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"On days like these, kids like you...
Should be burning in Hell."

An exclusive boss who only appears at the very end of the No Mercy run who tries to stop the genocide with a fool-proof method: making the player quit. With a knowledge of the game's mechanics and pure DETERMINATION, this boss was designed to frustrate the player with the longest and most difficult fight in the game.


But wait- what's his name? Who is this guy? Well, we normally go through trope's in alphabetical order, but let's get one thing right out of the way.

  • Walking Spoiler: The Genocide Final Boss's whole shtick relies on catching you off-guard with all his rule-breaking strategies, and in addition to that, his identity is not what players would initially suspect. The nigh-impossible, brutally punishing warrior who players only get a real chance to fight on the Genocide run? It's pun-loving Sans the Skeleton! All of the tropes applying to this character outside of the Genocide fight are under that character's folder on one of the other Undertale pages. Anything related to his identity and his special attack that ends up on this page will be tagged, but if you keep reading expecting not to get anything else spoiled, you're in for a bad time.

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  • Acrofatic: Despite being plump and lazy, the Genocide Final Boss is the only character in the entire game who not only has the ability to avoid your attacks, but do so completely effortlessly. That said, it's not clear if he has this ability naturally, or if, given the game's meta nature, he's simply the only character who thought to do a seemingly-obvious thing.
  • Almighty Janitor: He's a low-level grunt with no seat of authority. He also has a working knowledge of quantum physics, has figured out the timeline is being manipulated without a fully Ripple Effect-Proof Memory, and is effectively The Dragon to Asgore. Oh, and those stats? His attacks ignore Mercy Invincibility and apply a Damage Over Time effect, and he can dodge incoming attacks. IN SLIPPERS.
  • Alpha Strike:
    • When you fight him in No Mercy, he opens with his strongest attack right away; a combination of Gravity Screw, Spikes of Doom, and Beam Spam with Wave Motion Gun. If you don't know it's coming, you're almost guaranteed to die from it until you manage to learn the pattern.
    "Huh. Always wondered why people never use their strongest attack first."
    • He also throws a vicious barrage of bones and lasers at you at the very end; a full 30 seconds length barrage, in fact.
  • Anti-Frustration Features:
    • His Battle Intro is quite long and you can't fast forward it, but you will reach the point where he practically speeds up and skips it. This is not a good thing, as the first time he does it is to catch you off-guard with his opening attack.
    • The Damage Over Time effect can only bring your HP to 1 and cannot actually kill you. Similarly, his Interface Screw attacks do not have Damage Over Time effect and can only bring your HP down to 1.
    • He stops attacking and gives you a chance to reconcile midway into the fight, telling you that he believes there's still a glimmer of a good person inside of you, someone who was his friend in another timeline. After his multiple, intense barrages that give you little, precious opportunity to heal, you'll appreciate this moment to heal yourself. And you'll want to, as the alternative is getting dunked on.
    • He also manages to invert the trope by being able to "cheat" in quite a few ways during the battle. But seeing how his goal is to frustrate the player to Rage Quitting, it makes quite a bit of sense.
  • Armor-Piercing Attack: Yes, he can only deal 1 damage, but he also reconstructs Scratch Damage into this. When you fight him in No Mercy, the fact that his attacks hit you per frame while leaving Damage Over Time makes your recently acquired Armor of Invincibility absolutely useless.
  • Attack Drone: Uses giant, floating, demonic skulls called "Gaster Blasters". They fire wave motion beams.
  • Awakening the Sleeping Giant: The screwy time manipulation that's come about in the Underground has caused him to abandon hope of getting to the surface — but he's still watching how things are playing out. Attempt to kill everyone in the game, and he'll decide he "can't afford not to care anymore".
  • Awesomeness by Analysis: While his time manipulation isn't as strong as being able to save, he's very good at filling in the blanks based on how you act and react to the events around you. There's also the fact that he can dodge your attacks just by asking himself why no one else had ever tried it.
  • Badass Boast:
    • He's full of these when he gets serious, but one in particular stands out.
    "It's a beautiful day outside. Birds are singing, flowers are blooming. On days like these, kids like you... S h o u l d b e b u r n i n g i n h e l l."
    • Or this one:
    "Hmm. That expression...that's the expression of someone who's died thrice in a row. (beat) Hey, what comes after "thrice," anyway? Wanna help me find out?"
  • Badass on Paper: Inverted, checking his stats says that he has the worst stats in the game. While this is technically true, his ATK stat of 1 ignores Mercy Invincibility, his attacks are quite large and thus hard to avoid, and his HP of 1 is irrelevant — he dodges every single attack while fighting you.
