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 murdering every single monster in the Underground. It can be achieved in any playthrough with no pre-requisites, though once completed any subsequent Pacifist playthroughs will have their ending altered, even after a True Reset.

Method
The Genocide Route is achieved by killing every enemy in every region - The Ruins, Snowdin Forest, Waterfall, and Hotland/Core (for this route, Hotland and Core share the same enemy counter).

In this route, starting at The Ruins, the protagonist intentionally triggers encounters and kills monsters until encounters with monsters no longer occur, in a manner similar to level-grinding in other RPGs. After killing all the monsters in an area, encounters will still occur, but no monsters will appear, and the battle text will display the message "But nobody came." Upon seeing this message, the music for the area will be changed to a distorted-sounding ambient track, instead of the regular music both in and out of battle. After exiting the Ruins, all further overworld songs are lower pitched and slowed, and the process is repeated for the next area.

The number of remaining enemies is displayed at save points starting in Snowdin. The Ruins does not provide an enemy count at save points. When all monsters in the area have been killed, the save messages will change to simply "Determination." Additionally, to satisfy a Genocide Route, all "bosses" and "minibosses" must be killed (secret encounters like So Sorry and Glyde are not required).

If at any point the protagonist stops meeting the requirements for a Genocide Route, they will be shunted back into a Neutral Route, where the game's normal soundtrack plays and more NPCs can be found.

The protagonist can, however, spare or flee from most encounters (except monsters listed below) without any penalty as long as they eventually meet the kill requirement. With this method, the protagonist can choose which monsters to kill the most, with the purpose of maximizing EXP and GOLD. For example, one can kill Glyde 14 times to gain 1400 EXP & 1960 GOLD with this method, leveling up earlier and making future fights slightly easier.

Requirements
Once the protagonist satisfies the kill requirement, a save point will simply produce the message "Determination."
 * Exhaust the monster kill counter in every area. This must be done before approaching the final boss monster of the area.
 * Ruins: 21 kills (Toriel)
 * Snowdin: 16 kills (Papyrus)
 * Waterfall: 18 kills (Undyne the Undying)
 * Hotland/Core: 40 kills (Mettaton NEO)
 * New Home: 1 kill (Sans)
 * Kill every boss and miniboss (i.e. secret bosses So Sorry and Glyde are not required)
 * Toriel
 * First Snowdrake (doesn't have to be killed on the first encounter, but must be killed before the kill counter is exhausted)
 * Doggo
 * Lesser Dog (can be avoided if Dogamy and Dogaressa are encountered before him)
 * Dogamy and Dogaressa
 * Greater Dog
 * Papyrus
 * Shyren
 * Glad Dummy
 * Monster Kid (killing is not possible, but attempting to do so will trigger the fight with Undyne the Undying, which is mandatory in order to complete a Genocide Run)
 * Undyne the Undying
 * Royal Guards
 * Muffet
 * Mettaton NEO (once defeated, it is impossible to abort the Genocide Route)

Kill counters are a tricky issue. 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 and finish off the last monsters, refighting Papyrus will still proceed like on the Neutral Route. Once the Genocide Route is aborted, it cannot be returned to without reloading to a save before the run was aborted.

When approaching Undyne in Waterfall, however, if the kill count has not yet been met, the message " Strongly felt left. Shouldn't proceed. " will appear, warning the protagonist. This is the only time a warning message appears, however.

Ruins

 * If the kill counter in the Ruins is exhausted, Napstablook will vanish when the protagonist approaches them.
 * The Froggit and Vegetoid NPCs do not appear.
 * In Toriel's Home, if the player checks the kitchen, the protagonist asks " 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 is able to kill her with a single devastating blow. She is astonished by the protagonist's hatred before she succumbs to her wound, and tells them that by keeping them there, she wasn't protecting them, but rather everyone outside of The Ruins.
 * When attempting to talk to Toriel, the flavor text "Not worth talking to" appears.
 * In the next chamber, Flowey identifies the protagonist as the first human, seeing them as nothing more than an empty vessel like him but with a 'stolen soul', and proposes to destroy the world (and its inhabitants) together.

