Underfell is a popular Undertale AU where all of the monster's personality are modified to be more hostile or edgy. Although the monsters have a hostile personality, Flowey, in this AU actually has a SOUL of his own and guides the protagonist, aka Frisk and helps them on their journey. The common color motif is Red and Yellow and is worn by all of the Monsters, except for Flowey and Glad Dummy.
Sans is a grumpy, angry, hot-headed monster instead of the fun, relaxed guy we all know and love. In Underfell, Sans sticks to the motto, "It's kill or be killed." He looks the same but he has sharp teeth with one golden tooth, red sneakers, and a black jacket. Not to mention he has red pupils instead of white. He will try and get Frisk to attack him and other monsters because Frisk is supposedly sparing every monster they meet. It ticks him off, and he will do everything he can to try and change Frisk to be evil like them.
Instead of the loud, happy, energetic skeleton we know from Undertale, we get a loud, terrifying, uncontrollable rude, and selfish Papyrus in Underfell. His costume consists of the two colors red and black, and he has a few scars on his face near his sockets. Papyrus is very controlling of his brother, and if Sans even shows a little resistance, Papyrus will beat him or torture him to do his bidding. When he encounters Frisk, he keeps them "prisoner", when really he actually doesn't have the guts to kill them himself.
In Underfell, Toriel is completely different. She has yellow eyes with red pupils, she has clawed hands/paws, and she wears a black dress. Yes, she will act like the motherly goat mom in Undertale, but when Frisk tries to escape, she will end them like Asgore did with the other children. She wishes to give them, "A Painless End." As she would call it.
This route is also known as the "bad ending". It is when you do a specific number of things in order to unlock it, and when you don't kill any monsters in the process. It plays like the Genocide Route in the end, but however, after sparing Asgore, the final boss, everything is restored to normal, which, still results a "good ending".
This route is when you kill all the monsters and is considered to be the "good ending". It plays sort of like the Pacifist Route in the end, with either Asriel being the final boss (if Flowey is spared, also known as the "Neutral Route") or Omega Sans being the final boss (if Flowey is killed).
However, a "bad ending" occurs when the player kills every single monster they encountered, also known as the True Genocide route.
The game has not been created as of yet. Dates for the game is unknown.