  • Ballistic Bone: Emphasizing his role in avenging the innocent, the Genocide-exclusive boss will throw Papyrus's bones at you in giant barrages both to overwhelm you and to guilt you for what's probably the most heinous murder you've committed. They also inflict Karmic Retribution.
  • Battle Intro: As is traditional with any good Boss Battle. Problem is, he'll skip his Battle Intro to catch you off-guard if you survived his Alpha Strike previously.
  • Beam Spam: He liberally uses his Gaster Blasters to fight. Taken Up to Eleven in his penultimate attack, where he fires in excess of 100 beams in less than 10 seconds.
  • Beware the Nice Ones: He's ineffably polite and is more than willing to share a good laugh, but he will stop at nothing to prevent the destruction of the Underground.
  • Big Good: Apparently regarded as a threat by the game's main villain, Flowey (hence why he avoids letting him know he even exists), having a working knowledge of how saving/determination works, as well as being implied to have a straightforward time machine. He is on good terms with many other characters. He also knows the true nature of EXP and LOVE. He faces you head-on as the toughest boss fight in the game if you're playing on the No Mercy route (which ends in dire consequences that could never be fully recovered from in-game except through messing with the save files).
  • Blatant Lies: His combat description when you CHECK him says that he's the "easiest enemy in the game." This is completely untrue, and since he leads with his strongest attack the moment the fight begins, the player will know — from painful experience — it's a lie by the time they can read it. Going far enough into the fight will have the game actually confess that reading its messages doesn't seem to be all that useful for you.
  • Bloodless Carnage: Wwhen you mortally wound him, blood pours out from his torso and mouth, despite monsters not having blood. Maybe it's ketchup?
  • Bootstrapped Leitmotif: Toby Fox's signature song MEGALOVANIA is often attributed to him due to it being used as his boss theme.
  • Boss Corridor: The Last Corridor is completely empty until he suddenly appears and stops you in your track. If you go No Mercy route, he decides to actually step into the fray and giving you one hell of a bad time.]]
  • Break the Cutie: Despite all the atrocious things you'll do in order to get to that point, having to kill him is still one of the most emotionally jarring moments in the entire game because of this trope. He fights so, so hard to stop you from destroying the world, knowing full well that the entire world will be lost, but in the end, both of you know that he can't keep dodging forever. By the end of the battle, he's been reduced to an exhausted, hopeless, pitiful wreck of himself, yet he still doesn't give up until you literally drive him to the point of losing consciousness. And if you go through with that last fatal attack, the game gives you one final shock to try and get you to think about your actions — you find out that he's is the only character who visibly bleeds when wounded. And not only his death bloody, it's slow, with you being forced to watch every agonizing second as the reality of what just happened to him sinks in, he manages a few halfhearted parting words to you, and then laboriously drags himself out of the battle screen.
  • Broken Smile: He's all smiles throughout the battle due to his confidence in his own super-abilities, but it eventually drops into a frown. It's anyone's guess exactly when it reaches its breaking point, but it clearly happens somewhere — just look at his face (spoilers!) when he realizes he's about to die.
  • Casual Danger Dialogue: He has a lot to say to you throughout the entirety of his boss battle, and he does so with perfect confidence that he'll continue to be able to dodge your attacks.
  • Combat Pragmatist: He pulls just about every dirty trick in the book, and constantly breaks the rules to compensate for his low stats. Some examples include launching a powerful attack right at the start of the battle, dodging your own attacks, ignoring Mercy Invincibility, and hitting you during menu selection. In his second phase, he's also able to change attack patterns on the fly just to screw with you. He also delivers some absolutely brutal trash talk throughout the fight (and subsequent attempts) to remind you just how much of a scumbag you are, and even pulls an I Surrender, Suckers before delivering a One-Hit Kill.
    "Huh. always wondered why people never use their strongest attack first."
  • The Computer Is a Cheating Bastard: He can break the rules of the entire game by ignoring your Mercy Invincibility, randomly jumping from attack to attack, dodging, which is something other characters can't do and killing you when you accept his offer to spare him.
  • Confusion Fu: While ultimately limited, the Genocide boss has a wide variety of different attack patterns, perhaps the most compared to other bosses in the game. Then you reach the midpoint, and he starts to randomly interrupt his attack patterns into each other, and reveals even more attack patterns, to keep the surprise factor high.]] It may take a while before you get to see them all, so keep your guard up at all times.
