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. It can be achieved in any playthrough with no pre-requisites, although any subsequent Pacifist Routes will have their Epilogue slightly altered, even with 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, the player intentionally triggers encounters 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 trigger, but no monsters will appear, and the text "But nobody came." displays. 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 slightly lower pitched and slowed.

The number of remaining enemies is displayed at save points starting in Snowdin. The Ruins does not provide an enemy count at save points, however, their save messages will still change to "Determination." when all monsters in the area have been killed. Additionally, to satisfy a Genocide Route all bosses must be killed.

If at any point the player's actions are not heinous enough to meet the requirements for a Genocide Route, they will be shunted back into the Neutral Route, where the game's normal soundtrack plays and more NPCs can be found.

You can however flee from most monsters (except monsters listed below) without any penalty as long as you kill the amount needed in each area to continue the genocide run. With this method, the player can choose which monsters they would desire to kill the most in terms of gaining most EXP & gold from the limited amount of monsters you can kill. For example, one can kill Glyde 14 times to gain 1400 EXP & 1960 gold with this method, leveling up earlier and making future fights a bit easier.

Requirements
You will know that you have enough kills when interacting with a save point simply produces 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/5 LV (Toriel)
 * Snowdin: 16 kills (Papyrus)
 * Waterfall: 18 kills/10 LV (Undyne the Undying)
 * Hotland/Core: 40 kills/15 LV (Mettaton NEO)
 * New Home: 1 kill/19 LV (Sans)
 * Kill every normal boss (i.e. secret bosses So Sorry and Glyde are not required)
 * Toriel
 * First Snowdrake
 * Lesser Dog (can be avoided if you walk into Dogamy & Dogaressa without going into battle with him)
 * Greater Dog
 * Dogamy and Dogaressa
 * Doggo
 * Papyrus
 * Glad Dummy
 * Monster Kid (attacking them is required)
 * Undyne the Undying
 * Royal Guards
 * Muffet
 * Mettaton NEO

Kill counters are a tricky issue. If certain events are triggered before clearing an area completely of monsters, you will be automatically sent back to Neutral Route. For example, if you fight Papyrus without exhausting the Snowdin kill counter, the cutscene will proceed as if you were on a Neutral Route. Even if you let him capture you, go back and finish off the last monsters, you are still on the Neutral Route. Once you have left the Genocide Route, you cannot return to it.

Note that Mettaton NEO is a critical character for a Genocide run. If the player is not on the Genocide Route, Alphys' scenes will proceed as usual on the Neutral Route, and they will instead fight Mettaton EX. If the player is on a Genocide Route, Alphys will never appear, the traps will be deactivated, and the player will instead fight Mettaton NEO. Mettaton is also the point of no return for the Genocide Route. Should the player kill Mettaton NEO without whiffing their attack, there is only one boss left (Sans) who will kill you for sparing him.

Ruins

 * If the player exhausts the kill counter in the Ruins, Napstablook will vanish when the player approaches him.
 * The two Vegetoids hidden in the ground in the switch room (After the 4 Froggits room) do not appear.
 * In Toriel's Home, if the player checks the kitchen, the protagonist asks " Where are the knives. "
 * 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 you there, she wasn't protecting you, but everyone else.
 * 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 offers to destroy the world (and its inhabitants) together.

Snowdin

 * After leaving the Ruins, you encounter Sans, who is unsettled by your lack of any expressions. He then proceeds to attempt you to hide behind the conveniently shaped lamp, but you simply ignore him. Papyrus runs in, and leaves after Sans tells him he found a human, all while ignoring your presence. Sans tells you that his brother would really like to see a human, and then asks the protagonist to help by pretending to be one.
 * Doggo shivers with fear when the player approaches him.
 * Papyrus does not initially recognize the player as a human.
 * Sans does not inform the player of blue stop signs.
 * The Snowdecahedron is just an ordinary snowball.
 * You can continue to take snowman pieces from the Snowman until he is completely destroyed.
 * The player mostly ignores all of Papyrus' puzzles, and most of them are already solved.
 * The player walks through the zap maze before Papyrus finishes explaining it.
 * You can still check the word search, although the dialogue will be different.
 * The switch hidden in the snow is already depressed with vines.
 * The spikes are already retracted for the X --> O puzzles, and Papyrus is absent.
 * Frisk cuts off Papyrus' 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 Genocide Route, specifically mentioning the player's upcoming battle with Papyrus.
 * The NPCs in Snowdin Forest and Snowdin Town have all evacuated, except to the Monster Kid, who believes Undyne will be able to protect everyone.
 * You are able to take the Snowdin Shopkeeper's wares without paying, and also steal 758 gold from the counter.
 * When you encounter Papyrus, Papyrus immediately offers to spare you.

