UT/DR Blog

Undertale and Its Many Universes

Logo for Undertale, 2015

About Undertale and Deltarune

Undertale and Deltarune were developed by Toby Fox. Undertale's main theme is that violence isn't always the answer, and that resorting to it can lead to the worst outcome for everyone. While Deltarune is mainly about defying fate, even if it seems impossible. Both of these games have underlying secondary themes on their more violent routes. For Undertale, it's the no-mercy route, where the player goes out of their way to choose violence; it features the theme of completionism, exploring how far the player will go down this route until there is nothing left. The no-mercy route is designed to be unfun and boring to play through, where all of the fun you used to have is replaced with an eerie silence. Deltarune has the "Weird Route," which is its official name in the files, but most players referred to it as the "Snowgrave Route" when it was first discovered. The theme of this route is control; the player is in control of both Kris (the unwilling protagonist) and Noelle (Kris's childhood friend). The player forces Noelle to get stronger, to turn this game about defying fate into a typical RPG. It parallels the lack of control Kris has over their own actions. These videos are good explanations for the games, UNDERTALE LORE EXPLAINED in 40 MINUTES! and The DEFINITIVE Deltarune Iceberg Explained (Ch1-4)

Logo for Deltarune, 2018

About Fans

Undertale and Deltarune’s fanbase formed around the same core traits as the games. A lot of the audience treats both titles as mysteries to discover and decode. That allows its audience to form a high volume of theories. The community also has a strong fixation on morality, which extends from the game’s thematic focus on choice and consequence. Many of the people in the fandom tend to get quite toxic over their favorite characters, or if one of their favorite YouTubers doesn’t play the pacifist route before neutral. Creative output is another defining feature. The fanbase makes a lot of reinterpretations, alternate routes, and character studies at a rate not many others see.

About AUs

Undertale (and by association Deltarune) has many alternative universes created by its fandoms. I don’t believe Undertale was the first fandom to ever have AUs or the like, but it was one of the biggest games to make one for. This stems from the fact that Undertale’s saving and loading system is implemented in the game's metanarrative, allowing the player to reset the timeline to the beginning of the game at any time. Because of Undertale's vast amount of quirky characters, it’s easy to see how one might get attached to one and possibly want to see different versions of them.

AUs (short for alternative universes) are quite self-explanatory; they take the characters of this world and even the world itself and change them. Kindred Spirits was the first AU ever created and completed. It takes place after the Pacifist Route, where Frisk (The Protagonist of the game), in this version of events, links Asriel to their soul, preventing him from turning into a villain again. It was uploaded on Archive of Our Own by QuillSparks 

Logo/Cover art for Undertale Yellow, 2023

Currently, one of the most popular AUs is known as Undertale Yellow, which is a fan-made prequel to Undertale. MasterSwordRemix and their team created the game. It follows one of the previous humans to fall into the underground; they had the yellow soul, which represents justice. The fan game was given support by Toby Fox, Undertale’s creator, which made a lot of fans consider it as a “canon” prequel, which it isn’t, since the point of the unseen stuff in Undertale is that you are the one who makes it up, which means someone could see Undertale Yellow as being canon to the game.


Other well-known AUs are Underfell, where everyone in the underground is, quite frankly, cartoonishly evil. It is made by Vic the Underfella, and it was mainly active on Tumblr. Underswap, where each character has their personality/role swapped with another, for example, Toriel/Asgore, Sans/Papyrus, and Undyne/Alphys. It was made by  popcornpr1nce, who sadly doesn’t want to be associated with it anymore. And Glitchtale, a series uploaded on YouTube set after the game's pacifist ending, was created by Camila Cuevas and is unlikely to be continued. 

There are many other AUs out there, and many more being made right now. This fandom loves to be creative and make works of art. I’ve scrolled through several social media platforms and have seen beautiful pictures people have made with these characters. I think it’s amazing that these games can inspire so much joy and whimsy.

About Theorists

Works Cited

Crog. “UNDERTALE LORE EXPLAINED in 40 MINUTES!” YouTube, 8 oct. 2025, www.youtube.com/watch?v=Td0Ps1CeLoQ. Accessed 4 Dec. 2025.

‌M3ow. “The DEFINITIVE Deltarune Iceberg Explained (Ch1-4).” YouTube, 28 July 2025, www.youtube.com/watch?v=3SF0uJHYJSo. Accessed 4 Dec. 2025.

‌Chang, Temmie, and Toby Fox. The Logo of Undertale., 15 Sept. 2015, upload.wikimedia.org/wikipedia/commons/2/26/Undertale-logo.jpg. Accessed 4 Dec. 2025.

‌Fox, Toby. Official Logo for the Video Game Deltarune, an Undertale Spin-Off., Oct. 2018, deltarune.com. Accessed 4 Dec. 2025.

Collin. 9 Dec. 2023, upload.wikimedia.org/wikipedia/en/9/93/Undertale_Yellow.png. Accessed 4 Dec. 2025.