Performing a Undertale Genocide run has you play the game in the most evil way possible - by killing everything in your path. (Which, ironically, is the way you play most normal role-playing games.)
The Neutral and Pacifist endings are both very similar in terms of outcome and what needs to be done to achieve them, but saving everyone and solving the mysteries are not the only way to get things done - you can always kill everyone instead.
We'd strongly recommend you complete Undertale on Pacifist before taking on a Genocide run as it will have some subtle effects on any future playthroughs you make.
How to play a Genocide run in Undertale
The overall route through the game will be the same, so we'll only cover the differences - for everything else, refer to the Pacifist walkthrough for specific details.
You need to kill 20 monsters here before you fight Toriel; once you've reached your quota you'll get a message saying "But nobody came" and you can then proceed to deal with her - she'll die in a single hit. If you get stuck while exploring, our The Ruins page can help.
Your kill quota for Snowdin is 16 monsters, and they need to be killed before fighting Papyrus - most of the fights will be fairly straightforward, but if you encounter a Jerry in a battle just flee when it's only him remaining as he's a monumental pain.
There will be a few minor differences in the sequence of events as you work your way through, but the route is the same. It's worth trying to fill your quota before reaching the village as it means you can simply loot the shop rather than buying anything.
If you get stuck while exploring, our Snowdin page can help.
You need to kill 18 monsters here before your showdown with Undyne. As with Snowdin events will play out slightly differently but still follow the same general structure.
The boss fight with Undyne will be your first significant challenge, so fill your inventory with as many healing items as you can. She'll use the same attacks as in a Neutral run but they will hit a lot harder - two or three hits may be enough to finish you off - so the only real tactic here is practice, practice, practice.
Hotland and The Core
These three areas count as one in terms of your kill quota, and you'll need to kill 40 monsters before you take on Mettaton.
The main difference here is that you won't need to stop for any puzzles in the Laboratory or Core. The two boss fights - Muffet and Mettaton - will both be over in a single hit.
If you get stuck while exploring, our Hotland and The Pages can help.
This place is much the same, but the story and flavour text will be different. There's nobody to kill here, but we'd recommend going back to MTT Resort to restock on supplies as you're about to reach the final boss.
Genocide Sans boss battle
Unlike every other fight in the game, you don't get first go - as soon as Sans is done monologuing he'll attack, and if you die and reload he'll try and catch you out by attacking slightly earlier.
He starts with a long sequence of bones that you'll need to jump over, and then follow it by summoning a couple of dragons.
The rest of his attacks are mostly variations on Papyrus's with a much smaller margin for error, but he also has a trick of his own - the bones may be held together by some platforms which both help and hinder your attempts to dodge his projectiles. You can land on the platforms, but be careful of the bone supports as they will damage you.
Keep fighting (and dying), but try and be methodical - once you've learned his attacks patterns and got into a rhythm it will get easier (or perhaps more accurately it will get less impossibly hard).
Eventually he'll tire of your shenanigans and offer to spare you. DO NOT accept his offer as he'll kill you instantly.
The next phase of the fight adds additional hazards in the form of bones that will appear over your Fight/Act/Item/Mercy buttons. Using one while it's got a bone on it will cause you pain so you'll need to time your menu selections carefully.
After a few more rounds of this Sans will threaten to unleash his special attack before launching a long series of bones at you followed by some explosive boxes and then some laser-firing dragon skulls. Again, "get good" is the only real advice we can offer here - it's an onslaught.
Finally, he'll unleash his special attack which consists of a devastating... nothing. He'll just stand there, and all you can do is wait a couple of minutes for him to fall asleep at which point you can hit him once more to end the fight.
Playing Undertale? Our complete Undertale walkthrough and Pacifist guide will explain how to finish the game while sparing everyone, covering all areas including the Ruins, Snowdin, Waterfall, Hotlands and The Core and more. You can also learn about how to unlock every ending, access hard mode and play the game in the most evil way possible with a Genocide run.
After completing a Genocide run
Head to the throne room and you'll get some dialogue and a brief cutscene, and you will then be given a choice - we won't spoilt it by saying what you get to choose between, but we will say that picking either option will permanently alter the Pacifist game endings; the change is minor, but it's no longer possible to get a "happy" ending.
Whichever option you choose, the game will quit. When you reload it you'll get nothing but a black screen - wait 10 minutes, and you'll be offered another choice.
Saying "yes" will restore the game to a working state, and saying "no" will mean another 10 minute wait before you're offered the same choice again.