Genocide Route

The Genocide Route makes up one of the three main possible Endings to be found in Undertale. It can be achieved in any play through with no pre-requisites, although any subsequent Pacifist Routes will have their Epilogue slightly altered, even with a True Reset.

Method
The Genocide Route is achieved by killing every enemy in every region- Ruins, Snowdin Forest, Hotland/Core (For the Genocide Route the Hotland and Core have the same enemy count).

As a note, this does not mean that the route is achieved by killing every enemy that the player happens to encounter. Instead, the player needs to kill enemies in encounters until encounters with enemies no longer occur in each area. The number of remaining enemies is displayed at save points. After killing all encounters, encounters will appear, but they will have no enemies and the text 'But nobody came.'

Additionally, to satisfy a all bosses must be killed.

If the player doesn't meet the requirements for a then they will enter the Neutral Route, where the music is at normal speed and more NPCs are found in the overworld.

Main Story
In the ruins, the protagonist doesn't let Toriel finish speaking before fighting and killing her. She is astonished by the protagonist's hatred before she succumbs to her wound, and tells them that they will fit in with the rest of the Underground. Flowey identifies the player as the first human, and offers to destroy the world (and its inhabitants) together.

In Snowdin forest, the protagonist doesn't react well to Sans' pranks. He tells them that his brother, Papyrus, would really like to see a human, and, having inferred that they are a cruel person, asks the protagonist to help by pretending to be one. They skip Papyrus' puzzles and arrive at the town, finding it mostly evacuated. Papyrus notices the human's behaviour and offers them guidance, sparing them and leaving him vulnerable. The protagonist kills Papyrus, who tells them that they can still be a better person.

In the Waterfall, the protagonist upsets the Mad Dummy, causing it to take a corporeal form with the dummy, allowing them to be killed. Gerson remains in his shop, aware that as he is a shopkeeper he cannot be fought, and sells his items as usual. The Monster Kid approaches the protagonist, who attacks them, but Undyne takes the hit. Instead of dying, she recovers and becomes a powerful heroine, telling the protagonist that she will save not only monsters but humans as well as the entire world by stopping them. She is defeated, but 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.

In Hotland, the protagonist encounters Mettaton, who flees, and kills any other monster in their way, including Muffet. The MTT Resort is mostly abandoned, except for the MTT-Brand Burger Emporium employee, who does business as usual. The protagonist kills the monsters in the Core before leaving to fight Mettaton, who reveals his new form as Mettaton NEO before being killed with a single hit.

In New 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 realizes that the protagonist will kill him, and runs away. In the Final Corridor, Sans fights the protagonist, telling them that they are a threat to all the timelines of the world. He traps the player in his turn after many attacks and falls asleep, and the protagonist takes the opportunity to kill him, gaining enough EXP to reach LV 20.

In the Throne Room, the protagonist approaches Asgore, who was talking to Flowey, and automatically attacks him, 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 attacks Flowey repeatedly, killing him, and continues until there is nothing left.

Instead of leaving the black screen and returning to the Throne Room, The First Human appears in the black screen and talks to the player. They offer to destroy the world so that the two of them can move on another game, where they will continue their slaughter. At this point, the player is given a prompt to 'ERASE' the world, or 'DO NOT'.

Post-Genocide Game
After having completed a genocide route, if you choose the option to not erase the world, the first human will laugh, causing a jumpscare where their face deforms as the game screen vibrates, commenting about how you were never in control. If you do choose to erase the world, they will mention how you are a great partner and that you will be together forever. Either way, 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 and 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 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. This, however, permanently alters all following Pacifist Endings, to illustrate that the soul of the player is now owned by the first human. This is shown in the last cinematic of the true pacifist ending where the player will awake from their sleep with red eyes and a laughing sound will play, the ending will also show a red "The End" text instead of a normal white.

Consequence Avoidance
Alternately, to return to a playable state, the player can enter 'AppData/local/UNDERTALE' folder and delete or rename the system information file, which contains only the letter 'a'.

For Mac users, the equivalent is to find the folder ☀~/Library/Application Support/com.tobyfox.undertale/ and delete the same.

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.

Lots of files with the ending screen will appear, as well as a picture of a faceless Flowey.