  • Curbstomp Battle: In a genuine first playthrough of the Genocide run, expect A LOT of these to be coming from him. In the end, however, the situation is reversed.
    "Hmm. that expression... that's the expression of someone who's died thrice in a row. ... Hey, what comes after "thrice", anyway? Wanna help me find out?"
  • Damage Over Time: In addition to massive direct damage when you touch them, his attacks also leave a slow damage over time effect called Karmic Retribution. The more you get hit, the longer this effect lasts.
  • Death by Irony:
    • The Final Boss spends the entire fight playing on your expectations and trying to catch you off-guard. How do you defeat him? By catching him off-guard with a surprise attack while he taunts you after pulling yet another Nonchalant Dodge.
    • Midway through his fight, he tries to spare you. If you accept his mercy, he also offers a hug, then kills you while you're off-guard.
  • Death of a Thousand Cuts: Basically his combat strategy. Individually, his attacks are the weakest in the game. But they also avoid triggering your Mercy Invincibility, and he can attack a lot.
  • Determined Defeatist: When he decides to step in and take action, he’s pretty much resigned to death at your hand. But knowing that you’re about to destroy the world... he just can’t afford to not care anymore. He gives one hell of a final fight with the hopes of getting the anomaly (the player) to Rage Quit or reset to a more merciful game route, but at the end of the day, you have the Save Scumming ability that he has no counter against.
    • In fact, this mentality reflects on his special attack. He admits he'll never be able to beat you, so he refuses to let you have your turn and traps you in the battle screen as a last resort. How long is he prepared to wait for you to get bored? Forever.
  • Did You Just Punch Out Cthulhu?: Through sheer power of badassery, annoyance, and persuasion, he is the only character who even remotely stands a chance of stopping you on a No Mercy run.
  • Didn't See That Coming: He's is savvy enough to keep his guard up at all times, even when he's offering reconciliation or sleeping... but he didn't expect you to be able to attack two times in a single turn. The look on his face says it all.
  • Double Meaning: His pre-battle speech, specifically the bit where he said how you are "REALLY not gonna like what happens next", can easily be interpreted as how difficult the fight is going to be. However, reading his lines further, specifically "Seeing what comes next" and "Don't say I didn't warn you..." when you mortally wound him, also opens the interpretation that 'what happens next' is referring to what the player encounter after beating him and reaching the REAL ending of the Genocide run: the destruction of Undertale.
  • The Dreaded: Anybody who successfully invokes his wrath learns to fear him. Flowey states in a post-Neutral conversation that he is the only thing that has kept him from reaching Asgore; the only time Flowey gets the better of him is in the True Pacifist run by catching him and everyone else off-guard. The Child's narration, despite being the inner thoughts of an immoral, emotionless murderer, will express concern only when they have to fight this boss.
  • Dying Alone: By the time poor his last breath is given, every monster close to him has either died or ran away. He doesn't even share his last moment with the player and walks out of the battle screen, and dies a Sound-Only Death.
  • The Dying Walk: If you kill him at the end of a Genocide Route, he'll will walk off the battle scene and suffer a Sound-Only Death.
  • Exact Words:
    • The Genocide boss never actually lies to you during his boss battle, even though the implication of his words and his actions don't line up. When asking you to Spare him, he says it would "make his job a lot easier," which is technically true in that his job is to ensure your death, and once you do Spare him, he responds that he "won't let this go to waste," which he doesn't, because it's an opportunity to kill you quickly.
    • If you CHECK him in battle, he is described as "Can only deal 1 damage." This is true, but he dodges all your attacks, and his rapid-fire attacks override Mercy Invincibility and leave behind residual damage, meaning every frame that heart touches his bone attacks will drop your HP point-by-point very rapidly.
    • Most infamously, when he uses his "special attack", he says "here goes nothing." What happens next is... literally nothing. As in Sans refuses to end his turn, since he knows that your next move will be his last. So he simply waits, hoping that you'll get so bored that you'll give up and quit.
  • Failure Is the Only Option: Taken up to its logical conclusion with his fight, when he's aware he's in a Hopeless Boss Fight.
    He can't keep dodging forever.
  • Final Boss: Of the No Mercy route. After him, you move on to Asgore, who's automatically one-shotted, so he's the last true boss you'll fight.
  • Foreshadowing: Pester Flowey enough by completing multiple Neutral Runs, and he will eventually tell you to stay away from 'Smiley Trashbag' for causing him 'more than his fair share of resets'. Then you try to [[KillEmAll walk in Flowey's footsteps, and boy are you in for a surprise.
