Wham Line / Undertale

Please don't hide spoilers here. Wham Line is a spoilerific trope by nature. You Have Been Warned.

Considering that the game serves as a deconstruction of the RPG genre, Undertale drops quite a few shocking lines that radically change the story. Be also advised that due to the Sliding Scale of Idealism Versus Cynicism based on game-play and your actions, these lines will need to be given the appropriate context.

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Neutral

• After getting past Toriel, almost anything Flowey says (which depends on what you did) counts as one, but the one the game most strongly leads unspoiled players into encountering is for if you kill Toriel, then reload and spare her.
  Flowey: But don't act so cocky. I know what you did. You murdered her. And then you went back, because you regretted it. Ha ha ha ha... You naive idiot. Do you think you are the only one with that power? The power to reshape the world... Purely by your own determination. The ability to play God! The ability to "SAVE."
• During the dinner scene with Sans, he'll begin to talk about how he met a woman and made a promise to her. It's all rather lighthearted until he says this line:
  Sans: do you get what i'm saying? that promise i made to her... you know what would have happened if she hadn't said anything? ...buddy. ...You'd be dead where you stand.
• For players on a mostly Pacifistic playthrough that feel they have a chance of reasoning with Asgore and have him let them out of the Underground, Alphys drops the line:
  Alphys: I can't take this anymore. I... I lied to you. A human SOUL isn't strong enough to cross the barrier alone. It takes at least a human Soul... And a monster soul. ... If you want to go home... You'll have to take his soul. You'll have to kill ASGORE.
• In a meta sense, if you've been playing this game like a typical RPG, The Reveal during Sans' judgement is going to hit HARD. These lines completely change the way the player thinks about the game.
  Sans: What's EXP? It's an acronym. It stands for "execution points." A way of quantifying the pain you have inflicted on others. When you kill someone, your EXP increases. When you have enough EXP, your LOVE increases. LOVE, too, is an acronym. It stands for "Level of Violence." A way of measuring someone's capacity to hurt.
• On the other hand, if you killed his brother on the same run, Sans doesn't stop there. Instead, he expresses the suspicion that you have some kind of power, before promptly asking you a question.
  Sans: you act like you know what's gonna happen. like you've experienced it all before. this is a weird thing to say, but... if you have a special power, isn't it your responsibility to do the right thing?
  • If you pick 'yes':
  Sans: ah. i see. Then why'd you kill my brother?
  • If you pick 'no':
  Sans: heh. well, that's your viewpoint. i won't judge you for it. ... You dirty brother killer.
• One exclusive to the Nintendo Switch version of the game: throughout your journey, you might encounter a female NPC known as the "Clam Girl". She frequently talks about her neighbor's daughter, "Suzy", and says that the player will probably meet her and enjoy spending time with her; however, Suzy never appears in the game. Come the Playable Epilogue, and you can find the Clam Girl once again, who has one last little tidbit for you:
  Clam Girl: So you never met my neighbor's daughter. But please don't despair. Because the time that you will meet her...
  (Clam Girl suddenly turns gray, taking on a form similar to a Gaster follower)
  Clam Girl: ...is fast approaching.

True Pacifist

• Upon entering the True Lab, you find a series of five logs discussing Alphys (or possibly Gaster) researching the nature of the human SOUL and the energy that gives them their power. The fifth log has the speaker confirm that they have identified this energy, which gives humans the power to "change fate"...and then promptly reveal the name they have given it:
  Let's call this power..."Determination".
• While in the True Lab, you can find some tapes of memories past, primarily concerning Asgore, Toriel, and their son Asriel. The second tape opens with this line, revealing that there's more to the character you named at the start of the game:
  Asriel: Okay, X, are you ready?
• At the end, Toriel, Sans, Papyrus, Undyne, Alphys and Metatton arrive to break up the fight between you and Asgore before it starts. Once they do, Alphys asks a question that reveals that this was part of Flowey's plan to steal the human souls, as well as every monster soul in the Underground.
  Alphys: H-hey, that reminds me. Papyrus... YOU called everyone here, right? Well, besides, uh, her. Uh, anyway... If I got here before you... How did you know to call everybody?
  Papyrus: LET'S JUST SAY... A TINY FLOWER HELPED ME.
• Just before the True Final Boss of the Pacifist route. Flowey reveals his true name and identity - that of Asgore and Toriel's long-dead son Asriel.
  Child: X, are you there? It's me, your best friend.
  Boss Subtitles: ASRIEL DREEMURR
• Asriel attacks you with all his power, reducing your HP to 1, less than 1, and even your life-bar breaks! But after a few moments, your heart (life bar) joins again. Why?
  Game Subtitles: But it refused.
• A line shortly after the conclusion of the battle against Asriel confirms that the name that you gave was for the first child to fall down many years ago, not the Player Character.
  Asriel: You're not actually X, are you? X's been gone for a long time. Um... what... What IS your name? "Frisk?"

No Mercy

• Just as you've left the Ruins stonefaced for Papyrus's introduction, Sans takes a moment to explain to you that his brother would really like to see a human.
  Sans: so, y'know, it'd really help me out... if you kept pretending to be one.
• A bit later, Sans pulls you aside to give you some "friendly advice" about what you're doing:
  Sans: if you keep going the way you are now... you're gonna have a bad time.
• A line made many people abort their attempted No Mercy runs. To some extent, you can justify a No Mercy run early on as an act of self-defense - after all, most monsters are attacking you. Sure, you're going out of your way to kill them over and over until the area is depleted of them, but the resultant stat gains will only make it easier to survive against future struggles, right? However, when you get past Snowdin and you get into a "fight" with Papyrus, this is what the flavour text says after the "fight" starts:
  Papyrus is sparing you.
• Sans's last line before giving you exactly the bad time he warned you about:
  Sans: it's a beautiful day outside. birds are singing, flowers are blooming... on days like these, kids like you... Should be burning in hell.
  • Granted, he did give you a bad time, but what comes after you mortally wound him is incredibly creepy because him fighting you was not the bad time he warned you against:
    "so... guess that's it, huh? ...just... don't say i didn't warn you.^note The "bad time" is when the Fallen Human destroys the world of Undertale after you off Asgore and Flowey (regardless of what your choice was), and only brings it back if you sell your SOUL to them (Frisk's SOUL to be precise), and doing this leads to any future True Pacifist Run being ruined at the very end by the Fallen Human taking control of Frisk.
• When you try to attack Sans for the first time, he does something that no other monster in the game can do: he dodges your attack.
  Sans: what? you think i'm just gonna stand there and take it?
• Finally, there's whenever the text in the narration boxes turns red during the No Mercy route... Examining Toriel's kitchen? "Where are the knives." Examining the mirror in Toriel's house? "It's me, X." Examining the coffins before your encounter with Asgore? "(It's as comfortable as it looks.)"