Snowdin

 * 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 simply ignore him. Papyrus runs in, and leaves after Sans tells him he found a human, not once even noticing the protagonist. Sans tells the protagonist that his brother would really like to see a human, and then asks them to help by continuing to pretend to be one.
 * Doggo shivers with fear when the protagonist approaches him.
 * When Papyrus finally does notice the protagonist, he does not recognize them as a human at first.
 * Sans does not appear as an NPC outside of encounters with Papyrus.
 * Game of "ball" cannot be played, as there is no snowball.
 * The snowdecahedron is just an ordinary snowball.
 * The protagonist can continue to take snowman pieces (three) from the Snowman until he is completely destroyed into a "useless pile of snow".
 * The protagonist ignores all of Papyrus's puzzles, and some of them are already solved.
 * The protagonist walks through the electricity maze before Papyrus finishes explaining it.
 * The word search, when checked, has different dialogue.
 * The switch hidden in the snow is already depressed with vines, implying that Flowey pressed it, and is likely the reason puzzles throughout the entire underground are already solved.
 * 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 wouldn't 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.
 * All save points say " That comedian... " after the protagonist passes the bridge gauntlet (regardless of whether they interacted with Sans). This does not mean that the Genocide run is a failure, merely that the kill counter for Snowdin can no longer be seen. According to Toby, the message appearing so often is a bug. The 'comedian' in question is Snowdrake—if Snowdrake is spared, the save point text will change to "The comedian got away. Failure."
 * If the protagonist enters Snowdin Town before the kill counter is exhausted, it will be inhabited, and the protagonist will be able to use the inn as normal, but the dialogue will be different than on a normal run.
 * Once the kill counter is exhausted, the NPCs in Snowdin Forest and Snowdin Town will all be evacuated, except for the Monster Kid, who is still unaware of the protagonist's true nature.
 * The protagonist can no longer sleep at the inn.
 * Inspecting the Christmas tree at the center of town will result in the message "Nothing for you."
 * Inspecting the unfinished book in the Snowdin Library will display the message "Some unfinished book."
 * A decoy version of the Innkeeper's Child will still be present.
 * Due to the absence of the Snowdin Shopkeeper, the protagonist can steal all of her wares, as well as 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.

Waterfall

 * Nearly all of the echo flowers are silent.
 * All of the NPCs are absent, except Monster Kid, Gerson, 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 the Monster Kid offers to help the protagonist reach the ledge, the protagonist climbs on to the Kid's shoulders roughly. Monster Kid complains, saying that they shouldn't be so rough.
 * The protagonists actions upset the Mad Dummy so much that it causes the ghost in it to take a corporeal form within the dummy, turning them into the Glad Dummy. Since the protagonist helped the Mad Dummy become corporeal, it immediately offers the spare option. The dummy must be killed, else the run will fail.
 * Gerson remains in his shop, and sells his items as usual, albeit with different dialogue filled with disgust. 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's happening, and the Temmie that watches the protagonist from within the wall.
 * The Temmie 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. Doing so accomplishes nothing. There is no way to obtain the Temmie Armor in a Genocide Run.
 * On the bridge where Monster Kid approaches the protagonist, saying that Undyne told them the protagonist had done bad things and begins 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.

Hotland and Core

 * Alphys is absent in her lab. Instead, Mettaton acknowledges the protagonist's malice and flees, knowing that he would easily be defeated.
 * The console that shows the protagonist says "It's me,  ." when interacted with.
 * Inspecting the dog food will yield the message "You just remembered something funny." when interacted with.
 * The hole in the wall, upon being inspected, 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, and they attempt to avenge Undyne.
 * Mettaton's cooking show, news report, and musical performance all 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.
 * It is revealed during Muffet's fight through her dialogue that it was Alphys who blocked off most of Hotland after evacuating the area.
 * Muffet doesn't show up at her bake sale. Before she is fought, she doesn't consider the protagonist to be fit to eat but attempts to dispose of them regardless.
 * The Muffet fight still takes place, though the protagonist kills her instantly.
 * 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, due to the fact that 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.
 * The potted plant in the MTT resort judges the protagonist for their sins.
 * The elevator in the Core is functional from the start, and can be used to bypass the rest of the Core.

