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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 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.
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, Chara and Mad Dummy/Glad Dummy are pretty much the nicest characters/only nice characters (and some of the only monsters without black, red and/or gold in their character design). Monsters weren't always this way however, their evil nature is caused by the downward spiral the Underground got into. 
- 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.
- While in most depictions of this AU, Frisk can be seen wearing a black shirt with a red stripe, they canonically wear a blue shirt with black and pink stripes.
- Frisk/the human refuses to stop trying to make everyone like them.
- Sans calls Papyrus any variation of the word boss but in a very stupid way.
- Instead of saying "You're gonna have a bad time", Sans instead says "I'm going to have ONE HELL OF A TIME!!!"
- Instead of spaghetti, Papyrus will cook lasagna.
- Grillby is green with purple hues within while working, and purple when off work.
- Muffet is often depicted wearing red and black clothes, however she actually wears black and purple clothes.
- Mettaton is often seen having four arms in his box form, but canonically he only has two arms in his box form.
- Mettaton always has a smile on his face.
- Annoying Dog resembles a black cat in this AU.
- Instead of Scolding Papyrus will threaten Sans if he is away from his post.
- Toriel was the first character created for this AU.
- Underfell was originally supposed to be named "Underhell", but was scrapped due to something already being tagged as such that had nothing to do with Undertale.