  • Forgiveness Requires Death: If you accept his offer of friendship in No Mercy Run, he'll tell you the one way he can forgive you is if you stay down.
    "If we're really friends.... you won't come back."
  • Fragile Speedster: This trope is taken to its absolute conclusion. He is a One-Hit-Point Wonder, but good luck getting even one hit in.
  • Full-Contact Magic: He slams your soul into the walls and ceiling of the action box with a wave of his hand.
  • Glass Cannon: He's capable of rapidly shredding down your 92 HP, yet he himself only has 1 HP. To make up for this, he dodges each and every one of your attacks until the very end.
  • Glowing Eyes of Doom: One of his eyes glows as he uses various abilities during his battle, like Asgore.]]
  • Goddamned Boss: Invoked Trope. He knows that you can just use Save Scumming to keep trying until you take him down, so he deliberately makes his fight as frustrating as possible to make you Rage Quit.
  • Godzilla Threshold: You. The one thing that can make the Genocide final boss get off his ass and challegne you is the threat of a player character about to murder the last remnant of people that have managed to escape you.
  • Good Is Not Soft: He treats you absolutely ruthlessly; with a smug-as-hell attitude, he insults you, mocks you, torments you with deliberately-unfair attacks, cheats, lies, rubs his own power in your face — whatever it takes to discourage you enough that you stop playing, since he knows that's the only way he can possibly save lives at this point.
  • Go Out with a Smile: Despite probably being in immense pain from being slashed across the torso, he still manages to keep smiling even as he dies.
  • Grin of Audacity: His attitude and grin throughout his battle, when it's not Tranquil Fury, absolutely screams defiance in the face of a player character who has mowed down every other enemy in the game up to this point.
  • Guile Hero: He states that he believes that there's some good left inside you and he asks you to spare him so he can cheapshot you, which works on a few people.
  • Have a Nice Death: He'll mock you every time you die and come back against him, but his biggest one is the now-memetic line:
    "Geeettttttt dunked on!!"
  • Hero Antagonist: He's the last fight before Asgore and the only character besides Undyne to give you any trouble. The flavor text, music, and general atmosphere of the fight all emphasize how he's is the last line of defense between you and destruction of his world, and how the player deserves everything that's being thrown at them.
  • Hero of Another Story: When you face him in the Genocide battle, he can attack you at all times and avoid all attacks, almost as if he was the player character and you were the boss monster. If you complete a few neutral runs in a row, Flowey would tell you to steer clear, as he had barred Flowey's path multiple times before, and caused him to reset. Successfully.
  • Hoist by His Own Petard: After an entire fight of throwing Interface Screw after Interface Screw at you, you beat his 'special attack' by breaking the interface yourself, allowing you to press the Attack button in spite of it not being your turn.
  • Hopeless Boss Fight: The fight in the No Mercy run is this. Not for you, but for him. While he can (and will) kill you very quickly and messily, he knows it won't do any good since you can just reload your save and try again, and it's only a matter of time before you finally defeat him. The best he can hope for is to try to get you to Rage Quit from the difficulty.
  • Horrifying the Horror:
    • Both Flowey and The Fallen Child dread the moment he gets serious.
      Flowey: Say, if I have ONE piece of advice for you... DON'T. Let [him]. Find out ANYTHING about you. He'll... well... Let's just say. He's caused me more than my fair share of resets. Stay away from him.
    • If you kill him and then reset the battle twice, he will provide a unique piece of dialogue at the opening of each new fight which suggests that he might be feeling a bit fearful himself. Of course, that's just one way of interpreting it — given his lack of voice acting and very unreadable demeanor, they could just as easily mean he's impressed, surprised, or just outright disgusted.]]
      ... That expression that you're wearing... ... Well, I won't grace it with a description.
      ... That expression that you're wearing... ... You're really kind of a freak, huh?
  • HP to 1: Karmic Retribution will never reduce your health to 0. Similarly, you can't be killed by getting hit by the bones in the menu while you're taking your turn, and his penultimate attack when he's bashing you against the walls won't drop you below 1.

  • I Fell for Hours: Weaponized for his penultimate attack, combined with Gravity Screw and Spikes of Doom. And it's not even the deadliest part of said attack.
