Undertale is the main universe behind the rest of the Doodle Sphere, being considered the original universe of the entire multiverse. Despite this, the timeline that is featured in Underverse is not part of the traditional Undertale canon, instead being a corrupted, alternate timeline of Undertale itself.
This alternate timeline is the result of an incident in a Genocide timeline, and causes Sans to be the only individual who remembers everything from the past Genocide timeline.
The alternate timeline of Undertale is created when Sans accidentally kills Frisk in a portion of the Genocide Run to where he shouldn't have, causing the route to head towards a Pacifist Run instead and making him the "owner" of his own timeline. Although he could lie about everything to force everyone under his control, he felt guilty over this and tried to come with terms with Frisk about this unique timeline. The two confront Flowey, and when they knew how uncooperative he was between them, they decided to kill him instead, knowing how much of a problem he would be.
As of result of this changed timeline, XGaster, X-Event!Frisk, and X-Event!Chara take notice and begin exploring the timeline itself. The X-Event is completely confused as to why they're all acting happy despite the amalgamated timeline that they chose to live in. The X-Event would soon try to convince XTale!Sans (Cross) to kill them so that they could live in a timeline where they could be imperfect, by showing him how irregular the Undertale timeline is. This plead fails, however, as Cross does not accept the timeline that he was shown.
After XTALE is destroyed and their soul was taken by Error!Sans, Cross!Chara would manifest himself into the Undertale timeline once more to try to convince Sans to live in their universe. When this fails, they take out half of his soul, and uses to keep themselves alive. Sans does not take this thievery well, as he starts to see visions of Gaster and XGaster and is surprised to see his soul marked with an X. When Frisk confronts him about this, he admits that there's nothing wrong until a CHECK command by Frisk makes them reluctant to RESET the timeline. After calming down when Ink!Sans appears, Frisk gives Sans some word of encouragement as he and Ink attempt to find Cross and reunify his soul once more.
Moments after the destruction of the Doodle Sphere by Error!Sans and the rebirth of XGaster, XGaster sends Sans back to his timeline in the middle of the picnic, with the rest of the cast confused about this. XGaster himself, alongside the rest of the XTALE cast head over there later, and with his perfect story set in motion, he now has plans to do something in this timeline.
- See Classic!Sans
The owner of his timeline, he was forced into a journey with Ink after his soul as stolen by Cross.
- See Classic!Frisk
The main human of Sans' timeline, they made a deal with Sans to not reset the timeline unless something happened to him or their friends.
- See Classic!Gaster
The Gaster of this timeline, he warns XGaster on the potential failure that he's setting himself up towards.
- This is the portion where Frisk is supposed to make the final hit after Sans goes to sleep
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