AUs

Undertale AUs are what-if changes to the universe of Undertale.

AU categories are used to differentiate AUs and to avoid confusion between AUs (categories are not AUs themselves).

An AU can fit under multiple categories.

AU Types

The basic types of AUs.

Switch Up

Some or all of the canon Undertale characters swap roles and/or personalities with or shift into the roles of the other canon Undertale characters. e.g: Sans replaces Toriel. No new characters are added, and the story is relatively the same for the most part.

If only one character pairing is swapped, it doesn't fit in this type. In that case, the AU is most likely in the Character-centered type.

If there is an in-universe reason for characters to change roles from their original ones, it doesn't fit in this type either. In that case, the AU is most likely in the Concept Change and Special Event types.

Crossovers

Two unrelated universes/shows/etc combined into one AU.

Basic Change

Every single character in Undertale is changed while still remaining in the same basic story in some which way or form. AUs that only affect some of the characters are not found here. AUs that change only during the time Frisk is in the underground are not found in this category.

Concept Change

AUs that change the basis of the story, such as sequels and backstories.

Character-centered

AUs that are focused on a particular set of characters. These include headcanons, extensions, and flat out new ideas.

Self-insert

AUs in which the characters are replaced with real people.

AU Tones

The tone of AUs.

Serious

AUs that, as opposed to Comedic AUs, actually hold a serious story and such. Just because an AU has a serious tone does not mean that it must be serious all the time, just most of the time.

Semi-Serious

AUs that have a balanced mixture of Serious and Comedic elements.

Comedic

AUs that are mostly comedic in nature. Bear in mind that a Parody can be either dark or comedic so Parodies and Comedic AUs aren't necessarily the same thing. Nevertheless, they can also be both.

Fell

AUs that feature horrible, evil, and/or murderous Monsters.

Monstrous

Warning: Some of these AUs contain themes that may be disturbing to some viewers.

AUs that have a balanced mixture of Fell and Horror elements.

Horror

Warning: Some of these AUs contain themes that may be disturbing to some viewers.

AUs with the goal of creating something scary, insane, or disturbing. Some AUs that follow the genocide route are also found here, but not all are geno-route-only. Some possible themes include suicide, drug use, alcohol use, depression, death, gore, insanity, disfigurement, pain, etc. Horror can also include Terror, and effects on psychology. This could include things like dystopias and the like.

It is suggested you read the short bios of these before googling them!

AU Settings

The setting of AUs using the Grim/Noble and Dark/Bright Spectrum.

Noble vs. Grim

A noble setting isn't one where everyone is good, more like one where people are active.
A grim world, no matter what you do, an individual can't secure more than an individual victory, if even that, because the rest of the world is too big, scared, powerless, and/or selfish to act upon their impulse.

Bright vs. Dark

A bright world is one full of opportunity, of wondrous sights to behold. It doesn't mean there's no threat, it can be dangerous, but your first instinct when looking at a new location should be awe and wonder: people may adventure to save the world, but they leave town with a smile upon their face, eager to see what comes next. The shadow of Risk is largely erased by the glint of Adventure. In a bright world, it's quite possible for people to go on adventure just because, since the journey is its own reward.
A dark world is one where life sucks, and something or someone is poised to kill everybody else in the story, if it has its way everyone dies and they die for good.