  • "I Know You're in There Somewhere" Fight: His comments at the midway point of his boss fight has him call out to "someone who wanted to do the right thing", followed by a plea for them to let it end and reset. It's not made clear if he can remember a previous Pacifist run, can sense the Human Child's kind heart beneath the Omnicidal Maniac the Fallen/the Player, or if he's simply bluffing, but it's clear he knows this isn't how it's supposed to be, and he's trying to get Frisk to realize that too. Unfortunately, Frisk is either completely gone or still can't break free from your control, so the effectiveness of this plea comes down to it reaching the player, not Frisk.]]
  • Inexplicably Awesome: Despite the fact that he's never trained a day in his life and has (presumably) been around for a tiny fraction of monster history, he is far and away the single most powerful creature in the underground, and easily among the smartest. This, combined with his awareness of the game's mechanics and constantly-manipulated timeline, almost make him feel like he's on another level compared to his peers, so much so that it's not unusual to find fans who are skeptical if he's even a monster at all. Incomplete fragments of his backstory can be found scattered around the game, but we're clearly not getting anything definite anytime soon.
  • Interface Screw: Many of the No Mercy final boss's attacks abruptly change from one to the next, and after the halfway point, he'll send bones through the menu during your turns to hit your SOUL cursor. He's able to change the direction of gravity during the Blue Heart phases as well.
  • Ironic Echo:
    • Papyrus insists, in a Genocide playthrough, that anyone can be a good person if they try. Before the final boss, said boss asks you if you think "anyone can be a good person if they just try", then laughs before you can give an answer.
    • Also, if the player attempts a No Mercy run after doing Neutral or Pacifist, the final boss will echo something that Asgore said previously when they met the protagonist, but of course with a different meaning.
    "It's a beautiful day outside. Birds are singing, flowers are blooming. On days like these, kids like you... 'Should be burning in hell."''
  • I Surrender, Suckers: Do you enjoy sparing people only to kill them when their guard is down? Well, if you think this guy's is being sincere when he offers a truce to a mass murderer like you... prepare to get dunked on. Besides just slashing him again, there's something that first-timers can look out for to realize he's faking. Remember how when enemies are in the sparing range, their names turn yellow? His name does... but the "spare" option, which normally does as well, does not.
  • I Warned You: After you cut him down, before he stumbles offscreen and dies, he gives a defeated "don't say I didn't warn you." He's very aware what's about to happen if you continue through the next two rooms and finish the playthrough, but it's no longer in his hands.
  • I Will Fight No More Forever: This is the main point of his "Special Attack", where he decides to not end his turn, intending to bore you into quitting the game.
  • Laser-Guided Karma: While fighting him, your life bar is under a "KR" effect that slowly decreases your HP like poison every time you get hit. Given cut battle narration lines in the script stating you're doomed by KARMA, it's safe to assume KR stands for Karmic Retribution. He also attempts to kill you by feigning a truce, much like you easily could have done for Toriel and many ordinary monsters at this point.
    "You feel your sins crawling on your back."
  • Last Stand: His boss fight. Everyone that your power to reset means that he can't beat you. But after seeing all the death left in your wake, knowing that you'll go on to destroy other timelines, and that there's no one else who can fight you, he decides he can't afford to not care anymore. Sadly, despite his best efforts to get you to give up, his ultimate bravery ends up costing him his life.
  • Leaning on the Fourth Wall: More than a bit Medium Aware. He uses this to full effect when denying you your turn in the battle against him, thus denying you the ability to actually progress through the game. Also extended to how he treats your reloads, which he seems to observe from an in-universe perspective. While Flowey is actively aware of how you handle the timeline and abuses savestates to skip around, this boss seems to treat your different loads as alternate timelines and simply skips forwards in battle without any visible justification. The main method of figuring out you've reloaded? Not through hard evidence, but simply recognizing how you react to different things. "That look on your means something."
  • Lethal Joke Character: He's is described as the easiest enemy who can do only a single point of damage per hit. As it turns out, what this really means is 1 ATK per frame. And boy, does he take advantage of this to the extreme. Not only does he attack at ridiculous rates and in ridiculous amounts, but also, the 1 ATK means his attacks pierce Mercy Invincibility to deal constant damage and leaving a poison effect. And they're some of the most intense in the entire game, because of the speed and range of his attacks (in some cases, there's literally just one tiny spot in the box where you can hide). He leaves the 1 defense point moot by constantly dodging your one hit kill swings.
  • Let's Get Dangerous!: In the No Mercy run, he comes to realize he can't stand by anymore, and resolves to finally do his job.
    All I know is...seeing what comes next...I can't afford not to care anymore.
