The Original (Undertale) world's story:

A thousand years ago, Chara was a human who had fallen from up above into Mount. Ebott's home of the monsters. Prince Asriel found them and brought them back into his home where his family cared for Chara. Soon Chara and Asriel were like siblings and Chara was apart of the family. But one day Chara got sick. The doctor couldn't help because for healing monsters was different from healing humans. Soon the human passed away, but Asriel wouldn't give up. He took Chara's SOUL and passed through the barrier, Chara's corpse in arms, hoping the humans would be able to heal his friend. But when he got to the center of the village, the whole village saw Chara's corpse and thought he had killed Chara, so they attacked. But even as they attacked, Asriel wouldn't fight back. When they were done, when he was severely injured, he only smiled and walked back to the underground. When he made it back home he walked to the garden and collapsed in the yellow flowers, turning into dust.

The King and Queen were heartbroken over the death of both their children. King Asgore then made the decision that for his kingdom, every human who fell into the underground would have their SOULS taken, because if you took seven human SOULS you'd be able to break the barrier and all of the kingdom would be able to return to the surface. But Queen Toriel disagreed with his decision not wanting to harm any humans. So if he did that decision, she'd no longer be the Queen and she left him to live in the ruins, hoping to get the humans before Asgore could harm them.

The Phsycotale AU 's story:

Chara had been pushed down into the Underground by the humans to see if they could make it out through the barrier with a knife. Asriel found Chara injured from the fall because even though they had landed in a patch of flowers, they had stabbed themselves with the knife in the fall. Asriel immediately took them back to his home where they were tended in a bed. Chara was scared and paranoid at first, but seeing how well the monsters had treated them compared to the humans up above, they realized they were just like humans, innocent. Chara had felt so at home there and would never want to come back to the surface. But that dream to stay with the family forever didn't happen. Because as we said before, Chara was pushed down by the humans for an experiment. The humans had discovered the hole and found out that it was where some hikers went missing to. They knew that seven SOULS were all it took to break the barrier and that even one SOUL was enough to take one monster up. The humans were afraid that over the years the monsters got stronger and if they escaped the humans wouldn't be able to win. So they had sent Chara, their strongest and most violent child down and had agreed that if Chara didn't come out after a week it was official they had died and that the monsters were indeed stronger and much, much crueler with LV. So they would send in a Toxic gas to kill them all. And they did, after a week at the same time Chara gotten sick.

But the humans didn't have any monster test subjects to test the Toxic on. All they knew was it had killed all of their test subjects. When they blocked the hole and filled the underground with toxic, something else happened. Instead of dying, the monsters went mentally ill. In other words, they went insane. They became Phsycopaths, even killing their own kind. The Toxic first spread around the ruins, and the Royal family was safe for now. Refugees came to the castle, and some monsters who had breathed it too, to see if the Royal scientist could find a cure.

So far, it had only reached a bit of An.

Chara was filled with guilt since this was all their fault. So they decided they'd try to stop the humans. They had also been sent with a bag filled with supplies to set up video cameras around the landing area of the hole (so then if they sent in the toxics they could see the Toxic come in and the results.) Chara decided to travel back to the ruins, holding their breath as much as they could, breathing as little as possible, trying to keep their sanity. But Asriel followed his best friend.

For some reason even when Chara couldn't hold their breath anymore, the Toxic didn't do anything to them. They assumed it was because they had gotten sick and that somehow helped.

They almost got to through the ruins, but then they noticed Asriel collapsing right behind them. Chara went to help him, asking him why he decided to follow, but Asriel couldn't answer because he was slowly losing it. Asriel apologized, crying, saying that he thought he could help, but that he had failed and would soon become a phsycopath. He begged Chara to end him with their knife so then he wouldn't be a threat, but Chara regected. Chara couldn't do that, no matter how violent they were. So Asriel took the knife and did it himself, laughing at his own death. Chara wept for what seemed like hours, as Asriel slowly died. But then, Chara couldn't take it and took his SOUL into their locket. Chara slowly went insane because of this, and the sickness was no help. They still needed to stop the humans for Mom and Dad, so they stumbled to the cameras. Chara made a bunch of motions to the cameras to get the humans' attention. Chara doesn't know if they ever noticed but they got too weak because of sickness and fell into the flower patch. As they lay there, they took Asriel 's SOUL out looking at how bright his SOUL was. They cried black tears tainting the petals black, until both Asriel and Chara turned to dust.

A/N
Sooooo what do you think? I'm going to bring in more soon! :3