The monsters have finally escaped the underground and are now friends with Yuya and his friends but when evil arises, Gods are unleashed, and a being wanting to destroy the very fabric of reality its up to Frisk and Yuya and their friends to save the dimensions and stop Armageddon.

I randomly had the thought, “Who would win in a fight? Sans or The Batter?” So decided to write a story about their fight! Hope you enjoy.

Many stories start with a small change. Those small changes have lasting consequences. Frisk took a detour down an alley and was killed. The resulting ghost chased a ghost hunter named Johnny Ghost down into the underground as a replacement. Johnny isn't the one who wakes up. Jimmy Casket, world's most renowned murderer, is in the underground, and all will know his secret. (On AO3)

La vie de Léa change du tout au tout quand des monstres surgissent de nulle part pour l'attaquer et qu'elle se retrouve contrainte de vivre dans une académie loin des humains.

Peace has reigned since Monsters and Pure Ones allied, but tensions erupt when a group of humans called the Preservers threatens genocide against monsters in the name of humanity. As both owl and monster fight to stop them, Ginger, Asriel and the others begin to unravel monsterkind's vague origins...Something key to their lives beyond the mortal realm.(Co-written by TimeBlade-NEO.)

Because it's easy to destroy something even without intending to.

Juancho, temido terrorista que hackeo rayuela, y destruyo las torres gemelas, ha estado años sellado. Despierta y cae al subsuelo, donde esparcirá el terror a todos sus habitantes. En la superficie: Chamu se da cuenta de la ruptura del sellos, por lo que rápidamente contacta con sus antiguos aliados, Maricon y Nick Furia, para formar al equipo Rayuela. (No tomar en serio)

Heroes pride themselves with being capable of protecting those who cannot protect themselves, but what happens when something appears and is capable of killing them all?

Bree and Mysticat introduce Toriel and Asgore to their new grandchildren. :) Done as a request for HomestuckFangirl83. :)

What if Frisk from Undertale went to the world of One Piece? See what happens as Frisk goes on a new adventure!

The whole of City Z shook as a mysterious house appeared in the street. Little did they and their protector Saitama know that these new visitors would bring destruction, hope and determination. Pacifist ending. SPOILERS FOR BOTH SOURCES

a completely... "normal" undertale.

in this tale madoka falls down into the underground and has the power to save and reset and each time she does someone else is with her and monsters do not want to fight her ( M for future chapters )

Stories speak of a mountain where children have gone and never returned, unknown of the mystery which lies within it. Now history repeats itself as a child falls but with a twist as another joins...and to think a mild stroll got him into something thought of as fairy tales.

The Dwemer had their hands in a great many potential pies-and the Falmer were what finally let the dough rise. (Done as part of a crossover challenge from r/Fanfiction).

Long ago in one timeline the barrier broke in the pacifist route. The monsters broke free and made peace with the humans. Humans established the monsters as an independent nation. After they gained independence they became a type of new free system republic ruled by Alphys as head of the science department, Undyne head of the military, and Sans, the main senator of the council.

Sans the Skeleton ends up getting himself tossed into a world of Shinobi and Violence. In a world as crazy, and cruel as this one can be, he will need all the luck he can get if he wants to survive long enough to make it back home, and help out his friends that he has made along the way. The question is: what obstacles will they all have to face in order to accomplish their goals?

(Jaspertale) In the year 201X, an earthquake causes someone neither human nor monster to fall into the Earth. They land in the Underground, where a small family finds and takes care of them. Leaving the Underground is impossible, so they begrudgingly stay. They may be stuck here, but it at least gives them time to think and learn a little. (Contains Steven Universe Spoilers)

After an accidental fall, Jack finds himself in the domain of the monsters. However, he doesn't have to find his way out alone.

"Phantom" is a young boy with a rough life, falling into the underground to meet his true family he's been missing. "Fenton" falls many years later, reigniting the love and compassion that "Phantom" lost so long ago. Not much of a story, more an idea blurb

Sans has gotten a bit lost after certain events. Now, he has a choice to make. Will he help guide the future heroes of Remnant, or will he let himself fall into a rut of laziness as he did before? All he knows is that either way... he's going to make as many bad jokes as he can. Adopted from Mugiwara N0 Luffy. (ON HIATUS)

Under major edit Gaster lost one of his children, to the evil hydra. This is their story of loss and redemption with help from the main undertale aus and the avengers.

Taking place after the True Pacifist Ending of this universe's version of Undertale, and three years before Episode 2: Attack of the Clones, the Monster and Human home world of Earth are facing rising friction following rising incidents of attacks on both sides as two Jedi are sent down to solve this problem. [Rated T for Potential violence, language and dark themes.]

When Chara and Frisk Hawthorn first arrived to Mount Falls, they imagined a boring summer with their Dunkle Sans. But things are not as they seem. Along the way, they meet new friends, enemies, and find out that family doesn't always have to be related. Join the Hawthorn twins on an adventure of a life time as they find out the mysterious and secrets of a not-so-boring town.

An interdimensionally lost Touta Konoe jumps down a hole to find something, anything, that might take him home. But instead of an ancient artifact or magical gateway, he finds an adventure, and new friends. Spoilers for up to Chapter 120ish of UQ Holder! and for all of Undertale.