can spare as often as you want as long as you eventually kill him)). So, basically, you can let some minor monsters get away while making sure to kill enough to depopulate each area (as well as the major monsters).
• What the Hell, Player?:
  • Killing any significant character will have this sort of effect come from somebody, so as you can imagine, this becomes the game's constant attitude towards you during a genocide route.
  • Even if you spare every other monster in the game, killing Papyrus causes Sans to completely vanish from the game until the very end, where he doesn't hold anything back as far as letting you know just how much of a heartless decision that was.
    Sans: this is an odd thing to say, but... if you have some sort of special power... isn't it your responsibility to do the right thing?
    > Yes
    Sans: ah. i see. ... T h e n w h y ' d y o u k i l l m y b r o t h e r ?
    
    > No
    Sans: heh. well, that's your viewpoint. i won't judge you for it. ... Y o u d i r t y b r o t h e r k i l l e r.
  • Played for laughs when you take too much candy from the candy bowl, causing it to spill and the narration to call you scum.
  • The worst endings of the first zone with Toriel and Flowey's speeches also qualify — if you do a betrayal kill on Toriel after she's spared you, she'll laugh hysterically at her own foolishness at having wanted to protect you when you're evidently just as much a monster as the creatures outside the Ruins.
  • The ending narration for most of the less pleasant neutral endings will call the player out on their actions. For instance, if you killed Papyrus and spared Toriel, Sans will comment how her protecting you led to you murdering his brother, and if you killed all major bosses but aborted a full-on No Mercy run at Hotland, Alphys will bitterly say that she should have killed you when she had the chance instead of letting you go on to kill nearly everyone she cared about. If you killed all of the bosses and at least one minor enemy, but didn't carry a No Mercy run up to Hotland, the Undergound will instead fall into a hopeless anarchy, with Sans calling the player out for driving them to that point.
• When All Else Fails, Go Right: The starting point is the westernmost point in the game, and you largely move towards the east, with the end of the game being the easternmost point.
• "Where Are They Now?" Epilogue: The true ending credits display or tell what every character, monster encounters included, is up to after being freed. The neutral endings, meanwhile, have the characters themselves inform you of what happened next. Also, if certain conditions are met during certain monster encounters, when the ending credits roll describing what the monsters are up to now, they are replaced by yellow text doing something else.
• Where the Hell Is Springfield?: For presumably the same reason as most other ambiguities about the world, we're never told or hinted at where Mt. Ebott is in relation to the rest of the world. The culture the monsters have seems very American and the city we see in the background in the true ending looks decidedly American with the skyscrapers, but the backstory mentioning magicians and way the Underground is run by a monarchy suggest European, especially since it's implied the Monsters would have been sealed at the height of the middle ages.
• White-and-Grey Morality: The True Pacifist Route. The Player Character becomes an All-Loving Hero who ends battles peacefully, and the Random Encounters come off as Non Malicious Monsters. The only truly villainous characters in this route are the Fallen Child, Flowey, and Asriel. Out of these, the Fallen Child is an Ambiguously Evil human who actually brought a lot of hope to the kingdom of monsters before setting into motion an Evil Plan that resulted in the demise of both them and Asriel. Asriel himself is a Tragic Villain who just wants to be reunited with his lost friend and can't bring himself to move on from what happened. And Flowey is a reincarnation of Asriel, whose vile and sociopathic nature stems from the Lack of Empathy he got saddled with after becoming a flower. Everyone else is a Well-Intentioned Extremist at worst, and in order to succeed in this route, you yourself must go out of your way to portray the Messianic Archetype.
• Widely-Spaced Jail Bars: Papyrus attempts to block your path with a set of these, which Sans points out can't actually stop you. Later on, it becomes evident that Papyrus hasn't learned anything; if he defeats you, he "locks" you in a cell that you can literally walk out of.
• A Winner Is You: Subverted. After Flowey is spared or killed, the game goes straight to some rather boring credits. However, The Stinger flips this right on its head.
• A Wizard Did It:
  • How environments like Snowdin and Waterfall even exist in the Underground with functioning ecosystems and individual flora.
  • Papyrus floating (kind of) over the player after sparing him in battle.
• A Wolf in Sheep's Clothing:
  • The Player Character is a small unassuming child who’s capable of singlehandedly destroying the world.
  • Flowey, the Big Bad, is a jovial flower who acts like a typical peppy tutorial character. That charade doesn’t last long at all.
  • While not evil, Sans is definitely more dangerous than he seems.
• Worked Shoot: Subverted. During the final battle against Mettaton, he admits that your previous battles with him were an act. However, at this point, he has concluded that it's necessary to kill you for real. It's implied the shoot was starting to get un-worked as you entered the CORE.
• World of Pun: Boy, is it ever. Sans might be the most notable offender, but the puns are everywhere… Yes, even in your menu.
• Writer on Board: Happens a few times, both with the same character.
  • Near the end of the No Mercy route, Flowey has some choice words for people who watch the run online and are too cowardly to do it themselves.
  • After the final boss of the pacifist run, the player can return to the beginning of the Ruins and talk with a ghostly Asriel. If the player talks to him repeatedly, he'll go on a philosophical tangent about how in the outside world, unrelenting pacifism may not always be a viable option, and that sometimes, the best you can strive for is "don't kill, and don't be killed" (echoing Flowey's catchphrase).