  • Marathon Boss: Yes, he only has 1 HP. But you're forced to use 22 turns just to get a chance at hitting him, not including turns spent to heal.
  • Metaphorically True: His stat readout lists him at 1 attack and 1 HP. And while that is technically true, he not only refuses to let your attacks land, but his attacks continually deal 1 HP for every instant their hitboxes make contact with you and apply a poison effect that ticks down at — you guessed it — 1 damage per frame.
  • Mid-Battle Tea Break: Halfway into the battle, he will stop attacking, pleads for you to remember him as your friend from a different timeline and to stop the entire thing there and then. You can use this opportunity to heal yourself, and can even invoke the trope literally if you still have the Sea Tea. [And you'll want to, as the alternative is getting dunked on.
  • Mind over Matter: His ability to slam your character's SOUL against the inside of the battle box via hand motions seems to suggest he can do this.
  • Mysterious Stranger: A very unconventional example, in that he treats you like an old friend from practically the moment you meet and outwardly acts very friendly and laid-back. Because of this, it's not easy to notice just how little we find out about him as the story progresses or pick up on the subtle hints towards just how much he's hiding from you, but every once in a while, you get an unexpected reminder.
  • Nonchalant Dodge: Can sidestep your attacks. Well, why doesn't anyone else? ...But then it's subverted to disturbing effect he gets slices him in two.
  • Not So Stoic: He's mostly amused throughout the fight at the opportunity to frustrate you, but he'll lose his cool should you survive his penultimate attack. He, in a fit of what is clearly rage and/or desperation, turns your SOUL blue and begins slamming it around the battle box like a child flinging around a toy, a stark contrast from the quick-but-controlled way he'd been fighting the entire battle.
  • Offscreen Moment of Awesome: We never get to see it, but the Genocide final boss apparently caused Flowey 'more than his fair share of resets' in the past, so much so that Flowey will outright tell you that avoiding him is the best advice he's got.
  • Oh, Crap!:
    • His expression just before he is sliced in half.
    • Meta-wise, this will be the reaction of anyone playing No Mercy blind and realize just what they have signed up for.
  • Older Than They Look: His age is extremely vague, but he's clearly an adult. Despite this, his overworld sprite is almost eye-to-eye with the player character's, and his presumably more-proportionate battle sprite is still squat enough to give the impression that he's a child.
  • One-Hit KO: Partway through the fight, he'll will offer to spare you. This is a trap; if you accept this offer, the combat box will fill with attacks arranged in a jail-cell pattern that can't be dodged, taking off all your health and resulting in a game over.
  • One-Hit-Point Wonder: Has one HP, but still is the hardest boss in the game.note ]]
  • Orphaned Setup: Due to coding error, some of his lines from No Mercy doesn't have the intended punchline, as below (the missing parts are in bold) .
    That's the expression of someone who's died five times in a row. Convenient, huh? That's one for each finger. But soon... you'll need a cool mutant hand to count all of your deaths.
    That's the expression of someone who's died six times in a row. That's the number of fingers on a mutant hand. You'll need to find a mutant hand with even more fingers.
    That's the expression of someone who's died seven times in a row. Hey, that's good. Seven's supposed to be a lucky number. Who knows, maybe you'll hit the jackpot... and that number will multiply tenfold.
    That's the expression of someone who's died eight times in a row. That's the number of fingers on a spider. Wait, don't spiders have legs?]]
  • Overly Long Gag: He'll count how many times you've died to him on the No Mercy path, with a snarky comment for each one of the first eleven attempts. Only after 12 deaths in a row will he stop coming up with unique lines.
  • Pintsized Powerhouse: He's about as tall as the Child, and is undeniably one of the strongest bosses in the game.
  • Platform Hell: Most of his attacks are done while your heart is Blue; they're fast, tricky to dodge, and deal a crapload of damage.
  • Pre Ass Kicking Oneliner: "It's a beautiful day outside. Birds are singing, flowers are blooming... On days like these, kids like you... Should be burning in hell." This is even played with to mess with you even further — he'll cut the speech off mid-sentence and immediately jump into his opening routine entirely at random.
  • Post-Mortem One-Liner: His iconic one-liner when you fall right into his trap:
    "Geeettttttt dunked on!!"
  • Reality Warper: Very much so. He can manipulate the game's battle mechanics to make his boss battle as frustrating as humanly possible. He's the only character in the game who can attack you between turns, and can even abruptly change the attack he's using mid-turn just to keep you on your toes. If you survive his penultimate attack, he flips out and starts smashing you against anything he can see or summon, in what truly is the only unavoidable attack in the entire bullet-hell game.
