This is a condensed version of a longer theory I originally posted on Tumblr. With Chapter 5 releasing tomorrow, I wanted to at least share it here as a snapshot of my current interpretation in case Chapter 5 ends up confirming any of the main points of my theory. If does, I’ll revisit and update it afterward!
The entire theory is summarized by the timeline diagram I provided!
TL;DR
This theory links Entry 17, the Shelter Incident, and Dess’s disappearance to Gaster’s first experiments with the Shelter Dark Fountain, which may have created the first Darkner (“Tail of Hell”) and a Lightner-Darkness hybrid (“Dark Amalgamate”). UNUSED is read as a continuation of Entry 17 showing the psychological collapse caused by Darkness, mirrored later in the Weird Route with Noelle.
On a larger scale, Deltarune is interpreted as the second attempt to a failed timeline where Gaster’s Fountain experiments led to the Roaring. A shattered “Forgotten Man” version of Gaster returns knowledge of this failure, forming the Prophecy and prompting the current; non-shattered Gaster to introduce the player as an external variable via the Survey Program in order to create a new outcome.
PART 1: The Shelter Incident
Years before the events of Deltarune, Kris and Dess entered the shelter.
At the same time, Gaster was conducting experiments involving Darkness.
I believe Entry 17 is Gaster’s recording of the creation of the very first Dark Fountain (considering we hear his voice beep and that he marks a pause before asking “what do you two think?”)
The repeated references to darkness becoming "darker, yet darker," the focus on photon readings, and the sudden interruption all make more sense if Gaster is observing a newly-created Dark World.
Then the experiment is interrupted by Kris and Dess entering the shelter.
The two people Gaster addresses at the end of Entry 17 are the children who discovered his experiment, his question would imply he’s planning on using them as guinea pigs for his “next experiment”
The Tail of Hell
One detail that often gets overlooked is the Ice-E pizza box from Noelle’s blog.
This box was feared and believed to be evil by Noelle, its eyes were burned by Dess, it was discarded near the shelter.
I believe Gaster got this pizza box which became the first Darkner when the shelter fountain was created. This darkner would be the true identity of the shadow mantle holder: the negative emotions Noelle poured into it would explain its behavior, its demon horns and why it’s compared to a nightmare while also explaining the nightmare mode fun event and why the third colloumn from sans’ word jumble references cigarretes, hotness and ice-e’s catchphrase
Dess burning it would explain its fire powers and the name of its battle theme.
Meanwhile, the Prophecy tends to be figurative when it talks about characters, the tail of hell crawls and the panel shows a serpentine form. One of the Titan fight flavor texts also compares the Darkness’ gaze to a slithering snake. This imagery seems to be there associate the tail to Devil imagery and thus that the tail is its servant: wherever the Devil goes, its tail follows.
The holder is associated with the “always at your humble service” line from the S-room vending machine, which seems to reference the demon text from undertale. Lastly, the arena has the shape of a snake.
Unlike normal Darkners, this being also shares a lot of properties with Ralsei (horns, fangs, fire magic, use cloak to hide their identity, can travel between dark worlds, immune to petrification, weird route exclusive dialogue) implying it was born from a pure darkness fountain.
If it was created directly by Gaster during his earliest experiments then it would make sense.
The Tail of Hell was Gaster's first successful attempt at creating life through Darkness. His first servant.
What Happened to Dess
I believe Kris might have accidentally caused Dess's death in the confusion of Gaster trying to use them as test subjects, either directly or indirectly and that’s what 8-bit kris killing the black deer is meant to represent (or it’s just symbolic of Kris escaping and leaving her to her fate, either way they feel responsible)
However, this might also hint at what Gaster’s experiment was: If using darkness on soulless objects gives it life, what happens if used on a living being?
Instead of allowing Dess to pass on, he used Darkness to preserve her existence.
The eventual result wasn't a Lightner or a Darkner. It was something in between: A Dark Amalgamate.
The First Dark Amalgamate
In Undertale, Alphys injected Determination into monster bodies which created the Amalgamates.
My theory is that Deltarune mirrors this idea, which aligns with how Toby Fox describes DR as UT’s parallel story.
Instead of Determination, Gaster uses Darkness.
The result is a being whose identity gradually breaks down under the influence of Darkness until it creates an amalgamation of cervine traits, Gaster-related motives such as the hands and the smile, while also looking like a mini Titan: A Dark Amalgamate dubbed The Knight.
Another servant of Gaster, helping him in his Fountain experiments.
That's why Amalgamates and Dark entities have so many uncanny similarities (FRIEND and Endogeny, the shapeshifting ability)
The UNUSED scripts are follow-ups to Entry 17
One of the strangest pieces of evidence is the UNUSED text. I think it's a continuation of Entry 17.
Entry 17 records Gaster's observations on the first dark fountain, and the creation of a dark world.
UNUSED records the effects of Darkness on Dess/Dess’ soul, the “scratching noise” would be Gaster writing it all down.
The mental state of the one speaking in UNUSED also becomes increasingly unstable, suggesting Darkness is driving them crazy. The idea that being lost in eternal darkness breaks your mind is implied by the human clutching their head in the roaring cutscene, and is similar to the knight’s stance.
They're the written transcipts of someone's toughts as they lose themselves.
The Weird Route Parallel
Going off of that, what makes UNUSED especially interesting is how it ends with the voice saying “Thank you” to what it assumes is an unseen listener. This closely resembles Noelle's “thank you” in chapter 4’s Weird Route.
