SPOILERS ARE UNMARKED, READ WITH CAUTION!
- Let's start with a meta example: The moment when you find out how far the game goes in knowing what you've done through Save Scumming, particularly with Flowey's speech at the end of the Ruins.
- The entire game could be considered an Awesome moment due to how it approaches the subject of RPGs. The No Mercy/Genocide route, in particular, has been touted as one of the darkest plots ever put in a video game, comparable to the likes of Soul Nomad and Drakengard.
- Special mention has to go to the sheer lengths the game goes through to dissuade a No Mercy route. It's very trying on an emotional level, not just because you (usually) know and love these characters before trying the route, but it completely kills any player-based justifications by the time you take out Papyrus. No, you are not killing in self-defense. You're killing because you can. It's also not particularly satisfying from a gameplay perspective, either. Sure, your stats go up, but you don't earn anything in the way of new skills, and with two rather important exceptions, everything goes down in a hit or two anyway. Compared to the esoteric battles you can get trying to SPARE them, not particularly satisfying. Then there's the final boss, which, as stated elsewhere, is as controller-breakingly unfair as possible and mocks your maxed attack and defense by making them effectively worthless, all with the weakest stats in the game on paper. Basically, if you complete the run, you really do have no one but yourself to blame for the tainted future playthroughs.
- During the No Mercy route, a few NPCs still stand up to the player:
- Gerson, the turtle shopkeeper in Waterfall, openly trash-talks you because he knows you can't kill people in the shop screen, so there's absolutely nothing you can do to him.
- While it's obvious that Burgerpants just doesn't understand the gravity of the situation, threatening him leads to him smoking a cigarette and smugly replying, "Threats don't work on me. I can't go to hell. I'm all out of vacation days."
- Despite being scared shitless, Monster Kid stands in your way to keep you from hurting other people.
- At E3 2017, Sony announced out of left field they would bring the game to Playstation 4 and Vita!
- On a similar note, the March 8, 2018 Nintendo Direct revealed a port was coming to the Nintendo Switch!
- Sans is now a Mii Costume in Super Smash Bros. Ultimate, and unlike the rest of the Mii Costumes (aside from Cuphead and Shantae later), he comes packaged with a song: MEGALOVANIA, to be specific. To put this into perspective, Toby Fox originally composed that for an Earthbound fan game and now it's shown up in Super Smash Bros, alongside a costume that represents his masterpiece and characters, locations, and music from the very games that inspired that original ROM hack. To have one of your creations achieve that level of influence and fame is beyond the wildest dreams of most fans, but Toby Fox did it. One can only imagine how thrilled he was when that offer was extended to him. Sans' inclusion also technically makes him the first playable indie game representative in Smash Bros., which is a hell of a world's first to claim. Think about all the retro gaming and indie icons that preceded Undertale. Toby's work being included in Smash a mere four years after making his official debut as a game developer is mind-blowing.
- If you dodge Flowey's introductory shower of "friendliness pellets", he starts to get a little annoyed, and the second time, he resorts to shouting at you to run into the bull er, friendliness pellets. After the third time, he throws his facade away and wonders if you just wanted to make him suffer. Even as he attempts to make it clear that he intends to kill you via undodgeable bullet ring, it can be satisfying to see through his ruse and piss him off by refusing to play along with him.
- Immediately after, Toriel sends Flowey flying with a single flameball.
- Toriel's words right before the bossfight can count too. Essentially, she says if you are so insistent on leaving, you can... but only if you prove to her you will be fine out there with the certainty of Asgore killing you like he (or the rest of the monsters) killed the other humans who fell in if he gets the chance by fighting her first.
- If you decide to kill Dogaressa, Dogamy becomes depressed and hardly makes an effort to fight anymore. However, should you kill Dogamy first, Dogaressa snaps and becomes a much tougher enemy, hacking away with her axe.
- When Papyrus confronts you in a fight to the east of Snowdin Town, you finally get to see what he's capable of. He starts out with completely nonthreatening attacks that are incapable of hurting you if you're high enough in the box. "Guess he's as incompetent at fighting as he is at everything else", you think... and then he proceeds to turn you blue, forcing your soul to have "gravity" added to it and making those low attacks much harder to avoid. "Bonetrousle" kicks in. Game on.Papyrus: YOU'RE BLUE NOW! THAT'S MY ATTACK!
