Chapter Text
I wrote this to a friend over on Discord, doing a summary for the lore of Merging Timelines, so enjoy!
Like, the basic lore is Gaster wants to be able to check on his sons and see if they're ok, maybe even help them, and Bill takes advantage of this and lets him go between those timelines, creating a pathway between them as a link. He does this so that he can come back with Nightmare later on in order for Nightmare to recruit them and for them to get along. Time passes, Resets happen, and then Sans gets his first merged Reset with Killer's timeline, kicking everything off. At this point the Determination transferred over from Killer and into Sans allows him to remember, but I never mentioned this.
From here, Sans remembers these timelines but none of the previous ones, but it's confirmed more have happened in the past due to Riley telling Sans that they've been to Waterfall and swam a few times before, and this isn't the first time they've had in control. This tells everyone the loop has been going on for a while now, the deal Frisk made with Chara happened a while ago, and now Sans is suddenly able to remember and is completely breaking his script, with no memory of how he used to behave in the loop. As time goes on, more of the personalities show up, and notice how there's never a neutral route? All the neutral routes were completed, leaving the only "complete" and worthwhile endings being Pacifist or Genocide, since everything else has been done.
There was two Genociders in the timeline, Karter and Iggy, but the others were Pacifists. Well, they did neutral routes at first out of fear before realizing they could go without killing anyone, and then the others still wanted to kill everyone. Not directly stated either, but it's been implied. Chara is in control over the Reset button, yes, but so is Frisk and their personalities, the Pacifists just never use it because they don't want to make everything worse, and the Genociders just Reset after they hit the end.
As time goes on, Sans is remembering all of these Resets and none of the ones beforehand, since he didn't have enough Determination to be able to, and as time goes on he's getting more and more messed up by the nightmares and everything leaking over. Some of their behaviors are leaking into Sans too, mainly personality and body language. In the same chapters, Killer and Sans have had matching body language, and Sans and Dust have done this before too, and I don't remember if Sans and Horror did this. This could be chalked up to them technically being the same monster, but it's just a little too coincidental.
It's reinforced with personality and mental changes, which could also be thought of as them being the same, but this one has less grounds. Sans adopts Killer's ideology of no consequences and not thinking about repercussions for his actions, he gains Dust's temper and protectiveness for Papyrus, as well as his smoking tendency, and he also gains Horror's temper and protectiveness for Papyrus, which makes Sans increasingly prone to bursts of anger. Again, this can be chalked up to them sharing a mind, but Killer, Dust, and Horror never get these effects, they stay themselves, they just grow more tolerant over each other over time. Sans is the only one really being changed mentally and physically by this.
That's not even going into detail on how Killer's brainwashing from Chara is effecting Sans, with him protecting them on reflex or with how he just blindly trusts them. Sure, he's the only one who saw the brainwashing, as Dust and Horror weren't there yet, but regardless, just seeing all of that in one night isn't enough to cause that kind of conditioning. Now here's what I didn't mention. In MT, it's never stated directly on how long the Resets have been going on for, or if this is Sans's first nightmare.
This is the real kicker here. What I never stated was that these timelines have been going on for a while. Horror, Dust, and Killer saw Gaster as they were becoming who they are now. Meaning Bill made the deal with Gaster before this happened. Sans has been having these nightmares for an unknown amount of time, meaning that he's probably witnessed multiple brainwashing scenes and incidents that have been subconsciously effecting him in the form of... wait for it.... deja vu!
The same way everyone else is effected by the previous timelines, the effects of the nightmares Sans has been having are effecting him here in that way as well. So not only has Sans been slowly getting more and more brainwashed by the Chara in Something New, he could've been seeing everything while it was happening and he didn't know. It's why he's relatively calm with them and wants to attempt getting along, despite still being upset. It's not just because he's friendly, wants to believe in you, and isn't easy to upset.
No, that has it's limits and Sans will be much harder on himself than the human anyways. So, he only sympathizes with them because he himself probably lived through that with them and knows how it feels, otherwise he would be much more reluctant to get along with them. Sure, Reloads and Resets each have an effect on him, but it's never stated how many have happened. It could've just been the ones in MT, but it wasn't this could've happened several times, and Sans has been slowly getting Determination, not just from Killer but probably also from Dust and Horror.
That's also an explanation for why Sans was easy to attack Undyne to protect the human, because he witnessed Horror's timeline, doesn't remember, and that bit of a grudge is still there. So Sans not only has been gaining Determination overtime, he's been gaining enough for not only to be able to remember these current Resets, but he's also got enough Determination and magic in his body for Dust, Killer, and Horror's Determination to latch onto and transfer over. Sans is literally acting as a vessel for Determination, not strong enough to Reset, but strong enough for Killer, Dust, and Horror to be transferred over. Them manifesting on another layer of reality to Sans? Yeah, that isn't entirely a lie, but it's not entirely true. See, they're not manifested on another layer of reality, and I actually hinted at this. Monsters are made up of what?
Magic. What does Sans currently have a lot of in his body? Magic. His soul itself is literally allowing Dust, Killer, and Horror to manifest by drawing a tiny amount of power out of him, and as the magic increases overtime, so does the magic they can access to manifest, being able to gain their own perspective, etc. This is hinted at when Sans manifests himself when Killer has control. Sans can feel his soul manifesting himself using his own magic, he can literally feel it happening. It's not another layer of reality only he can see, no, Killer, Dust, and Horror are cold to the touch.
They're like a phantom, like a ghost. The others can feel them, but Sans can only see them because they're manifested from his magic. Alphys, when she used those headsets to allow the others to see and hear them, she didn't tune a frequency for reality, she tuned the frequency for seeing magic. That was all that was.
No other layer of reality, nope, just seeing the magic being drawn out of Sans to manifest them. Then, the end. This is the icing on the cake. The Reset Button. How Killer broke the Reset button, and they somehow gained physical forms. That can't make much sense at first glance, can it? Or does it?
When Killer broke the button, there was a bright flash of light. Kind of like what? An explosion. An explosion of magic coming from the Reset Button being destroyed, which allowed Chara and Frisk separate forms and gave Frisk their pieces of their personality back and unified. This also brought magic manifestations and gave them physical forms, and the amount of magic that exploded out of the Reset Button once it was destroyed soaked into Sans's body, since he was right there too. This allowed them to gain enough magic to manifest bodies.