Asundertale is an Undertale AU created by Elvenkin6603 on January 20, 2018.
Most of the story is told by Headmaster Olo Terraglyth to some of his students.
It starts at the end of the Genocide run (Though a Neutral run has also happened as hinted at later on). Frisk had just killed the last of the monsters in their way, and Flowey had appeared to grovel for his life. In one last effort, Chara attempts to stop it but in the end, they were forced to kill Flowey by a mysterious voice. Her mind is now broken, causing them to turn on Frisk and in a fit of rage and wished away all humanity in the world.
The destruction drew the attention of Jay Herah, a wondering mage who in an attempt to make amends for his past misdeeds, wonders the universe helping people. He confronts Chara, but sensing the conflict in her, decides to help her instead of banishing her. He cures them of the corrupting effects of LV and fixes her damaged soul. Exhausted, he soon leaves but not before trading names with her. Then the newly purified Chara Resets the timeline.
Upon the Reset, Frisk attempts to atone for their actions by doing the Pacifist run and succeeds. Things go exactly as a normal pacifist. Frisk runs up to when Asriel destroys the Barrier, this draws the attention of powers of both good and evil. Then, he restores life to all the people whose souls he used including the Six humans, this also accidentally brings Chara back to life as well.
The monsters have a feast to celebrate the destruction of the Barrier, giving much praise to Frisk as most do not remember the past timelines. Just as they are doing so, Chara walks in and is enraged at Frisk, a person they watched murder so many and is praised as a pacifist, attacks them. This fight goes badly for both sides as Frisk is slashed across the face by Chara. Sans blasts Chara across the room nearly killing her before Asgore intervenes. Asgore takes Chara under his protection and abdicates the Throne to his ex-wife, Toriel, so they get to her to see his point of view. Then, Asgore and Chara reunite and left.
One year passes as tensions between humans and monsters begin to rise. In an effort to bring peace to the situation, a group from the Council of Magic, lead by Olivia Volenza and Tyberius Innatarsh, arrive to mediate the peace talks. Meanwhile, Chara and Asgore are living alone far away from the rest of the monsters. Other then Asgore having nightmares due to the guilt, he felt over ordering the deaths of the six humans they lived in relative peace until Chara finds an injured Jay in their garden.
As they nursed Jay back to health, Undyne came to their shack. She originally came looking to hire a gardener but upon Finding out that he was Asgore, her former king and father, figured out that she begged him to return. While she fails to convince him to reclaim the throne, she is able to get him to come to a feast. Jay offers to watch Chara while he's gone and tells Chara several stories. Chara then responds by telling her own. Upon hearing of her brother's misfortune, he takes Chara to his old magic school to look for clues. While there they meet several interesting Characters, including Olo Terraglyth, who tells them that there are dark things at work. Meanwhile, Alphys is warned by her former mentor of a being know as Lady S.
Soon after, Sans became enraged by the attack of his brother and swore revenge. He gave a speech to monster kind and many joined him in defeating what the believed was their "greatest threat". Sans attacked Chara, who was tired from the content hate, refused to fight and was quickly brought down. As Sans attempted to finish off Chara, he was stopped by Jay. Soon, Jay and Sans were locked in a fierce duel to the death, which only ended when sans shot many Gaster Blasters at Chara and Jay. Jay blocked the Gaster Blasters, knocking him unconscious. Sans still was unable to strike down Chara, as Asgore appeared and destroys Sans' Gaster Blasters with his trident. Defeated, Sans fled off. Chara, Asgore, and Jay fled to the latter's old home town the ancient city of Torisal. Once they reached the city, they met the guardian of the town Nisir Savaî, who allowed them to stay a few days in the Inn.
Chara, also known as "the First Fallen Human", is the adopted child of Asgore and Toriel Dreemurr, has died in an attempt to free all monster kind. They are also the narrator and scapegoat of Frisk. They were saved from madness by Jay and brought them back from the dead as a result of the barrier's destruction. They live with their father (formerly the king of the Underground) Asgore away from monster society due to being blamed for the Genocide run.
Asgore is the former King of the Underground. He renounced the throne to raise his only surviving child, Chara. He is plagued by guilt-ridden nightmares due to the deaths of the Six humans. Perhaps a young man in tattered robes can help him redeem his honor.
Alphys is the Royal Scientist and adviser to the royal family. Despite being kind to many, Creatures trade their origins to her, from the horrid Amalgamates to Flowey. This horrified her so much, that only her current work and her relationship with the captain of the guard keeps her from going insane.
Jay Herah is a Rogue Sage and also an Wondering hedgemage and healer. Jay is the Former apprentice of the Sage of Galavunt, he felt the destruction of the world of Undertale and decided to face the being who destroyed it. But when he found Chara, instead of defeating them, he healed them of their corruption. Before Chara restored the world, Chara promised to one day that they will aid him. Has that day now come?
Sadraia is "the Nightmare Queen". What darkness lies at an evil heart? What malice drove the deaths all every monster? Who is this voice in the shadow? What does it want?
He is the wizard and headmaster of the School of Magic. After hearing the inaccurate stories about Mt. Ebott, he decides to tell his student the true story (which is this story).