  • Reasoning with God: You, the player, are effectively the god of the world of Undertale and are treated as such in-universe. In battle, the boss reaches out to you personally and pleads with you to call off your current playthrough, reminding you that you were once friends.
  • "The Reason You Suck" Speech: Delivers one to the player in the second stage of his battle, questioning why they want to destroy his world so badly. Given that this happens after his attempts to stop the player through trickery and just plain making the battle difficult have failed, doubles as an attempt to Break Them by Talking.
    I always thought the anomaly was doing this cause they were unhappy. And when they got what they wanted, they would stop all this... but that's ridiculous, right? Yeah, you're the type of person who won't EVER be happy.
  • Red Oni, Blue Oni: The blue to Papyrus' red (blue clothing, calm personality).
  • Ripple Effect-Proof Memory: Although most characters in the game have a severely downplayed example of this, only allowing them to retain subconscious, deja-vu-esque memories from previous timelines that serve no practical purpose, he's able to, among other things, keep track of the exact number of times you've died up to your eleventh attempt just by glancing at your face.
  • Roaring Rampage of Revenge: He unleashes hell on you for all the innocent lives you've taken.

  • Scratch Damage: His attacks only deal one point of damage per hit. This means it practically ignores your Defense, and considering it hits per frame while leaving a Damage Over Time component, this is a lot more dangerous than it sounds.
  • Screw the Rules, I'm Doing What's Right!: When facing him at the end of a No Mercy Run, he disregards many of the established mechanics in the game and fights dirty — interrupting his dialog to open with a brutal first strike, ignoring Mercy Invincibility with his attacks, dodging your swings, feigning surrender, attacking while you're in the menu, and using his last turn to bore you into submission. All as a last-ditch effort to stop you from killing everyone.
  • Sealed Evil in a Duel: Attempted. His 'Special Attack' is simply to start his turn... and never end it. He outright says that he intends to just sit there until you get bored and turn off the game. Unfortunately, he falls asleep after a while, allowing you to finally kill him.
  • Sheathe Your Sword: What his special attack turns out to be — he just never ends his turn, meaning you never get yours, unless you find a way to cheat the system.
  • Smash Cut: He can teleport by smashing in a few black cutaway frames and then cutting back to reality. He liberally begins using it during the second phase of the fight against him in order to smash cut you out of and into various attack patterns.
  • Smug Super: If the player loses to him (which they will, a lot), he'll keep rubbing it in their face the first twelve times after reloading a save. Afterwards, he'll ditch the showing-off, and go straight into obliteration mode.
    "Do you wanna have a bad time? 'cause if you take another step are REALLY not going to like what happens next."
    "You look frustrated about something. Guess i'm pretty good at my job, huh?"
    "Woah, you look REALLY pissed off... heheheh... Did I getcha?"
    "Hmm. That expression... that's the expression of someone who's died eleven times in a row. Well, give or take. There's nuance to this stuff. Don't think i'll be able to count very well from here. Count for me, ok? We'll start from 12."
  • SNK Boss: :A non-fighting game example. He breaks damn near every rule of the game (and of turn-based RPGs in general) when fighting you and is far and away the hardest fight in the game to boot.
  • So Proud of You: To Frisk during the judgment if you don't kill anyone.
  • Sound-Only Death: The only character you can kill who doesn't die onscreen. After you deliver the final blow, he resigns to his fate, then shuffles offscreen. Seconds later, you hear the "monster turns to dust" sound.
  • Squishy Wizard: Taken Up to Eleven with the Genocide final boss, who has 1 attack, 1 defense, and 1 HP, but who uses convoluted magical attacks with no Mercy Invincibility to compensate.
  • Standard Status Effects: Karmic Retribution has a damage-over-time effect, similar to a poison/bleeding effect. The more you've been hit, the more damage you take from it.
  • Tactical Suicide Boss: There's always a way to dodge his attacks, allowing you to weather his barrages and exhaust him. But he's shown to be capable to perform inescapable attacks too, so there's really no reason why he'd provide platforms for you to survive his attacks other than a purely gameplay perspective.
  • Talk to the Fist: He's the only enemy in the game who gets the first turn in combat, very likely catching you off-guard. And if you think you can try skipping his intro speech, he'll gladly oblige and cut off his speech abruptly straight into his opening barrage.
  • That One Boss: In-universe; according to Flowey, whose sociopathy and ability to SAVE drove him to start treating the Underground like an RPG, this one boss caused him "more than his fair share of resets." He's likely to do the same to you in a Genocide run.
