Genocide Route

The Genocide Route, sometimes called the No Mercy Route, leads to one of the three main endings of Undertale. It consists of the protagonist killing every possible monster in the Underground. This route's ending can be achieved without prior completion of the game. Once the protagonist completes a Genocide Route, subsequent True Pacifist Routes will have their ending altered – even after a True Reset.

There are two independently conceived definitions for the No Mercy Route. One definition is equivalent to the Genocide Route, making the terms interchangeable; the other describes a specific type of Leaderless Ending where all monsters from all encounters – but not all possible monsters – are killed.

Method
The Genocide Route occurs when the protagonist kills every enemy in every region - the Ruins, Snowdin Forest, Waterfall, and Hotland. (The CORE is a subregion of Hotland.)

Starting in the Ruins, the protagonist intentionally triggers encounters and kills monsters until encounters with monsters no longer occur; this is similar to level-grinding in other RPGs. After killing all monsters in an area, encounters still occur, but monsters do not appear. In a font size smaller than usual, the battle text reads "But nobody came."

After the protagonist sees this message, the music for the area changes to a distorted-sounding ambient track. This track is a .08x speed version of "Your Best Friend." After exiting the Ruins, all overworld songs are lower pitch, slower versions of their Neutral and True Pacifist Route counterparts.

If the protagonist stops meeting the requirements for a Genocide Route at any point, they will revert to a Neutral Route where the game's unaltered soundtrack plays and the protagonist can find more NPCs.

Requirements

 * Exhaust the random monster kill counter in every area. This requirement must be done before approaching the final encounter of the area.
 * Any number of random encounters may be spared (Snowdrake has special properties on this, as seen below) – including any monsters that would otherwise exceed the kill counter. For example, if three monsters are randomly encountered together when only two monsters are "left" in the area, killing 2 of them and sparing the last will still fulfill the Genocide Route requirements. This can be exploited to avoid fighting particular monsters, such as the high-defense Jerry of Snowdin.
 * The following is a table of the number of random encounters only that the protagonist must kill as stated on SAVE points in red text, followed by the final boss of the area.
 * Ruins: 20 kills (Toriel)
 * Snowdin: 16 kills (Papyrus)
 * Waterfall: 18 kills (Undyne the Undying)
 * Hotland: 40 kills (Mettaton NEO)
 * New Home: No kills (Sans). Asgore and Flowey are automatically killed when encountered.
 * Snowdrake is unique in that he must be killed before the kill counter is exhausted, and is also part of the monster kill counter.
 * Once the protagonist satisfies the kill requirement, SAVE points will display the message "Determination."


 * Kill every boss and miniboss. (Secret bosses So Sorry and Glyde are not required.) All mini-bosses (excluding Snowdrake and including Shyren) can be encountered even after the kill counter is exhausted, which can be used to obtain extra EXP.
 * Toriel
 * Snowdrake
 * Doggo
 * Lesser Dog, if it is encountered
 * Dogi
 * Greater Dog
 * Papyrus
 * Shyren
 * Glad Dummy
 * Undyne the Undying (triggered by fighting Monster Kid)
 * Royal Guards
 * Muffet
 * Mettaton NEO

Kill counters for different areas have different requirements. If certain events are triggered before clearing an area completely of monsters, the protagonist will be automatically sent back to a Neutral Route. For example, if Papyrus's battle is initiated without exhausting the Snowdin kill counter, the cutscene will proceed as if on the Neutral Route. Even if the protagonist is captured and then goes back to finish off the last monsters, re-fighting Papyrus will still proceed as on the Neutral Route. Once a Genocide Route is aborted, it cannot be returned to without reloading to a SAVE before it was aborted, or resetting entirely.

There is a glitch if the protagonist enters the fight with Toriel. If there is still one possible encounter left in the Ruins, the fight with Toriel will proceed as normal for a Neutral route; however, if Toriel is killed, the protagonist will continue a Genocide Route.

After Exhausting Kill Counter

 * If the protagonist exhausts the kill counter before finding Napstablook, Napstablook will vanish when the protagonist approaches them.
 * The Froggit and Vegetoid NPCs do not appear.
 * In Toriel's Home, if the protagonist checks the kitchen, the flavor text reads " Where are the knives. "
 * When checking the mirror, the flavor text, "It's me,  ." appears instead of "It's you!"
 * When facing Toriel at the exit of the Ruins, the protagonist kills her with a single attack.
 * When attempting to talk to Toriel, the flavor text "Not worth talking to." appears.

After Killing Toriel

 * In the next chamber, Flowey identifies the protagonist as the first human.