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 you 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.
 * The protagonist upsets the Mad Dummy so much by their actions, which causes it to take a corporeal form within the dummy, turning them into the Glad Dummy. Since you helped the Mad Dummy become corporeal, it attempts to let you go.
 * 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.
 * Interestingly enough, he appears to have some meta knowledge, stating that he knows you can't hurt him, and thus your threats mean nothing - this is due to you not being able to fight shopkeepers.
 * Temmie Village is empty, except for the Temmie Shopkeeper, who acts normally, and the one Temmie that watches you 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, Monster Kid approaches you, and says that to continue, you will have to fight them. After you attempt to attack them, Undyne jumps in and takes the 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, she 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.
 * All of the NPCs, other than Burgerpants, are absent.
 * You are able to take Bratty and Catty's wares without paying.
 * Instead of meeting Alphys, you encounter Mettaton, who flees, knowing that you would easily defeat him
 * 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 destroy 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.
 * The elevator in the Core is functional from the start, and you can use it to bypass the rest of the Core.

New Home

 * The Worn Dagger present box in Asgore's Home now contains the Real Knife, to which the protagonist thinks "About time."
 * The Heart Locket present box in Asgore's Home now contains The Locket. "Where it belongs."
 * Examining various objects throughout Asgore's Home will yield unique descriptions.
 * Padlocks: " He leaves them in the kitchen and the hallway. "
 * Beds: "My Bed" and "His Bed."
 * Bookshelves/Asgore's Closet: : " Nothing useful. "
 * Refrigerator: " No chocolate. "
 * Kitchen Note: "I've read this already."
 * Photograph: " ... "
 * Journal: "The entries are always the same."
 * Drawing of Flower: " My drawing. "
 * Closet: "Our clothes."
 * Asgore's Dresser: " Still has that sweater. "
 * Mirror: " It's me, . "
 * Whilst walking through Asgore's Home, Flowey talks to the player, telling them about his life ever since he was a flower, their ability to SAVE and why they started killing. 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 if they want to have a bad time should they continue going down their path. He will fight you, attempting to stop you from destroying the timelines of all worlds.
 * Within the room with all of the coffins, if you inspect the red one, you are greeted with the following message: " It's as comfortable as it looks. "
 * Within the throne room, you attack Asgore, who was talking to Flowey, depleting 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 then attacks Flowey repeatedly, 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 you. Chara explains that you were the reason they came back - the soul and determination that allowed them to destroy everything - they came from you, the player.
 * 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, you are given a prompt to 'ERASE' the world, 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', and then the game will crash.

Misc.

 * If you call Toriel after you have killed her, it states "But nobody came."
 * After the ruins, all of the areas you go through will play a slowed, slightly distorted version of the theme that would play in a non-genocide run.
 * When you encounter "But nobody came" instead of an enemy encounter, the music will change to a very slowed down version of Flowey's theme.
 * 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
On a genocide run the player can act mercifully: complete an area by sparing the boss, or reach certain points in the story without exhausting the kill counter, and the genocide run will end. Future areas will be in the Neutral Route but any previous areas will remain affected by the genocide run.
 * If Papyrus is spared, ending a genocide run, 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 before the fight. In the ending's phone call, Sans hands the phone to Alphys.
 * If the Genocide Route is aborted midway through, Flowey's dialogue at the end of the Neutral Route will be different. Usually, if you spare Flowey, he'll give you advice on how to complete the True Pacifist Route, but after aborting a genocide route, he will instead tell you that he's surprised by your choice of resetting and having so much mercy, and will question your motives for doing so.

If the player has gone past Genocide's point of no return, they can simply reset their game so long as they do not approach Asgore.

Post-Genocide Game

 * After having completed a genocide route, once you've chosen whether or not to erase the world, and after the corresponding dialogue, the first human strikes at the screen, destroying the world. This closes the game.
 * Upon a relaunch, you'll happen upon 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, the fallen human will speak to you. Reminding you that you're the one responsible for the world's destruction and ask if you are above consequences. If selected YES, the Fallen Child will simply say, "Exactly". If selected NO, the Fallen Child ponders the player's motives for seeking to destroy the world in the first place.
 * Regardless of your choice, they come up with a compromise: they will give you the option to restore your world, at the cost of your SOUL.
 * If the player chooses to decline these terms, they will mock the player to remain in the abyss and leave, forcing the player to wait an additional 10 minutes to have the deal re-offered.
 * If the trade is accepted, then the game will crash once more and your SAVE file will be deleted. You will be able to restart the game with everything restored. However, the game will be permanently altered from now on.

Changes in Post-Genocide Game

 * At the end of the Pacifist ending, you find out your bargain with the first human has allowed them to take 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 with glowing red eyes and a more 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 - everyone's faces except Frisk's has been crossed out with a red X, implying Chara has killed them all. This could be considered the true bad ending, as Frisk had successfully fulfilled Chara's objective to escape the Underground, and will now destroy all of humanity without anyone left to stop them.
 * "The End" text changes from white (or yellow) to red. Also, 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 wants to escape and destroy the Human world, they are encouraging the player to complete a Pacifist Route)
 * If the player selects "DO NOT", Chara will claim "You already made your choice long ago.", leading to the same cutscene like the first time.
 * 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 Run.

Save Locations
Default save locations are as follows:
 * Windows: %SystemDrive%\Users\%USERNAME%\AppData\Local\UNDERTALE and USER/AppData/Roaming/Steam/CODEX/280360/remote/System_information_#
 * 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.