  • The Fourth Wall Will Not Protect You: He's really showing it in No Mercy. A lot of his boss fight mechanics are designed to screw you, the player playing the game, instead of the in-universe character. Of particular notes are him skipping his Battle Intro straight into an Alpha Strike, throwing attacks at your cursor browsing the menu, and his ultimate attack of doing nothing so you never get your turn to end the battle. He also calls you, the player, as 'The Anomaly', and outright tells you his Hidden Depths and calling out your actions across the game.
  • The Tape Knew You Would Say That: If you get dunked on and refuse his offer this time around. More effective when you really think he's being sincere.
    "Woah, you look REALLY pissed off..."
  • This Is Gonna Suck: If you survive his opening barrage in the boss fight, he immediately opens his eyelights again and says "here we go".
    "You feel like you're going to have a bad time."
  • Tranquil Fury: He's grinning with one hand in his pocket as he pulverizes you with the other one, one of his eyes literally flashing with rage.
  • Turns Red: Once you reject his offer of mercy, he'll step up his game. This is the part where he outright starts cheating, unleashing combos that require you to do different things and attacking you while it's your turn.
  • Unblockable Attack:
    • If you fall for his false offer of mercy, he kills you with a cage that fills up the Bullet Board and is impossible to dodge.
    • The final attack in his ultimate gauntlet of attacks has him slamming your SOUL into the borders of the Bullet Board again and again, depleting your HP rapidly. Fortunately, it cannot actually kill you.
  • Unexpected Gameplay Change: Don't like platform games? Too bad. This guy does!
  • Unexplained Accent: As mentioned above, he seems to have some variety of New Yorkian accent. Pretty weird coming from a guy who's spent his entire life underground among monsters who mostly lack readable accents, especially since his own brother doesn't even have one.
  • Up to Eleven:
    • If you died from him for more than 11 times in a row, he stops counting how many times you died.
    • His moveset in general mirrors another fight on steroids: Papyrus. Right down to forcing the player to jump over/dodge fast-moving bones and stay still to avoid blue attacks.
  • Video Game Cruelty Punishment: If you've been going for the Kill 'Em All route, he bars your path as the single toughest boss in the game.
  • Wave Motion Gun: His "Gaster Blasters", which are skull-like cannons that fire off massive beams in a straight line with little to no time spent to let you get a sense for their angle. He's capable of summoning multiples at once at varying sizes and patterns to catch you out, they're easily one of his hardest to dodge attacks, and given the entire fight is ridiculously hard, that says something.
  • Weak, but Skilled: Technically speaking, he really is the weakest enemy in the game, having only 1 HP and only capable of doing 1 damage. He makes up for it through clever abuse of game mechanics. He only has 1 HP, but he dodges all of your attacks instead of just taking them like every other monster in the game. He knows 1-damage attacks don't trigger the invincibility period, so he does 1 damage per frame. That's not even getting into his SPARE trick, or his "special attack". Indeed, the only reason you can even kill him is by breaking the rules yourself — dragging your SOUL over to the ATTACK button during his turn.
  • Why Won't You Die?: Near the end of his boss fight, he tells you that now would be a good time to die. note  And when you manage to weather his final barrage of bones and lasers, he goes ballistic, his eyes glowing as he slams your SOUL repeatedly to the arena borders. That look on his face is the wordless version of this trope.
  • Wide Eyes and Shrunken Irises: The lights in his eyes become very small pinpricks of white after you mortally wound him.
  • World's Best Warrior: There is not a soul in the Underground more deadly than him. Sure, he might be the physically weakest monster there, but he'll essentially break reality to take you down.
  • Would Hurt a Child: If it means saving the Underground, he's more than willing to kill the ten-year old child trying to commit genocide.
    "It's a beautiful day outside. birds are singing, flowers are blooming. on days like these, kids like you... Should be burning in hell."
  • Wowing Cthulhu: Very few things even remotely faze the protagonist during the Genocide route, let alone impress or frighten them. He's one of those thing. He's also the one and only character Flowey, out of fear, respects on any level.
  • You Have Researched Breathing: He's the only character in the entire game who even makes an attempt to dodge your attacks. Sure, by that point in the game you're evil incarnate and could kill anything you want with a single blow, but you're still a small child wielding a kitchen knife (or whatever the Child has equipped) — jumping out the way shouldn't be rocket science.]] He even smugly points this out, which makes sense given his awareness of other mechanics in the game.


Example of: