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Underfell is an AU (Alternate Universe) 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 this hell. It presents a universe where the Underground monsters are darker versions of their originals. Some fans refer to these characters with 'Nega' 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".
About the AU
Underfell was foremost created as a design aesthetic, so there isn'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 UF Sans getting easily flustered and irritated by flirting, puns, and having an obsession with mustard or UF Papyrus being a cold individual with no patience for failure (especially not from his brother).
All of the monsters are essentially malicious/more violent than in Undertale. They attack one another and don't all appreciate the fact that Frisk is befriending them. Instead of promising to protect the human, Sans is dared to kill them. Instead of the wide arrange or friendly colors, most characters tend to share a fashion of black, red, and gold. Flowey is pretty much the nicest character/only nice character (and the only monster without black, red and gold in his character design).
- As of now, Underfell’s creators are leaving it up to the community to create canons. This may change over time.
- Frisk and Flowey are Pacifist-Run characters in a world of Genocide-Run based monsters, trying to befriend the characters who are trying to kill them.
- Sans asks if Frisk/the player wants to have a good time, as opposed to a bad time.
- Sans hates to admit he likes jokes and puns, often becoming flustered and irritated if caught laughing.
- Frisk/the human refuses to stop trying to make everyone like them.
- Grillby has two accepted designs, either blue or purple.