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 "Where it belongs."
 * Examining various objects throughout Asgore's Home will yield unique descriptions.
 * Padlocks: " He leaves them in the kitchen and the hallway. "
 * Padlocks, while only having one key: " There are two keys. "
 * Padlocks, while unlocking: " I unlocked the chain. "
 * Closet: "Our clothes."
 * Beds: "My Bed" and "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. "
 * Whilst walking through Asgore's Home, Flowey talks to the protagonist.
 * 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 goes forth 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 run 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 himself 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, then, with his face and voice turning to Asriel's, he begs the protagonist not to kill him. The protagonist ignores him, attacking Flowey repeatedly and without mercy, killing him, and continues until there is nothing left.
 * Instead of leaving the black screen and returning to the Throne Room, an ominous track begins to play as Chara appears in the black screen and talks to, not the protagonist, but the player themselves. Chara explains that the player's determination was the reason that Chara came back; the soul and determination that allowed them to destroy everything. 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.
 * After the giant slash animation, the entire window of UNDERTALE will shake violently side to side, implying that the attack was damaging the game itself.

Misc.

 * After the Ruins, all other areas play a slowed, slightly distorted version of the theme that would play in a non-genocide run.
 * When "But nobody came..." appears in place of an enemy encounter, a certain track begins to play, and will play in all areas afterward (with some exceptions).
 * After Snowdin, 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.
 * After the Ruins, a kill counter is added to the Stats menu.

Aborting a Genocide Run
The Genocide run 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 run.
 * If the Genocide run 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. 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 run, 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 run (Mettaton NEO fight), they can simply reset their game so 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 inputs do nothing here. There are no menus. No buttons, nothing. The world is no more.
 * After waiting approximately 10 minutes in this screen, Chara notes the player's return to the game. They remind the player that they're the one responsible for the world's destruction, observe that the player cannot accept that they caused it to happen, and then remark coldly that they think they're above consequences.
 * If the player affirms by selecting 'YES,' Chara will simply say, "Exactly."
 * If the player selects 'NO,' then Chara questions why the player even is in the void.
 * 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, 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. The player's SAVE file is deleted, and the game is now permanently altered. From that point onwards, any future playthroughs of the True Pacifist Route and the Genocide Route become known by fans as the "Soulless True Pacifist" and "Soulless Genocide" Route. Unusually, this does not affect the Neutral Route endings at all.
 * If the player chooses to decline these terms, Chara will end the conversation, forcing the player to wait an additional 10 minutes before having the deal re-offered.

Changes in Post-Genocide Game

 * At the end of the True Pacifist Route, Chara has taken over Frisk's body:
 * If Frisk chooses to stay with Toriel, after 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 and a lower pitched version of Flowey's laugh will play.
 * 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", stating that time after time, they will appear, and with the Player, they will eradicate the enemy and become strong, listing 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. (Chara, who wants to escape and destroy the human world, is encouraging the player to complete a True Pacifist Route.)
 * If the player selects "DO NOT" this time, Chara will point out that this very feeling is exactly what they had been talking about. Chara then comments to the player that unfortunately, "YOU MADE YOUR CHOICE LONG AGO." This causes another jumpscare.

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: 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.
 * Delete or rename the file "system_information_962". This will remove any effects of having done the genocide route. This file being present without "system_information_963" is what causes the game to load 'destroyed' until the player sells their soul.
 * Create a file with the name "system_information_963". The presence of this file will load the game normally, but with all post-genocide route effects.
 * 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. In addition, all 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.