Forum Thread

Ending Theory 1
The ending has made a TON of theories, but the one that most people seem to think could be true is this one.
Basically, the theory says that the player (you) has been controlling Kris the entire time.
There's quite alot of evidence for this.
1. In the beginning it tells you to create a vessel, only for it to delete it and give you Kris instead. This could be interpreted as it giving you Kris as a vessel. Plus, it uses the word 'vessel' the whole time and that's often used to describe something that holds another thing (in this case the player SOUL) and when something is possessed it's often referred to as a vessel
2. Cant remember exactly where these were, but some characters near the end do mention that Kris is acting a bit different lately. One example is i think someone mentioned Kris looked paler than normal and that's usually a sign of possession. Another is apparently multiple characters are surprised that Kris is talking to them first which could mean that normally Kris wouldn't engage in the conversation first. (Sorry about all the 'i think's and such, i cant find where these parts were and i have to go off of memory right now)
3. At the end when the SOUL gets thrown in the cage, you can still move around as it. This likely means that the SOUL is you and that Kris is a separate character who you were controlling.
So all of this makes it seem pretty likely that the player was controlling Kris the entire time. This likely means that Kris put you (the SOUL) in the cage to stop you from controlling them.

Random thing but im not sure yet if the cutscenes where Kris moves on their own are you making that happen or Kris being able to move for a second. Also should be noted that when Susie fights Lancer in the castle and Lancer attacks, you still use the same SOUL as before. This either might just not mean anything or could mean that Kris was un-possessed for a bit during that battle.

Might add more to this theory when i find more evidence.

BIG SPOILERS FOR UNDERTALE, TINY SPOILERS FOR DELTARUNE
TL;DR: Sans isn’t a monster, he’s a Darkner from the Dark World, and that’s why he bleeds, as well as what’s up with his room and his seeming teleporting.
OKAY so everyone’s aware of the whole thing where, when you defeat Sans at the end of the Genocide Route, the wound it leaves looks a whole lot like its bloody. Combined with the fact that we don’t actually see him die, and his awareness of the nature of SAVEing/LOADing, many have speculated Sans isn’t fully monster, and has some human element. But that was all just blind speculation around a seemingly anomalous character who both uses magic and has an organic body.
Until now. ‘Cause, see, Deltarune introduced us to beings who can use magic and have blood: Darkners.
Lancer knows, or at least thinks, the heroes have blood. He says he’s gonna “turn them into blood” and even has a bucket to clean up said blood. Susie obviously wouldn’t, but she doesn’t correct him because the conversation is framed around Lancer learning to be more badass/intimidating from her, so she’s not concerned with the accuracy of the statement. And we know that people from Kris’s world know what blood is, and have the same rules as in Undertale, that being that only humans have blood. This is shown by someone in the town recognizing Kris as “the human who lives at the top of town” and asks them if it hurts to be made of blood.
Yet, Lancer naturally thinks that crushing the heroes would leave a bloody mess, and King says Lancer would splatter it he dropped him off the castle ledge. We’re suspiciously unable to kill any Darkners, so we never see anyone die or turn to dust. In fact, Lancer even knowing what blood is raises suspicions, because Darkners otherwise seem identical to monsters, and there’s no indication they know what a human is.
But most importantly, to me, Ralsei doesn’t correct Lancer. Just about every other time Lancer says something erroneous, Ralsei calls it out. But he doesn’t so much as comment on Lancer thinking there’d be blood from crushing the heroes, meaning that, as far as Ralsei knows, there would be.
What this means is that Darkners can use magic and bullets, even manipulate the space a SOUL-heart moves in (in much the same way Sans does), but also have blood.
y'all
Y'ALL
SANS ISN’T A MONSTER
SANS IS A DARKNER
THE DARK WORLD IS WHERE SANS ORIGINALLY CAME FROM
Want more evidence? Darkners have colored dialogue portraits, while monsters from both Kris’s world and Frisk’s world have black-and-white dialogue portraits.
Sans, who would be the only Darkner in Undertale if this theory is true, conveniently has an actually black-and-white head. In other words, we wouldn’t know if his portrait was coloured.
And on top of all that, the fast-travel doors in the Dark World have the exact same shifting light under them as the door to Sans’s room in Undertale, a room that Papyrus said “looks like another world,” and which is totally black when you first enter. Combined with Sans’s mysterious ability to teleport around via “shortcuts,” it seems highly likely he’s using the Dark World to get around.
EDIT:
Adding more fuel to this, people have been speculating that the photo of three smiling people in Sans’s workshop could be the heroes of Deltarune… the ending theme of which closes with the lyric “Don’t forget, I’m with you in the dark,”… so this photograph would be of the Fun Gang who traverse the Dark World… and spawns based on FUN values relating to a character who talks about her neighbor Suzy… and this photograph has “don’t forget” written on the back…
AND to top off all of this, the primary way to dispatch Darkners peacefully is to get them tired and make them fall asleep, just like you how you beat Sans. Their names even turn blue when they’re tired, for god’s sake. And they can actually dodge your attacks, unlike every monster but just like Sans.

the Dark World was all just a dream, as evidenced by the classroom at the end of the game. There are playing cards, a mat that resembles the color scheme of the Field of Hopes and Dreams, a checkerboard with pawns (Ponmen) strewn across it, a stuffed animal resembling Seam, and a closet with the same color scheme and (I think) number of 'levels' on it, among other evidence. Also, at the beginning, if you choose to go right back to bed, you end up in the Dark World immediately, suggesting that it's a dream world.

I was reading some comments on a video and someone mentioned a part of this theory and i thought about it and it made a bit of sense. So later i was reading a page of the Deltarune wiki and some people (their usernames were unclever title and jokerjay779 btw, wanted to give them credit) mentioned the theory too. Basically those 3 brought up the idea that the dark world DOES exist, but its some imagination realm or something. By that they meant that the characters in the dark world directly correspond to things in that classroom (E.G. Seam is the stuffed animal, the cards being various characters, etc). Not sure how it exactly works (especially with things like the pencil turning into a sword and such) but maybe its like toy story or inside out or something where if someone made a character with one of those objects it became an actual darkner in the dark world i guess? That could also explain why Ralsei looks similar and has a name similar to Asriel even though they can't be the same person since they could either be created from the red horn headband Kris had or maybe just some imaginary friend that Kris or Asriel forgot about?
Also, when they go into the dark world they go through the closet while when they leave it they end up in an abandoned classroom. I dont see why a school would have a closet leading to another room.
I think this would explain alot but there isn't really much evidence. The whole 'sleeping to skip to the dark world' thing might just be for speedrunners, though. Idk

Edit: Another one i heard recently is that maybe Kris just has a really strong imagination and that somehow manifested in the closet when they went inside? I dont think this is it but might as well mention it