Underfell

Underfell is an AU where the monsters in the Underground are evil and Flowey helps Frisk escape the Underground safely. It is mostly a collection of designs, with very little personality information. Underfell's creator encourages people to make their own stories. Most of Underfell's character designs use mainly red, black and gold

The monsters, especially Toriel and Papyrus, don't hold back, making the battles much harder.

Underfell's creator disapproves of NSFW Underfell fanart and suggests that if people wanna drawn NSFW content anyway, they should at least use #underfrick or any other tag not close to #undertale or #underfell.

Only 9 character designs were revealed before the Underfell blog shut down, and no new designs were revealed. However, Underfell's creator has since revealed designs for other characters on their new blog; including the likes of Frisk, Asriel, Napstablook, and Muffet.

History
Underfell's creator started posting Underfell designs on their personal blog shortly after Undertale's release, but upon Underfell getting popular, they created a blog for it: underfell.tumblr.com, which they mostly used to reblog Underfell fanart. The blog has since shut down, and then opened again, but with all previous content removed and a minimalistic theme.

Since leaving Underfell, Underfell's creator has requested to stay anonymous.

Trivia

 * Though some believe Underfell to be the first Undertale AU created,
 * Underfell is called Underfell because the Tumblr tag "Underhell" was already taken.
 * Underfell's creator has no idea why Underfell became so popular.
 * Underfell's character designs reminds Underfell's creator of McDonald's, as they both use a similar color scheme. Because of this, Underfell's creator has imagined Sans working at McDonald's, which they've drawn.
 * It's possible that all of the dogs in Undertale become cats in Underfell, so it's harder to spare them.
 * Underfell's blog was managed by several people, not just one person. A few mods were minors.
 * Underfell was attacked by some people for "ruining the game" right after its creation.