- How does Papyrus learn your cell number? He calls them all sequentially until he gets to yours. He may be thick, but you've got to admire his dete uh, or rather, perseverance.
- Papyrus is by far his most heroic on a No Mercy run, how? He doesn't attempt to fight you; instead, he confronts you with optimism, friendship, and mercy: even if it gets him cut down, he dies stating his belief that you can still do the right thing. Keep in mind that many people give up a No Mercy run at Papyrus, and you'll realize this is every bit as effective a deterrent as both Undyne and Sans, and unlike them, he doesn't even have to raise a violent finger towards you.
- When Undyne finally takes off her helmet and confronts you face-to-face.
- On the No Mercy run:
- She comes in to intercept you at the last second when you attack the helpless Monster Kid, suffering a fatal blow in their place.
- Despite the fact that she's mortally wounded in a single hit like nearly every other monster you've fought up to this point, she channels the power of Determination to resurrect in a new form and give you hell. Yes, this girl went so anime that she channeled a power only humans are supposed to have.
- And then, there's what she says before the fight:Undyne: My body... It feels like it's splitting apart. Like any instant... I'll scatter into a million pieces. But... deep, deep in my soul. There's a burning feeling I can't describe. A burning feeling that WON'T let me die. This isn't just about monsters anymore, is it? If you get past me, you'll... You'll destroy them all, won't you? Monsters... Humans... Everyone... Everyone's hopes. Everyone's dreams. Vanquished in an instant. But I WON'T let you do that. Right now, everyone in the world... I can feel their hearts beating as one. And we all have ONE goal. To defeat YOU. Human. No, WHATEVER you are. For the sake of the whole world... I, UNDYNE, will strike you down!
[in a blinding flash of light, she turns into Undyne the Undying]
Undyne the Undying: You're gonna have to try a little harder than THAT.
*The heroine appears.
*The wind is howling...
And the piano begins to play...
- Undyne becomes the only monster in the game from there on that you cannot defeat in one or two hits. Your attacks only take off small increments of her health, while every hit you receive has absolutely brutal effect; even more so by how fast her barrages of attacks come at you.
- If (or when) you lose to her, a new SAVE point is created right before the bridge, so that you don't have to make such a long walk on subsequent attempts. As SAVE points are the manifestations of your Determination, one way to interpret this is that after having the No Mercy run handed to you by multiple easy boss fights, you've become so full of excitement and Determination from your first real challenge that you want to get back into the fight quickly if you get killed again.
- When (or if) you finally beat her, she's still so full of Determination that, unlike all other monsters, she begins to melt instead of simply turning to dust, like the Determination-infused Amalgamates. She dies with the widest grin you'll ever see on her, confident that Asgore will stop you and avenge her death.
- Even after you deplete Undyne's health completely on a Neutral run, she keeps standing. She somehow manages to keep going, even as her attacks slow and the music grows dreary, determined to not lose to you. In the end, she melts like in the genocide route, screaming her refusal to die. Sadly, it isn't enough.
- Really... everything Undyne does is awesome. The woman is living anime. Even cooking is done with burning passion! Making friends has never been so intense! Shame about the house (it's still burning when you come back in the epilogue).
- Papyrus gets one when he manages a clever Batman Gambit to get Undyne to be friends with you, proving he can actually be pretty smart when he wants to be.
- If you rescue Monster Kid when they fall off the bridge, they put themself between you and Undyne, their hero, and outright tells her that if she wants to hurt you, she'll have to go through Monster Kid. This from a tiny kid who can't run without falling over and hero-worships Undyne. Kid's got some serious balls.
- Muffet's attack where she attempts to feed you to her pet, the spider-muffin-thing. It's one of the longest attack sequences in the game, and it's more dynamic than most, having its own build-up section.
- The boss fight at the end of the Core, Mettaton EX, is pretty cool, especially if you're trying to win non-lethally. The fight includes a "ratings" counter that you have to get over 10,000 (or 12,000 if you don't want to blow his limbs off), which is actually challenging as they drop fairly quickly.
