This AU will be told by three ways: a) the song, b) the song description, c) the thumbnail

Songs will highlight important events, and may sound different than the OST replacements.
Song descriptions will likely be around a paragraph, with a short story to explain the song.
The thumbnail will show deisgns/aesthetics

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Character -> Role
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Frisk -> Frisk
Flowey -> Mad Dummy
Chara -> Sans
Asgore -> Napstablook
Toriel -> Chara
Sans -> Toriel
Papyrus -> Papyrus
Undyne -> Asgore
Alphys -> Undyne
Mettaton -> Alphys + Mettaton
Napstablook -> Asriel

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Location -> Replacement
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Ruins -> New Home
Snowdin -> Hotland
Waterfall -> Waterfall
Hotland -> Snowdin
Core -> Ruins
New Home -> Core

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Alphys, Mettaton, Napstablook, Asgore, Asriel, Chara *roles* explained:

Asgore was the original king and Toriel was the queen. Mostly everything was the same upto this point, until a human child fell down. The king found them and introduced them to monsters in a more positive note. Soon, the child found themself a part of the royal family. However, they felt lonely without anyone else around. The king and queen decided to take the human out around the kingdom one day.

There, the human child met Napstablook. The two became close friends after the human was able to help with one of Naspta's more depressing moods. Because of how close these two became, the whole Blook family and Dreemurr family became close allies with each other. Because of this Mettaton was granted access to study materials only the royal scientist would have access to (though, he only did this to get himself a robot body and rise to stardom).
However, one day, something seemed off about the human. They had a pretty terrible nightmare that humans might discover the monsters underground and finish them off. So, they decided to confront Napstablook about this. The two then devised a plan; if Napstablook had possesed them, maybe the two would be able to cross the barrier with their combined monster and human power, and they could stop humans before it was too late.

This was a grave mistake. As the two reached the barrier and began their plan, Toriel walked in. She tried to stop the two, but it was too late. In trying to possess the human, Napstablook forced their SOUL out and turned them into an empty husk. Toriel had accidentally absorbed the SOUL by touching it. The burst of sudden emotion turned them nearly corperal, but they managed to escape in time, at the cost of their SOUL. It wasn't strong enough to escape the lifeless husk. Now a former shell of theirself, Napstablook fled in panic. They would eventually come across a Dummy and turn bitter towards everyone, including theirself.

Asgore discovered his wife in a horrible state. The fusion was going all wrong, Toriel's body had started to deform, transform into some horrifying beast. But with a little time and Asgore's comfort, Toriel and the human were able to stabilize. The human, however, still wanted to enact their plan. After convincing Toriel, the two went to cross the barrier in hopes of establishing peace.

The humans did not hesitate to slay this unknown beast.

With their life nearly over, the two managed to meet with Asgore. Toriel's last request was to for her dust to be spread on the other side of the underground. Asgore agreed to this and began his journey to the other side of the underground. However, by the time he had reached the Core, he had been too struck with grief. He felt he was no longer fit to rule as king and hid himself deep in the Core after spreading Toriel's dust.

With no royal blood left, Mettaton was suggested to rule because of his close ties with the family. However, he didn't feel he was worthy of that responsibility (and was still overcoming Blooky's disappearance) and suggested the current captain take that role, Undyne. Undyne was more than willing, and immediately declared war on humanity for their sins against monsters upon becoming queen. Her powerful words moved most of the kingdom, but there were some who were against it. These were the people of Hotland, and popular center of culture that tended to glorify humanity in the form of cheap anime.
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Sans, Papyrus, and Toriel *roles* explained:

With the humans falling in various locations, Undyne was able to gather six SOULs. However, when Chara fell down near Hotland, Papyrus found them. They were badly injured from the fall, and Papyrus being Papyrus, decided to take them in. Sans was living with him at the time and was very skeptical about keeping a human hidden in their home, but eventually obliged.

The two grew very close with Chara, almost as if they were a family all their life. Sans was working with Mettaton time at the time as the assistant scientist, until he assigned Sans to monitor inside the Core. The reason behind this was because Undyne wanted to start tightening security around the underground, and leaving the energy machine all alone felt like a terrible (though, it didn't really need much tending to besides ice). So, he had to move into it, unfortunately behind closed doors. Papyrus and Chara would still often visit the door to talk with him, or slide under letters, but Sans wasn't able to leave most of the time.
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Undyne role explained:

Alphys was a close friend (and crush) of Undyne's from the beginning. She wanted to keep Alphys close by still, and because she knew of her intellect and potential tactian skills, she assigned her captain of the royal guard. Alphys doesn't entirely like the job, but she does it for Undyne.