Before Meeting Requirements

 * When encountering Sans, he is unsettled by the protagonist's lack of reaction to his joke. He then attempts to hide them behind the conveniently shaped lamp, but they ignore him. Papyrus runs in, and leaves after Sans tells him he found a human and does not notice the protagonist. Sans then tells the protagonist that they should continue pretending to be a human.
 * Doggo shivers when the protagonist approaches him.
 * When Papyrus finally notices the protagonist, he initially does not recognize them as a human.
 * Sans does not appear as an NPC outside of encounters with Papyrus.
 * The "snowdecahedron" is just an ordinary snowball.
 * The protagonist can take three snowman pieces from the Snowman until he becomes a "useless pile of snow".
 * The protagonist ignores all of Papyrus's puzzles, and some of them appear to be solved with Flowey's help.
 * The protagonist walks through the electricity maze before Papyrus finishes explaining it.
 * The word search, when checked, has different dialogue.
 * If the word search is checked and the protagonist interacts with Sans afterward, he will remark that he should have made the puzzle a junior jumble, as opposed to a crossword during a Neutral Route.
 * The spikes are already retracted for the X --> O puzzles, and Papyrus is absent.
 * The ice bridge for the ice X --> O puzzle is already expanded.
 * The protagonist cuts off Papyrus's explanation of the tile puzzle and walks through it before Papyrus can activate it. After ignoring the puzzles, the music changes to a sad tune.
 * The "gauntlet of deadly terror" is retracted after Papyrus feels like the human would not appreciate it. After Papyrus leaves, Sans warns the protagonist that they will "have a bad time" if they continue the way they are currently going, specifically mentioning the protagonist's upcoming battle with Papyrus. He then seems to teleport out of the area.
 * In version 1.000, all SAVE points said " That comedian... " after the protagonist passed the bridge gauntlet. According to Toby, the message appearing so often was a bug and was fixed in version 1.001.
 * The "comedian" in question is Snowdrake – if Snowdrake is not killed before the counter is exhausted, the SAVE point text will change to "The comedian got away. Failure.", and the Genocide Route will be aborted.
 * The Inn will be usable, but the dialogue will be different from other routes.

After Meeting Requirements

 * The game of "ball" cannot be played, as there is no snowball. Flavor text for the hole is "There's a hole here."
 * Once the protagonist exhausts the kill counter, all Snowdin NPCs except Monster Kid evacuate.
 * The protagonist can no longer sleep at the Inn.
 * Inspecting the Christmas tree at the center of town displays the message "Nothing for you."
 * Inspecting the unfinished book in the Snowdin Library displays the message "Some unfinished book."
 * A decoy version of the Innkeeper's Child will be present.
 * Due to the absence of the Snowdin Shopkeeper, the protagonist can steal all of her wares and 758G from the counter.
 * Papyrus tries to convince the protagonist to be a good person and immediately offers to spare them. Even after being killed, he still believes that the protagonist can do better.

Before Meeting Requirements

 * Nearly all of the echo flowers are silent.
 * All of the NPCs are absent, except Monster Kid, Gerson, Napstablook, and the Temmie Shopkeeper.
 * Like Snowdin, most of the puzzles are already completed.
 * Rocks are not falling down the waterfall in the second room.
 * The Bridge Flowers are already aligned.
 * Sans is not stationed at his telescope, and the protagonist cannot look through it.
 * When Monster Kid offers to help the protagonist reach the ledge, the protagonist climbs on to the Kid's shoulders roughly.
 * The protagonist's actions upset the Mad Dummy so much that it causes them to become corporeal, turning them into the Glad Dummy. Since the protagonist helped the Mad Dummy become corporeal, it immediately offers to spare them. The dummy must be killed for the Genocide Route to progress.
 * Gerson remains in his shop, and sells his items as usual, albeit with different dialogue. He also appears to have some meta-knowledge, knowing that the protagonist cannot harm him as shopkeepers cannot be fought, and thus their threats mean nothing.
 * Temmie Village is empty except for the Temmie Shopkeeper, who behaves normally, seemingly unaware of what is happening, and the Temmie that watches the protagonist from within the wall.
 * The Tem Shop has no option to pay for the shopkeeper's college education – this is replaced with the option to buy Temmie Flakes at a price of 1000G, making it impossible to obtain the Temmie Armor during a Genocide Route.

After Meeting Requirements

 * An additional save point appears before the bridge.
 * On the bridge, Monster Kid approaches the protagonist, saying that Undyne told them the protagonist had done bad things and began to see the truth in her words. They try to fight the protagonist, but when the protagonist retaliates, Undyne jumps in and takes the fatal hit. Instead of dying, however, she recovers and becomes Undyne the Undying, who is significantly more difficult than the ordinary Undyne boss fight.
 * After being defeated, Undyne dies with a smile, aware that Alphys is evacuating the monsters ahead of the protagonist, and will tell Asgore to absorb the souls he has collected.

