Since its release, the game has inspired a wide range of fan games, created by enthusiastic players and fans of the game. These fan games vary in genre and style, ranging from puzzle games to action-adventure games. Many of these fan games are made with the same art style and mechanics as the original game, while others take a more creative approach. Regardless of the type of fan game, they all share the same passion for the world of Deltarune.
Deltatraveler is a fangame of Deltarune that places the game's protagonists Kris, Susie and Noelle in the world of Undertale. Version 1.0.0, containing the section set in the Ruins, was released on 13th November 2021. Version 2.0.0, adding the section set in EarthBound and Hard Mode, was released on 27th June 2022.
Each section has their own Genocide and Pacifist routes, though it's currently unknown if they have effects on the follow sections.
Underfell is an AU of the well-known role-playing video game Undertale where all of the good-natured monsters who don't actually want to hurt the protagonist, are replaced by vicious versions with opposite intentions and vice versa. One prime example of this is Flowey, who, normally shown as being an antagonist in Undertale, actually wants to help the protagonist escape the Underground safely. It presents a universe where the Underground monsters are darker versions of their originals.
Some fans refer to these characters with 'Nega' but must refer the characters as 'Fell' as a prefix to differentiate from canon. There are many interpretations of this universe, but most share similarities such as Frisk falling from Mt. Ebott and meeting Flowey, who isn't evil but rather a guide through the Ruins, and instead, King Asgore is the one that has instated the motto "Kill or be killed" and the one who poisoned Chara rather than themselves.
Underfell was foremost created as a design aesthetic, so there originally wasn't a canon backstory to the characters outside of the idea that they are an alternative, evil version of the regular Undertale canon characters. However, there are some various ideas about the characters that are widely accepted, such as Sans getting easily flustered and irritated by flirting, puns, and having an obsession with mustard or Papyrus being a cold individual with no patience for failure (especially not from his brother). Later on when the creator of Underfell returned actual canon personalities got created.
Undertale: The Final Run is an AU that takes place after an unknown (yet high) amount of Genocide Routes caused by the Eighth Fallen Human. After so many runs, one little flower got fed up and shared what he knew with other monsters. No one believed him, except for two skeleton brothers. They shared this info with the king of the Underground, who decided to prepare for the human's next arrival.
Since Flowey was able to share what he remembered of the Resets and previous Routes, King Asgore decided to make preparations for when they arrive in the Underground and others who realize what the player will do will change how they see them. As such, Papyrus still believes in them, but to a lesser extent.
Through the use of Sans's shortcuts, monsters are able to evacuate quicker and leaving less out in the open to kill, reducing the amount of determination the player has and making them easier to kill.
This is a fangame where u can fight vs Susie and ralsei in deltarune chapter 1 genocide in the Castle release on 2022