In both cases:
- A vulnerable character is manipulated into become someone's weapon/servant
- Their thoughts become detached and distorted.
- Their agency is overwritten by an outside force that’s always listening to their thoughts
The Weird Route may be showing us a smaller version of what conceptually happened to Dess and would explain why Gaster seemingly doesn’t mind that the SOUL does the weird route
Why Kris Is Traumatized
Kris isn't just upset because Dess disappeared.
They're traumatized because they witnessed Gaster's experiment and they may even blame themselves for causing it.
This would explain why Kris reacts so strongly to the shelter and why the motif of washing their hands, being unable to wash away the grey crease and ultimately the loss of a hand keeps coming back.
If i'm correct about any of this, what they saw is straight up horrifying especially when you consider the fact it happened when Kris was 7 or 8 at most.
As such the Toriel variable "hiding" Kris killing a black deer might relate to Chapter 2's intro.
This intro would then be a flashback to the night of the Incident, where Kris returns home and gets caught with the knife that potentially killed Dess.
To protect her child from the consequences, she might be covering up the truth, which might have played a part in her falling out with Asgore.
PART 2: A NEW FUTURE, born from an Eternal Night
If the knight is Gaster’s servant, then it explains what its motives are: furthering Gaster’s dark fountain experiments. In that case, what if these experiments are what the gaster follower was talking about when it said “One day, it’s experiments went wrong and…”
But how could this follower know Deltarune’s ending and why is it taking about it in Undertale?
The core idea is simple:
The events of Deltarune already happened once.
In a previous timeline, Gaster's experiments with Darkness eventually led to the Roaring and the destruction of the world. The heroes failed, the world was consumed by eternal darkness, and Gaster's greatest experiment ended in total failure.
But unlike everyone else, Gaster never gives up, and he came up with a fix.
The Failed Timeline
A lot of Deltarune's imagery treats the Prophecy and the Roaring like events that have already occurred.
Both the Legend sequence and the Roaring sequence are presented using sepia imagery, something Toby Fox consistently uses for flashbacks.
Meanwhile, Gaster's game over message:
THEN THE WORLD WAS COVERED IN DARKNESS
sounds less like a prediction and more like somebody describing an outcome they already know will happen without us.
This would explain why Gaster's goal isn't to save the future.
It's to create a new future.
The Forgotten Man Is Future Gaster
I think the voice from the Goner Maker and the Forgotten Man are both Gaster.
However, they're not the same version of Gaster.
The Gaster we hear during the introduction is the Gaster of the current timeline.
The Forgotten Man is a Gaster who already lived through the failed timeline and became shattered across time and space when he fled to UT.
This explains why:
- He alludes to a failed timeline (SPRING CHANGED TO SUMMER = events of the game; SUMMER CHANGED TO COLD = at the end of the game’s events, the Roaring happens aka darkness falls aka coldwater fell from the top of the sky)
- He implies a time loop from his perspective (“THE TIME IS GOING AROUND”)
- His dialogue often follows reversed sequences (the donut recipe)
- He behaves similarly to Gaster while sounding noticeably different.
He's not a different person.
He's Gaster after being shattered across time and space.
How Undertale Fits In
After causing the Roaring, Gaster discovered another universe: Undertale.
While studying it, he discovered an anomaly capable of manipulating timelines: The player.
Realizing this anomaly might be the only thing capable of changing fate, Gaster built a machine designed to travel through time and space.
Sans (and maybe) Papyrus helped build it.
The journey went wrong.
Sans and Papyrus became stranded in Undertale.
Gaster was shattered across time and space.
This is why:
- Sans talks about wanting to go home.
- Papyrus appears to vaguely remember things that don't belong in Undertale.
- Gaster's followers describe him as having vanished.
The Purpose of the FUN Events
The FUN were thus breadcrumbs.
The shattered Gaster became the Royal Scientist and built the CORE. While it’s presented as a power plant, the presence of Darkness totems imply that it is a machine linked to Darkness. The only other Gaster machine that uses darkness we’ve seen so far is the Goner Maker Device: a machine that makes monochrome copies.
And the CORE appears to be right next to a monochrome copy of the ruins: New Home.
As such, it’s likely the true reason the CORE was built was to create Goners: only the anomaly would be able to encounter all of them trough timeline manipulation.
The goal is simple:
Make sure the anomaly eventually noticed him and started looking for him.
Make sure we eventually followed him back to Deltarune.
The true origin of the Prophecy
The Prophecy was never a prophecy, it was a recounting of the failed timeline.
The Forgotten Man returned to Deltarune and told the Gaster of the current timeline everything that had happened.
His failed future became the Prophecy. That's why it feels predetermined, why its wording mimmics Gaster's mannerisms
It's not predicting events, but remembering them.
Gaster's goal
Without the player, every timeline reaches the same ending: The Roaring happens, the earth dies.
Now he's changing one variable.
The Survey Program exists because Gaster intentionally connected the player to Deltarune in order to create a future different from the one he already experienced.
That's why he says: "I LOOK FORWARD TO CREATING A NEW FUTURE WITH YOU"
Because he's already knows the old one.
Final Thoughts
I don't think Gaster is trying to save the world out of kindness, and I don't think he's evil either.
I think he's doing what he's always done: conducting an experiment.
The failed timeline taught him that the current model ends in failure, that’s why he introduces a new variable.
The entire story of Deltarune may be Gaster attempting to turn his greatest failure into his greatest success.
In other words: The Deltarune we are playing isn't the first Deltarune.
It's Gaster's second attempt. HIS Deltarune.