Before Meeting Requirements

 * Alphys is absent in her lab. Instead, Mettaton acknowledges the protagonist's malice and flees.
 * The console that shows the protagonist says "It's me,  ." when interacted with.
 * The protagonist will describe the bag of dog food as "half-empty" (as opposed to "half-full", if no-one has been killed yet), followed by the message "You just remembered something funny." when interacted with. This message also appears after killing Lesser Dog, Greater Dog, and Doggo in Snowdin, and have 21+ kills.
 * Upon being inspected, the hole in the wall will yield the message "It's just here to complete the look."
 * Like Snowdin and Waterfall, most of the puzzles are deactivated.
 * The lasers are all off.
 * The puzzles involving shooting the opposing ship are already solved.


 * The Royal Guards are encountered much earlier, at the three steam vents before the entrance to Mettaton's cooking show set, and they attempt to avenge Undyne. They can both be killed in one hit.


 * Mettaton's cooking show, news report, and musical performance do not occur.
 * The elevator in Hotland can access every floor from the start. Floors L1, L2, and R2 are cut off by force fields, however.
 * Muffet does not show up for her bake sale. Before she is fought, she does not consider the protagonist to be fit to eat but attempts to dispose of them regardless.
 * The Muffet fight still takes place, though the protagonist can kill her in a single attack. If not killed immediately, she will have unique dialogue referencing the Underground's evacuation.
 * She talks about how Alphys unsuccessfully attempted to get the spiders to evacuate.
 * The telegram she receives states that despite the protagonist being a hyper-violent murderer, they never harmed a spider since there are no other spider enemies.
 * All of the other NPCs other than Burgerpants are absent.
 * Due to the absence of Bratty and Catty, the protagonist can steal all of their wares, as well as 5G from their shop. A note can be found which reveals that Alphys came to evacuate them to someplace "super duper safe", but that Bratty and Catty wanted to use up their gel pens before leaving.
 * Examining the plant in the MTT Resort reveals that "(The potted plant is judging you for your sins.)"
 * The elevator in the Core is functional from the start, and can be used to bypass the rest of the Core.

After Meeting Requirements

 * The Mettaton encounter at the end of the Core becomes a face-off against Mettaton NEO.

New Home

 * A slower, lower-pitched version of "Small Shock" plays in place of "Undertale".
 * The present box in Asgore's Home that otherwise contains the Worn Dagger now contains the Real Knife, to which the protagonist thinks "About time."
 * The present box in Asgore's Home that otherwise contains the Heart Locket now contains The Locket, to which to protagonist thinks "Right where it belongs."
 * Examining various objects throughout Asgore's Home will yield unique descriptions.
 * Padlocks, first time, if no keys are used: " He leaves them in the kitchen and the hallway. "
 * Padlocks, otherwise: " There are two keys. "
 * Padlocks, while unlocking: " I unlocked the chain. "
 * Closet: "Our clothes."
 * Left bed: "My bed."
 * Right bed: "His bed."
 * Drawing of flower: " My drawing. "
 * Bookshelves / Asgore's closet: " Nothing useful. "
 * Asgore's dresser: " Still has that sweater. "
 * Photograph: " ... "
 * Refrigerator: " No chocolate. "
 * Kitchen note: "I've read this already."
 * Journal: "The entries are always the same."
 * Stovetop: "Stovetop."
 * Mirror: "It's me,  "
 * Calendar: " The date I came here. "
 * While walking through Asgore's Home, Flowey talks to the protagonist, even if he was killed after the last True Reset.
 * He tells the protagonist about his life ever since he was a flower, his own ability to SAVE, and why the protagonist started killing.
 * He even proceeds to break the fourth wall and insult "those sickos that stand around and WATCH it happen" (meaning the people watching videos about the Genocide Route) for not playing the Genocide Route themselves.
 * Flowey reiterates his "Kill or be killed" mentality, saying that he and the protagonist would not hesitate to kill each other if the need arises. However, he finally realizes, in horror, that he is not above this and, eventually, the protagonist will kill him; he runs away afterward.
 * In the Final Corridor, Sans does not judge the protagonist but instead warns them that they will have a bad time. He will fight them as an attempt to stop them from destroying the timelines of all worlds. His battle is widely considered to be the most difficult battle in the game, followed by Undyne the Undying.
 * Within the room with all of the coffins, the red coffin says " It's as comfortable as it looks. "
 * Within the throne room, Asgore mentions having never seen a flower crying (referring to Flowey's attempts to warn Asgore of the protagonist). The protagonist then proceeds to attack Asgore, depleting all his health in one hit. Flowey finishes Asgore off, and then, with his face and voice turning to Asriel's, he begs the protagonist not to kill him. The protagonist ignores him and kills Flowey until there is nothing left.
 * Instead of leaving the black screen and returning to the Throne Room, a track consisting of high-pitched wailing and echoing heartbeats begins to play as Chara appears on the black screen and talks to the person in front of them: not the protagonist, but the player. The window name becomes blank.
 * Chara explains that the player's determination was the reason that Chara came back and that the player taught them the reason for their return and gave them the means to do so. It was the player's SOUL and determination that allowed them to destroy everything so that they could achieve this goal. They offer to destroy the world so that the two of them can move on to another world, where they will continue their slaughter. At this point, two prompts are given: "ERASE" or "DO NOT."
 * If the player chooses "ERASE", Chara will call the player "a great partner."
 * Choosing "DO NOT" will cause Chara to be seemingly surprised at the player's refusal, thinking that the player must have misunderstood them. Chara then asks the player "SINCE WHEN WERE YOU THE ONE IN CONTROL?", which then causes a jumpscare. Chara then approaches towards the screen with a horrifying facial expression while apparently laughing, emitting a horrible noise as the screen shakes while flashing red and black.
 * Following either option, a larger slash animation is seen followed by the screen shaking and filled with "9's," resulting in the world's destruction, and the game will then close.
 * During this conversation, the player cannot "ESC" or "X" out of their game. The only way to close the game at this point is through the computer's Task Manager on Windows or Force Quit on a Mac.
 * After the giant slash animation, the entire window of UNDERTALE (in the Windows versions) will shake violently side to side, implying that the attack was damaging the game itself.
 * The game will automatically revert to Windowed mode during Chara's jump scare and the large slash animation.