- The ratings meter, especially as it increases, implies that everyone in the Underground is watching the battle. Ending the battle via the ratings meter (rather than killing the boss) has Mettaton commenting on his record ratings and a slew of viewers calling in to tell him how deeply his shows have moved them.
- You still have to survive the boss's attacks although taking damage can also improve ratings, especially if you used the right ACT.
- Alternatively, another ACT has you boast that you won't be hit, which makes the ratings increase during the boss's turn as long as you stay untouched.
- Many types of items boost ratings when used, including changing your armor and even using the Stick, if you kept it in your inventory.
- At one point, the boss asks you to type something flattering, and reacts differently depending on what you write.
- Distinct from using the FIGHT option to deal damage, you can also attack the boss during some of their turns and cause his limbs to blow off.
- But most of all, why does this Glory Hound of a boss want so badly to defeat you? He knows that if you go on to fight Asgore and lose, Asgore will get the last SOUL needed to go to war against the humans Mettaton wants to prevent that.
- The beginning of the fight with Asgore, which the game has built up as the final obstacle of your entire journey. Then Asgore pulls out an Interface Screw of a degree that hadn't been seen in the game yet: destroying the MERCY button.(A strange light fills the room.)
(Twilight is shining through the barrier.)
(It seems your journey is finally over.)
(You're filled with DETERMINATION.)
- And then there's what comes next, especially for players who expected they were getting the best ending possible: everything about Photoshop Flowey, from the fight itself, to his trollface after loading his SAVE, to finally beating him.
- There's something to be said about how Frisk defiantly steps forward to fight Photoshop Flowey even after he threatens to use the SAVE function to endlessly tear them to bloody pieces over and over.
- Special note goes to the six human SOULs uniting around you to give you the power and support you need to finally stand up to Photoshop (fan name Omega) Flowey, coming in with the track switch from the dismal and frightening "Your Best Nightmare" to the rousing and inspiring "Finale", Flowey's defense is shattered and he DESPERATELY starts to throw everything he can at you. What seemed like a hopeless battle has turned. And after Flowey resets his SAVE and then tries and fails to reset it again, the SOULs reappear and attack Flowey themselves!
- Even though you've been forced to fight, the game still gives you the option to SPARE the boss. Flowey insists that the only way to end this for good is for you to kill him, even threatening to kill you and everyone you love if you let him go. If you persist despite having every reason to give in, he breaks down and flees, as you've shattered his worldview of "kill or be killed" which he's maintained from the start.
- Lets say you succumb to the temptation to kill him — given the power he's shown, and the threats he's giving, you may legitimately consider his existence something of a Godzilla Threshold. So you do it... and his last words are "I knew you had it in you." Thereby taking away your moral victory.
- On the other hand, killing him results in Flowey avoiding you on subsequent playthroughs, similar to how he avoids Sans - a.k.a. the one who forced him to reset the timeline the most.
- Pretty much everything about the final confrontation of the Pacifist route.
- The build-up in which you get to see all of the friendships you've made, including gathering together all of the minor monster encounters, all to confront Flowey, despite it going badly.
- When the six big characters are first entrapped by Flowey's vines, notice Undyne. While the others appear stunned, she's fighting against the vines the entire time.
- Undyne, Asgore and all others are covered in two vines: Papyrus, however, is covered in four. Flowey also mentioned that it took him a while to get bored of Papyrus if you keep doing a Neutral Run and Undyne mentions that Papyrus is "pretty freakin' tough". The implications are obvious.
- While everyone is trapped and judging by their expressions in pain, they still protect you from Flowey's attacks. Even though Undyne is the only one fighting the vines, everyone is doing what they can to protect you who they have come to love so much.
- The build up right before the True Final Boss fight. After Flowey absorbs all of the monsters' SOULs from the Underground in a flash of light, the player is alone in a black void with a monster child. He then begins to introduce himself: cue turning around, appearing as a Boss Monster... and then transforming into an adult version of himself, the words ASRIEL DREEMURR appearing letter by letter in stylistic bold text.
- During the battle, the background shines with all the colors of the rainbow, the boss' attacks are extremely over-the-top and awesome, and he even calls them!
- And the background doesn't start doing that until the first crescendo in the boss theme, "Hopes and Dreams."
- Dying in the fight against the boss yields this message: "But it refused." You can't die, as your SOUL simply mends itself together and refuses to let the fight end here and now, no matter how much damage it takes.
- The final phase of the fight where the button to ACT becomes one to SAVE. You then confront the Lost Souls of the many friends you made throughout the game, absorbed by Asriel and fighting against you... but by reminding them of the times you have spent together, you are able to rescue them from hopelessness and get them back on your side.
- Also from the final boss fight: after the player saves all of Frisk's allies and tries to SAVE Asriel himself, the boss is reduced to firing a huge Wave-Motion Gun while bawling Berserker Tears, begging Frisk to let him win, while Frisk's determination is so strong that they survive their HP going into the fractions, all the way down to 0.0000000001/20.
- The final boss is even more awesome if you're trying for a No-Damage Run on top of True Pacifist. Because now Asriel is a legitimate challenge instead of a Foregone Conclusion, and his over-the-top attack patterns are almost as hard to avoid as Sans'. And, yes, it is possible to avoid everything save for the final rainbow laser.
- An understated one: At the turning point of the final battle, Asriel's broken out his One-Winged Angel form, and the sheer power of his presence alone leaves the player unable to move. The narration specifically points out that you're reaching for your SAVE file, but can't. Saving the game is impossible. But then the narration suggests that, with what power you have remaining, you can save something else...
- At the very end of the game after the True Final Boss, as you are free to travel and revisit characters, you discover that the biggest stealth badass of all was the one nobody expected: NAPSTABLOOK. Due to Asriel having learned your name with everyone's souls inside him, everyone now knows your name, except that one, single character. Why? When Flowey unleashed that flash of light that reached through all the underground, allowing him to absorb every monster, Napstablook saw the snails on their farm vanishing from their window and just shut the blinds. That's right: while Frisk was valiantly facing off against Asriel for the fate of the timeline, Napstablook was just chilling at home, same as always, no worse for wear!
- During the New Home walk on No Mercy, as Flowey talks to you, one of the points he makes is how he and you are better than the "sickos" who prefer to watch acts of cruelty happen rather than having the guts to do it themselves, i.e. Take That, Let's Play Viewers! It can catch viewers off-guard in a spooky way, but damn, Flowey is Genre Savvy personified.
- Playing through the No Mercy route will make you feel like a horrible person, but the final fight is awesome nonetheless in many ways. You'll definitely have to use some Villainous Valour of your own to win against this one, and it feels great when you do:
- Absolutely everything about the fight is openly designed so that it's as rage-inducing as possible starting with stupidly hard-to-dodge attack patterns that rack up immense damage from the smallest mistakes, and throwing in several Interface Screws and a number of fake-outs. Who's responsible for putting you through this hell? It's Sans, who finally decided to get serious. Turns out he's known all along that you're an "anomaly in the timespace continuum" with the power to reset the world, and he was hoping you might stop if he just became friends with you. But he also knows that this time is different, and if he doesn't stop you here, everything will end.
- The first thing they do, before the music starts, even before the battle menu comes up, is hit you with a rapid-fire series of attacks including giant laser volleys that are nearly impossible to dodge unless you know they're coming. After you finally manage to survive this, they comment that they "always wondered why people never use their strongest attack first." No other enemy does this. If you start the battle again, he'll interrupt his own monologue to sucker-punch you!
- And speaking of that monologue: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.
[cue Sans' strongest attack]
- The above in itself is a CMOA for the boss himself when you realize what the opening line in the dialogue box means: "You feel like you're going to have a bad time." At this point, the Fallen Child has fully assumed control of the character, and has become corrupted to the point of now being a horrific, soulless abomination that Was Once a Man, who thrives on the dust and blood of murder and even toying with hope of being spared, if you choose to be that sadistic. But Sans? Sans legitimately frightens this travesty of nature after the abrupt onslaught he unleashes so casually. He's the only character who scares the Fallen Child. For good reason.
- Who stopped Flowey every single time he got close to a Genocide run or something like it, and forced him to reset the most? You guessed it.
