Undertale has a variety of endings, depending on your actions over the course of the story.
These actions are essentially who you spare and kill on your adventure. They essentially fall into three categories - Pacifist (sparing everyone), Natural (some healths) and Genocide (kill everything).
As the game keeps track of how each playthrough ends up we'd strongly suggest a Pacifist walkthrough first followed by Genocide in order to get the standard version of each, and then you're free to experiment. There's also a bonus 'Hard Mode' to discover, too.
Of course, before you progress, expect spoilers ahead.
Undertale endings explained
The game has two "proper" endings - True Pacifist in which you save both the underworld and humanity, and Genocide in which you ruin everything for everyone - but it also features a wide range of "neutral" endings triggered by killing and Sparing specific characters.
To get the Pacifist ending, you must not kill a single creature, plus spare (and date) every main character you come across. Doing so will see you access new content - so it's well worth playing first.
To get a true Genocide ending, you must kill a certain number of monsters, as well as their bosses before moving on. Our Genocide run page explains in more specifics how this works.
A Neutral playthrough is something in-between the above two endings; not sparing everyone, but not quite slaughtering everything in your path, either.
There are many subtle variations to be had, but we've listed the ones with the most significant differences below, along with the effects they have.
Again, expect spoilers below. We recommend playing Pacifist (see above) first, and then experimenting with the many different variations.
- Do everything required for the Pacifist ending, but kill between 1 and 9 monsters. This causes the game to end before the additional Pacifist additions, so if you've played as Pacifist you've seen this already.
- Kill a monster before meeting Undyne.
- Kill Toriel and spare Undyne.
- Kill 10 or more monsters and Papyrus. Killing or Sparing Mettaton will slightly affect the outcome.
- Kill 10 or more monsters and Undyne. Spare or killing Papyrus will result in a slightly different outcome.
- Kill Undyne, Mettaton, and Toriel.
- Kill Undyne and Toriel.
- Kill only Undyne, Mettaton, Papyrus, and Toriel, but no monsters.
- Kill Undyne, Mettaton, Papyrus, and Toriel, and some monsters. This ending is a huge downer.
- Start a Genocide run then miss your quota in Hotlands.
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.
One final "ending" available is Hard Mode - it's currently an incomplete feature and is likely to remain so, but for the sake of completeness it's worth playing.
To access it enter your name as 'Frisk' when you start a new game, and then play through the Ruins as normal. Enemies will all have been replaced with tougher versions from later in the main story, none of the switches will be labelled, and some of the dialogue will be slightly different, but otherwise the experience is the same until you face Toriel.
During her fight an Annoying Dog will appear and it'll all be over. There's otherwise nothing to it - though it does unlock a bonus border, according to the Undertale subreddit, as a bonus for anyone attempting it.