The butterfly effect is known as "a property of chaotic systems by which small changes in initial conditions can lead to large-scale and unpredictable variation in the future state of the system." The smallest of changes make the largest of impacts. So… What if when Angel and Frisk fell… they fell into different locations? Squeal "One World, Two Souls", and "One World, Many Souls".

After the death of the sixth human, Toriel makes a desperate request: seal the entrance to the Underground. Sans reluctantly agrees, but stopping Frisk's arrival sends the system into chaos, and the soul of Determination finds a new host—Sans himself. Now burdened with immense power, a destruction-hungry phantom, and an unquenchable thirst for Determination, how will Sans navigate

Eran las primeras vacaciones que se tomaba. Sans pensó que serian relativamente tranquilas, claro, si se ignoraba los gritos de su hermano, Papyrus, y el enamoramiento de este con Frisk; la molesta y deportista Undyne, intentando que se levantase del sofá y moviese el esqueleto; además de la familia Dreemurr, conocia a Toriel, pero sus hijos eran otra cosa. [Chara x Sans] Resubido

After falling into the Underground, Frisk is found by a young Monster and taken home, where they find friendship and family, and make a new life for themself in the Kingdom of Monsters. But despite the happiness, something lingers in the back of their mind, and soon, they find a way to repay the kindness which they have been shown.

Just because your father is "Renowned Scientist Dr. W.D. Gaster" doesn't mean everything in life goes according to plan. Corbel H. Gaster knows this entirely too well. If trying to balance school, friends, and a life-altering disease wasn't hard enough, Corbel must also face the pressure of living up to his father's legacy. AU story for "The CORE of the Matter" CONTAINS SPOILERS!

A Multiverse Phenomenon drags Nightmare to a place that should be unreachable. Yet it changes the course of this Multiverse irrevocably.

In a reality where existence is a jigsaw puzzle of shattered worlds, a certain doctor has survived the collapse of his own universe and now embarks on a mission to build something new. Join one W.D Gaster utilizing the Observer, a creation of his own design, and navigate a realm where perfection is a mirage and every piece of a broken world holds the key to a bold new beginning.

At the end of the war, monsters don't have the choice : they have to join Mont Ebott for survive another day. Sans and Papyrus, forced to run away after the attack of their village, are delivered to the horror of the war without any help and this one objective. But as sons of the royal scientist, W.D Gaster, they are an easy target. Both of them start a dangerous journey.

There are thousands of legends about the Underground, most of them telling the story of the human who managed to break through the barrier and free the monsters. On this occasion, it is time to go back in time, a story that is thought to be abandoned in the face of existence, one where we are presented with a new name, a new soul, a new identity, but the same righteous ideal. Is t

It's only thanks to you that I could change from a sadistic ogre to what I am now. I feel alive for the first time. I can't thank you enough, but I can write down my story. - This story follows Frisk and Chara from a genocide run to a truly happy ending!

After Chara is freed from the Void, reunited with her friend, Gaster, and gone through a Reset of time, she must now help guide young Frisk through the Underground, showing her how to get through without harming anyone. (Female Adult Chara, Female Frisk)

Gaster lost count of the days since he first entered the Void, but since that day, he had watched and analyzed each and every timeline there was. Just a few more resets, he calculated, and the timelines would start to mix. Worse, they would start to destroy each other. He had to make things right, stop the resets, and to do that, he had to leave the Void. UPDATE 2024: NEW EPILOGUE!

Asriel didn't deserve this. To be back, to be nearly himself again, to be able to feel. But it was only because of the souls inside him. When he was about to release them, a voice made him question everything. The human children who died because of the fury he had caused wanted to live. He would right that wrong, no matter what. But bringing them back wasn't as simple as he thought

Littleverse: a multiverse in which most, if not all, of the main characters are children. Small children. Some of which have never had parents to guide them. FMD Ink: a psychopath. What will happen when these two forces meet? Well..a lot of adoption, fluff, and angst, apparently. We do not know how this happened. Go check out Clichély on AO3 and Wattpad!

A human who was once tainted with hatred and distaste for monsters has started to see the wrongness of her actions when she had gone to Aynor Village, the last monster town damned to be liquidated by human hands..

The power to reset the timeline, and the determination to move forward no matter what the cost. These are the two things that keep a young girl moving forward in the mysterious world of the underground. She could be the key to their salvation, but would it all be worth it? Especially if the one person that matters most remains trapped and alone. (I don't own Undertale.)

After getting to the surface countless times Frisk finally decided to tell Sans and Alphys what would inevitably happen, though that's when a desperate plan was suggested. If one soul isn't powerful enough to stop the resets, then maybe three will be. The only question is, how can those three souls meet, and will even that power be enough to stop the endless cycle of resets?

After Toriel mysteriously doesn't return from her vacation to the Spooky island resort one of her friends Undyne receives an invitation to the island and she takes the opportunity to find out what happened, but she finally finds Toriel, Undyne quickly realizes something is definitely wrong.

An overworked human therapist and a posse of monsters hailing from deep underground. Need more be said? THIS IS JUST A TRAILER, UNTIL CONTENT IS SUFFICIENT TO PUBLISH.

The Underground is divided. Evil lurks in the darkness as hope slowly fades away. Only a human, a monster, and a dead prince can restore this corrupt kingdom, but not if it corrupts them first.

Frisk no longer exists. Chara has become the main character. After 1000 years of genocide, something new happens, and Chara is thrown into a whole new underground. Will Chara manage to survive this new environment? Well, let's see how much determination they'll have after exploring the new routes this new game has to offer! Rated T for minor adult themes.

"El jugador" ha muerto... Y ahora todos quieren JUGAR. En la peligrosa Ebott City, radica una constante inconformidad por habitar monstruos y humanos en un mismo lugar., desatándose conflictos delictivos que atentan con la vida de los habitantes. Sin importarle las advertencias de su madre, Frisk decide ir a la ciudad y tener su florería junto con su compañero y mejor amigo Flowey

Asriel is miraculously revived and is living happily with Frisk, Toriel, Asgore, and the other monsters. But Frisk has a secret, and Chara is tormenting them for it. 3 years later, they go back to Ebott and help Chara regain their compassion after having an empty soul for so long. A year after, Frisk is still the tiniest bit resentful towards Chara cuz..y'know. Fem Frisk Male Chara

A household full of skeletons are competing over the attentions of one love. This isnt that house though. Unknown to the group secretively wrestling with their unpredictable love lives and universe-meddling machine, another house a few doors down has been gifted with its own skeleton posse clamoring for the attentions of their own little lady. A little old lady by the name of Mabel

The little Sansy is odd, even for a Bitty Monster. He just wanted to escape his terrible reality... Turns out, he had a quirk. A random Bittybones request fic off my tumblr for reblogslol4545. And for anyone else who might enjoy it. Genderless OC called Y/N. -I mean 'Yin'. TW for blood (stains on clothing), some minor swearing, and some unfortunate rats.