Genocide Route

The Genocide Route, sometimes called the No Mercy Route, makes up one of the three main possible endings to be found in Undertale, which consists of the protagonist murdering every single monster in the Underground. It can be achieved in any playthrough with no pre-requisites, though once aborted although any subsequent Pacifist Routes will have their Epilogue slightly altered, even after a True Reset Function.

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 player 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 player 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 player can however flee from most encounters (except monsters listed below) without any penalty as long as they meet the kill requirement. With this method the player 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 player 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: 20 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
 * 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
 * Undyne the Undying
 * Royal Guards
 * Muffet
 * Mettaton NEO (Once defeated, there is no longer any leaving the Genocide Route)

Kill counters are a tricky issue. If certain events are triggered before clearing an area completely of monsters, the player 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 player 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.

Ruins

 * If the player exhausts the kill counter in the Ruins, Napstablook will vanish when the player 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, it says, "It's me, ." 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.
 * In the next chamber, Flowey identifies the player 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 player. Sans tells the player 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 inform the protagonist of blue stop signs.
 * 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, indicating 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.
 * The "gauntlet of deadly terror" is retracted after Papyrus feels like the human wouldn't appreciate it. After Papyrus leaves, Sans warns the player that they will "have a bad time" if they continue down the way they are 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. Contrary to popular belief, 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 more than once is actually a bug. The 'comedian' in question could be either Sans, or Snowdrake.
 * If the player enters Snowdin Town before the kill counter is exhausted, it will be inhabited, and the player will be able to use the inn as normal, but the dialogue will be different that 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 player's true nature.
 * The protagonist can no longer sleep at the inn.
 * Due to the absence of the Snowdin Shopkeeper, the player can steal all of her wares, as well as 758 gold from the counter.
 * Papyrus tries to convince the protagonist to be a good person, and immediately offers to spare them.

Waterfall

 * Most of the echo flowers are silent.
 * All of the NPCs, other than the Monster Kid, the Temmie Shopkeeper, and Gerson, are absent.
 * 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 player 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 player's 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 player helped the Mad Dummy become corporeal, it immediately offers the spare option.
 * Gerson remains in his shop, aware that as he is a shopkeeper he cannot be fought, and sells his items as usual, albeit with different dialogue filled with disgust. He also appears to have some meta knowledge, knowing that the player cannot harm him as shopkeepers cannot be fought, and thus their threats mean nothing.
 * Temmie Village is empty, except for the Temmie Shopkeeper (who acts normally, behaviorally unchanged in this route) and the one Temmie that watches the player 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.
 * On the bridge where Monster Kid approaches the player, saying that Undyne told it the player had done bad things and begins to see truth in her words. It tries to fight the player, but when they retaliate, 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

 * The "Welcome to Hotland" sign is deactivated.
 * Alphys is absent in her lab. Instead, Mettaton acknowledges the player's malice and flees, knowing that he would easily be defeated.
 * 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.
 * Right and Left Floors 2 in Hotland are cut off by force fields, yet the elevator can access every floor from the start.
 * It is revealed in the Muffet 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 fought, she doesn't consider the human to be fit to eat, but still attempts to dispose of them.
 * She talks about how Alphys unsuccessfully attempted to get the spiders to evacuate.
 * The telegram she receives states that despite the player 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 the Bratty and Catty, the player can steal all of their wares, as well as a little bit gold from their shop.
 * The elevator in the Core is functional from the start, and can be used to bypass the rest of the Core.

New Home

 * 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 player, telling them about his life ever since he was a flower, his own ability to SAVE, and why the player started killing. He even goes forth to break the fourth wall and insult "those sickos that stand around WATCH it happen" for not playing the Genocide run themselves. Flowey reiterates his "Kill or be killed" mentality, saying that he and the player 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 player will kill him; he runs away afterward.
 * In the Final Corridor, Sans does not judge the player, but 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.
 * 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. The player 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 player not to kill him. They ignore 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, Chara appears in the black screen and talks to the player. Chara explains that the player were the reason 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', they will call the player 'a great partner.'
 * Choosing 'DO NOT' will cause a jumpscare, where they tell the player that they were never in control, coming closer towards the screen with a horrifying facial expression while apparently laughing as the screen shakes and flashes to 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 then the game will crash.
 * 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.

Misc.

 * Calling Toriel's Phone after killing her states "But nobody came."
 * After the ruins, all other areas plays 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, the music will change to a 1200x slowed down version of Your Best Friend.
 * After Snowdin, instead of an "!" appearing over the player's head, a smiley face will appear instead when encountering an enemy.
 * A kill counter is added to the Stats menu.

Aborting a Genocide Run
The Genocide run will end if the player either completes an area by sparing the boss, or reaches certain points in the story without exhausting the kill counter. Future areas will be 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 player 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 player.
 * Sparing the Monster Kid results in the game entering the Neutral Route, and the player 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, Flowey, at the beginning of the game, wonders why the player had reset before telling them to do whatever they want. At the end of the Neutral Route, 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.
 * There is a large amount of altered dialog only used in an aborted genocide run.

If the player has gone past Genocide's point of no return, 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 only now 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 and remarks coldly that they think they're above consequences.
 * If the player affirms by selecting 'YES,' the Chara will simply say, "Exactly."
 * If the player selects 'NO,' the Chara ponders the player's motives for seeking to destroy the world in the first place.
 * Regardless, Chara comes up with a compromise: they will restore the world, in exchange for the player's SOUL
 * If the player chooses to decline these terms, Chara will mock the player to remain in the abyss and leave, forcing the player to wait an additional 10 minutes before having the deal re-offered.
 * If the trade is accepted, then the game will crash once more and the player's SAVE file will be deleted. The game can be restarted with everything restored, but is now permanently altered.

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, they will turn over in bed in the final scene and face the camera to reveal Chara having take 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. This could be considered the worst ending, as Chara's objective to use Frisk as a proxy to escape the Underground was fulfilled successfully, Chara will now destroy all of humanity without anyone left to stop them.
 * "The End" text changes from white (or yellow) to red, and the Annoying Dog does not show up to sleep under the text.
 * At the end of another Genocide ending:
 * Chara chastises the player for their "perverted sentimentality" and suggests that should they recreate the world 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," Chara will claim "You already made your choice long ago."
 * If a Genocide Route playthrough is done again, Chara expresses confusion as to why the player keeps destroying the world they keep bringing back. Chara even states that they don't understand the player's feelings and motives from their sold SOUL anymore. However, Chara still feels obligated to bring back the world once again in hopes that the player completes a True Pacifist Route.

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
Note that this method will not work for players of the Steam version.

To return to a playable state, the player can enter their save data folder and either:
 * 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.
 * Copy the above file or create a new file and (re)name it "system_information_963". The presence of this file will load the game normally, but with all post-genocide route effects. Essentially this file is noting you have sold your soul to the First Human.
 * Alternatively, right click either file, select properties and then security. Remove [Your username] and [administrators] rights to READ the file. Click apply.
 * Alternatively, one can completely uninstall the game and do a fresh reinstall.

Steam

 * Undertale automatically syncs with the Steam Cloud before starting up - even disabling Steam Cloud or disconnecting from the internet is not sufficient. There is always a local Steam Cloud cache available. However, the Steam Cloud cache files can be deleted as stated here.

Undertale Genocide Remover

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

Demo
In the Undertale Demo, once you do this, the ending screen appears with red text saying 'That was fun. Let's finish the job.' and ominous music plays - this music is the pre-battle theme slowed down significantly.

The pages of the manual (except for the first and last) will be replaced with said ending screen, and the last page will be replaced with a picture of a faceless Flowey.