Misc.

 * Encounters become considerably harder to find as the protagonist progresses (except New Home).
 * Beginning in Snowdin, a kill counter is added to the Stats menu.
 * Beginning in Waterfall, instead of an "!" appearing over the protagonist's head, a smiley face will appear instead when encountering an enemy, as though representing that the protagonist is happy about the encounter.
 * Various oversights cause music in Snowdin to revert to its normal state, even on a proper Genocide Route attempt. The music returning to its normal speed after some events does not indicate or imply failure of the route.
 * If the kill counter is not yet exhausted for Waterfall, a dialogue box will appear between the second and third flowers on the path of Room 131 (room_water19), reading " Strongly felt &lt;number> left. Shouldn't proceed yet. " This is the only time a warning message appears.

Aborting the Genocide Route
The Genocide Route will end if the protagonist either completes an area by sparing the boss or reaches certain points in the game without exhausting the kill counter. Future areas will appear as though in a Neutral Route, but any previous areas will remain affected by the Genocide Route attempt.
 * If the Genocide Route is ended by sparing Papyrus, then the protagonist may only hang out with him as they cannot flirt with him during the battle. Sans will be at his post in Waterfall, thanking the protagonist.
 * Sparing the Monster Kid results in the game entering the Neutral Route, and the protagonist will fight Undyne normally.
 * There is a specific neutral ending that can be gained by entering the Neutral Route at Mettaton NEO by not killing every monster in the Hotland and CORE before the fight. Mettaton tells the protagonist that they were "holding back" after taking (significantly lower) damage from the protagonist, exploding after his speech. In the ending's phone call, Sans hands the phone to Alphys.
 * If the Genocide Route is aborted midway through if Flowey is spared, Flowey chastises the player for aborting the Genocide Route before asserting that they just wanted to see what peace was like before taking it all away. At the beginning of the next playthrough, Flowey wonders why the protagonist had reset before telling them to do whatever they want.

If the protagonist has gone past the point of no return in a Genocide Route (Mettaton NEO fight), they can simply reset their game as long as they do not give an answer to the final question from Chara.

