X-tale, also known as Underverse, is an alternate universe created by Jael Peñaloza. It is characterized by its monochromatic world with sparse colors, and it features a unique power called "OVERWRITE" that allows certain characters to alter timelines and the universe itself . The story revolves around XGaster, the creator of the X-tale universe, who is on a quest to create a perfect story across ten different timelines . X-tale introduces a variety of characters, each with their own roles and stories within the timelines, such as X!Frisk, X!Chara, and XGaster himself . The AU is known for its complex narrative and the exploration of different themes, including the struggle against the creator's control and the pursuit of a perfect existence.
Underswap is an alternate universe (AU) of Undertale created by Popcorn Pr1nce, where many characters' roles and personalities are swapped. In Underswap, Sans and Papyrus undergo a significant personality swap; Sans becomes the excitable, confident human hunter, while Papyrus is the laid-back, protective brother following Chara (who is swapped with Frisk). The AU explores these character swaps in depth, offering a fresh take on the original Undertale narrative.
Underfell is an alternate universe (AU) of Undertale where the monsters in the Underground are portrayed as evil, with the notable exception of Flowey, who helps Frisk escape the Underground safely . This AU is primarily known for its dark and hostile take on the characters, with a color scheme that predominantly features red, black, and gold . The creator of Underfell, Fella/Vic, initially posted fanart and character designs on Tumblr, and while the AU started with minimal backstory, it has since grown to include various fan-created narratives and character developments . Underfell is also recognized for its shared universe approach, encouraging fans to contribute their own stories and interpretations .
Sudden Change is an Undertale AU that features a dramatic shift in character personalities and storylines. It's known for its intense and often darker portrayal of familiar characters, with a focus on the consequences of choices made in the game. The AU explores themes of chaos and the potential for characters to change drastically under different circumstances.