UNDERFELL

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. 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".

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 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).

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, with red also being the color often used for battle sprites. Flowey is pretty much the nicest character/only nice character (and one of the only monsters without black, red and/or gold in his character design).