Prologue.
Chara. The first human to fall down into the Underground, in which the monsters were imprisoned. Their hatred for humans that mistreated them resulted in reckless plan to bring about the freedom of Monsters and demise of humanity. The failure of this plan resulted in the deaths of both them of their best friend. Later, as the result of twisted curiosity of a child named Frisk, Chara brought about The End of the world.
However, in another timeline, under different circumstances, things turned out quite differently...
Chara, an orphan with no real attachments or grudges, a child genius, incredibly intellegent and mature, climbs mt. Ebott in curiosity, and falls down into the Underground, in which the Monsters have been imprisoned for centuries. There, they find the joys and wonders of having a family, courtesy of Asriel Dreemurr and his parents. Later on, their keen intellect and thirst for knowledge catch Doctor Gaster's eye and they quite eagerly become his apprentice.
Time passes, and Chara falls ill. Monsters hardly knew anything about human bodies and how they worked, therefore they could not treat the illness, and Chara eventually ends up on the deathbed. Chara only regrets one thing: not being able to help the monsters, their freinds and family, regain freedom.
This is when Asriel Dreemurr makes a rash and reckess decision. As the human has drawn their final breath, he absorbed their soul and crossed the barrier, intending to get six human souls and free the monsters as Chara wished, so that their death wouldn't be in vain. However, as he approached the village of the humans and challenged them, his resolve crumbled due to his kind and gentle nature and he could not bring himself to hurt any of them. He stumbles back home, only to turn to dust in the castle's throne room.
Time passes. Chara is awakened by determination of a child named Frisk, and the original timeline's journey repeats itself.
Then, Firsk gives into their curiosity and resets for the first time. Reset after reset, they have achieving every possible "ending", until only one remained. Chara could only watch helplessly in dismay and disgust, as the child toyed with their friends and family shamelessly.
...In the end, LOVE has twised and corruped Frisk beyond recognition, turning them into a demented monster who has taken sick pleasure in killing. Chara, who has gained it alongside them, however, resisted LOVE's sweet whispers and bent its power to their will. As Asgore and Flowey has fallen before them, Chara managed to wrestle control from Frisk and take their body and soul as their own, completely destroying Frisk's conciousness in the process, reasoning that they will attempt to reclaim what's theirs if allowed to linger, only to bring about more death and suffering in their LOVE-driven dementia.
...With that done, Chara considered their next move. They knew that the fate of the world rested upon their shoulders from now on. They were overflowing with Determination.
They decide that the best thing to do would be to reset the timeline once again. Not to Frisk's starting point, however, but rather, their own. Then, they would ensure that Frisk would not gain control of the timeline again, as that would bring about nothing but pain, misery and death. Clever and cunning as they migth have been, Frisk was still a child, and they did not understand the true magnitude of the power they posessed, and used it carelessly; treated reality as a game. Played god, much like Flowey did before them. Chara, however, knew that with great power comes great responsibility. Responsibilty to do the right thing.
So they reset the timeline. However, things did not go the way they expected.
Rather than going back a couple years, they instead have thrown themselves hundreds of years into the past, before The War between humans and monsters took place. Not at all disheartened, Chara intends to make the best of this situation, prevent the war and carve out a new, brighter future for both Humans and Monsters...