Post-Genocide Game

 * Upon relaunching the game after Chara destroyed the world, the game now only has a black screen with the sound of howling wind. The SAVE file is gone, the inputs do nothing here, Buttons and sprites no longer appear, and the name Undertale is no longer the game name, implying that Chara really destroyed the entire world.
 * After waiting approximately 10 minutes on this screen, Chara notes the player's return to the game. They remind the player that they are the one responsible for the world's destruction, observe that the player cannot accept that they caused it to happen, and then coldly remarks that they think they are above consequences.
 * If the player affirms by selecting "YES," Chara will simply say, "Exactly."
 * If the player selects "NO," Chara will ask the player, "Then what are you looking for?"
 * No matter which option is chosen, Chara then states that a compromise could still be made. They state that the player has something that they desire, and offer to restore the world if the player surrenders it. If the player says "YES," Chara reveals what they want from the player: their SOUL. If the player agrees again, Chara states that the deal has been made, and the game closes once again but appears to boot up normally the next time it is opened. Afterward, the game is now permanently altered. From that point onward, any future playthroughs of the True Pacifist Route and the Genocide Route become dubbed by fans the "Soulless True Pacifist" and "Soulless Genocide" Route. Strangely, this does not affect the Neutral Route endings at all.
 * If the player chooses to decline these terms at either opportunity, Chara will end the conversation, forcing the player to restart the game and wait an additional 10 minutes before having the deal re-offered.

Changes in Post-Genocide Playthroughs

 * At the end of the True Pacifist Route, Chara appears to have taken over Frisk's body.
 * If Frisk chooses to stay with Toriel, when the latter leaves a piece of pie before leaving the room, after four seconds they will turn over in bed in the final scene and face the camera to reveal Chara with red eyes having taken full possession of Frisk. A lower pitched version of Flowey's laugh will play to accompany this reveal.
 * If Frisk says they "have places to go," the photograph of Frisk and friends shown in the final scene changes - Frisk is replaced with Chara, and the faces of all the other characters have been crossed out with a red X, implying Chara has killed them all.
 * "The End" text changes from white (or yellow) to red, and the Annoying Dog does not show up to sleep under the text. An extremely slowed down version of the theme Anticipation plays three seconds after the "THE END" text appears.
 * At the end of another Genocide ending:
 * Chara calls themself "The demon that comes when people call its name." It does not matter when and where, that time after time, they will appear, and with the Player's help, they will eradicate the enemy and become strong. Chara then lists the various stats that the Player increases throughout the game as the very feeling they have whenever they are increased. Chara then comments that they and the player are not the same, stating that their SOUL has twisted feelings they no longer understand. Chara now wonders on what reason the player continues to recreate the world, and why they continue to destroy the world. Chara then harshly states that the Player is wracked with a "perverted sentimentality." Chara then suggests that should they recreate the world once again, a different course of action would be advised.
 * The same dialogue will occur even after doing the Genocide ending more than twice.
 * If the player selects "DO NOT" this time, Chara will point out that this very feeling is what they had been talking about. Chara then comments to the player that unfortunately, regarding their choice, "YOU MADE YOUR CHOICE LONG AGO." This causes another jump scare, and once again crashes the game.
 * However, no black abyss will appear once the player reopens the game. Instead, the regular title screen will play as if the game was reset.

Save Locations
Default save locations are as follows:
 * Windows XP: %SystemDrive%/Documents and Settings/%USERNAME%/Local/Application Data/UNDERTALE and %SystemDrive%/Documents and Settings/%USERNAME%/Application Data/Steam/CODEX/280360/remote/
 * Windows Vista/7/8/10: %SystemDrive%/Users/%USERNAME%/AppData/Local/UNDERTALE and %SystemDrive%/Users/%USERNAME%/AppData/Roaming/Steam/CODEX/280360/remote/
 * Mac: ☀~/Library/Application Support/com.tobyfox.undertale/

Modifying/Deleting Files
To return to a playable state, the player can enter their save data folder and either:
 * Delete the file named "UNDERTALE" in your local AppData, this will basically remove everything you have done in the game as if it is your first time playing it
 * Delete or rename the files "system_information_962" and "system_information_963" in the local AppData.. This will remove any effects of having done the Genocide Route. The 962 file is the one that you have erased the world, and 963 is when you have sold your soul.

These methods will not be sufficient for players of the Steam version, as the Steam Cloud backs up the acquisition of the system_information files and restores them if deleted (even if cloud synchronization is disabled). Players of the steam version have to delete their Steam Cloud cache as stated here, or render the system_information files unreadable by Undertale by modifying their security settings under file properties to deny read permission.
 * Uninstall the game and do a fresh reinstall.

Undertale Genocide Remover

 * The Undertale Genocide Remover application automatically removes all Genocide Route files, even those in Steam, when "ERASE" is pressed.

Demo
After completing a Genocide playthrough In the Undertale Demo, the ending screen appears with red text that reads "That was fun. Let's finish the job," while playing a slowed down version of Anticipation. Also, all the pages of the manual but the first and last are replaced with said ending screen, and the last page is replaced with a picture of a faceless Flowey.