- At the halfway point, Sans offers you a chance at mercy. Should you choose it, Sans will reward you by straight-up killing you and telling you to "get dunked on". This seemingly dick move is a test on Sans' part; if you fall for his taunt, all it did was prove that you believed accepting his offer was the easy way out. By saying "if we're really friends... you won't come back", he's saying that if you cared about him or the other members of the Underground one bit, you'd reset and start going for the Pacifist route. If you go back to him and reject his offer, then congratulations: you've been exposed.
- Fridge Awesome, with a dash of sadness takes effect when you consider who Sans is talking to as he offers mercy. He's not talking to the Fallen Child, but Frisk; acknowledging that they're still there deep down, despite everything. It makes a lot of sense to consider that Sans isn't so much trying to bluff The Fallen into letting down their guard, but appealing to the person trapped within to take over and suppress the evil just long enough to perform one last good action by letting him end it mercifully.
- The boss shows their insane level of ability in an entirely dissimilar fashion to anything thus far: being the most Genre Savvy opponent you can face. They sidestep your attack every time, asking why they should just stand there and take it. When you finally overcome their best gauntlet of attacks, they unleash their special attack: nothing. Literally nothing. They're perfectly aware they can't beat you, and one blow from you will finish them, so they decide to do nothing, trapping you in their turn so you can't attack. There's only way you can win: dragging the borders of the attack zone with your SOUL so that you can reach the FIGHT command. But you still have to wait for the boss to fall asleep from exhaustion or else they'll reset your position. And even then, the boss evades your attack, so you have to break the rules further and attack twice in one turn to actually land the hit.
- Everything about the boss in this fight is badass. Their cool moves, their lack of care, their insanely complicated trolling, and their unwillingness to move an inch while firing these attacks at you demonstrate just how absurdly powerful they are.
- Perhaps the most amazing part is that this boss has, on paper, the worst stats in the game. 1 ATK, 1 DEF, 1 HP. "The easiest enemy," according to CHECK. But this enemy knows that tiny 1-damage ticks won't trigger your Mercy Invincibility. That means you're taking 1 damage per frame! Compare this to the Fallen Child's obsession with stats and power, and that by this point, you have items that give you maximum ATK and DEF. In this boss battle, your stat boosts are absolutely useless. Considering you are about to finish a complete monster genocide, you feel "your sins crawling on your back", and being hit incurs residual damage in the form of poisonous Karma, you might suddenly realize that "No Mercy" means no mercy for you, either.
- Again, playing the No Mercy run will make you feel awful. But after dying literally dozens of times to this enemy, nothing will feel sweeter than finally killing him off. One of the more awesome moments, simply because of how unbelievably difficult it is.
- Sans makes fun of the player for each defeat during his fight and come back to try him again. Kill him once and restart just to fight him again?Sans: that expression that you're wearing...
Sans: you're really kind of a freak, huh?
- There are several bosses that can change the color of your SOUL. But they all have a certain trick. Papyrus sends you through a whole turn's worth of blue bones, Undyne strikes you with her spear, Muffet pours tea on you, and during the Mettaton fight, you use a phone program that Alphys wrote. But there's one boss that can manipulate your SOUL at will, changing you between blue and red in the middle of a turn. Sans.
- After finally taking out the above monstrosity, you get a rather twisted moment yourself during the confrontation against Asgore. Just before he is cut down, Asgore mentions he was warned about you by a certain flower. After killing him, a desperate Flowey strikes the final blow and begins begging you for mercy. You're so crazy, you manage to put fear back into a deranged, sociopathic abomination.
- While most players tend to focus on the fact that you can choose (in vain) not to erase the world in the end, it's entirely possible to go along with Chara's suggestion, even if you're fully aware that doing so might involve them "killing" you (implied by the slashing animation). You can choose to be someone filled with determination to destroy everything, up to and including yourself, proving that sometimes, the cruelest and most destructive people are those who, unlike Flowey, don't fear death.
- It's actually possible to delete the save file flags that trigger the Corrupted Pacifist Ending. By doing this, you essentially manage to beat the Fallen Child at their own game.
- And if you are feeling especially clever, note that the Fallen Child will not ERASE the world until you make a final decision, ERASE or DO NOT. There's nothing stopping you from just quitting the game and starting over, cold feet and consequences be damned...