Trope Y-Z

• Year X: The opening cutscene takes place in "201X." The game itself takes place much later.
• You Are Better Than You Think You Are: During a Genocide run, Papyrus will try to stave you off your destructive path by assuring you that you're a great person at heart and, if you just try, no matter how little effort you put into it, the great person you are will shine through. Should you kill him anyway, despite his speech, he'll even use his last breath to promise you that he knows you can do better, even if you don't think so yourself. The amount of faith he puts in you has proven to be just as effective as Undyne's and Sans's beatdowns when it comes to making people abort their Genocide runs.
• You Are Too Late: In any neutral ending where she is alive, Toriel will arrive just after Asgore dies. It doesn't matter how long you take, or if Asgore kills himself, you kill Asgore, or Flowey does the deed. The only run with the subversion is the True Pacifist run, where she shoots Asgore with a fireball and ends your fight with him before it begins. It's implied that the time you spend in the True Lab ends up buying you enough time for her to mount a rescue/intervention.
• You Bastard!:
  • The game repeatedly tries to invoke this against the player for taking violent options. Both implicitly, with plenty of Video Game Caring Potential and Player Punches, and explicitly in Flowey's speeches after the encounter with Toriel.
  • As listed under Take That!, even players who don't actually play the "kill everyone" route aren't safe from being shamed, as Flowey suggests that Undertale owners who are watching a playthrough of the route just to see what it's like are too cowardly to try for themselves, and are arguably worse than those that do for indulging in this version of the story while thinking that they're still above the consequences.
  • Notably, a rare example of a game which invokes this for actions taken in past playthroughs, such as the crime of trying to achieve 100% Completion when doing so requires you to reset the Golden Ending you've achieved and instead murder all the people who were once your friends. The game not only calls you on your belief that you can do these things with no consequences, but is designed to ensure that any attempts to get the Golden Ending afterwards are tainted forever by what you did if you follow that path to the end.
• You Could Have Used Your Powers for Good!: If you kill Papyrus without going full genocide, Sans will confront you just before the final battle and indirectly give you this accusation by phrasing it as a question.
  Sans: sometimes... you act like you know what's gonna happen. like you've already experienced it all before. this is an odd thing to say, but... if you have some sort of special power... isn't it your responsibility to do the right thing?
• You Mean "Xmas": Snowdin has a tradition of giving presents under a tree, which originated when some pranksters started putting decorations on Gyftrot.
• Your Size May Vary: Characters' battle sprites aren't drawn to the same scale relative to each other as their overworld ones. It's not too noticeable for most of the game, mostly because major characters tend not to appear in-battle with each other, but during the True Pacifist ending, all of the main characters are shown together both in the battle interface and on the overworld, and there are obvious differences. For example, Asgore is, judging from his overworld sprite, supposed to tower over everyone except Toriel, but in-battle Undyne is almost a full head taller than him.
• Zero-Effort Boss: Most bosses and enemies on a No Mercy run can be killed before they even get to act.

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