BeatTale (UnderTale AU)

BeatTale is an AU that works similarly to DanceTale, but instead of different kinds of dancing every monster has a different kind of music. Their kind of music will play directly from their soul during a fight, and whichever side has the better rhythm or best music in general would win. In Frisk or Chara's case, when a monster's music is defeated they can either choose to have mercy on them like usual, or send out a sound wave of their music that shatters the monster's soul.
Monsters (and in Frisk's case, humans as well) bond through sharing their music together. They will enter a mock fight, in which they will both play their music to one another. If they get along, their souls with work together to create music that has the dominant trait of one monster's music, and it harmonises with the other monster's melodies. Even if their music isn't compatible, if monsters are determined to make music together, they can change the kind of music their soul emits to make it compatible with their friend's kind of music. Only when making music together will the monsters' souls change to that kind of music.
In this AU, monsters are generally more friendly to a peaceful human, but more violent with a killer. Even killing one innocent monster will turn everyone against the killer, making their journey through the underground significantly more difficult. If the killer has been captured, they will not be killed, but be kept away from any other living being until it was found out why they committed the crime. If it's found that their heart is not filled with evil intent, or if it was an accident, they will be released under watch by a monster placed in charge of them. If their intentions are found to be evil, however, they are terminated in as humane a way as possible and their bodies are buried with the flowers. The monsters respect them, because even if they wanted to kill, they must have had people who cared for them. Monsters know that they probably won't escape from the underground anytime soon, as no human souls have been collected yet. Because they know that they probably won't get to the surface soon, all monsters are very close with one another and they're all good friends. 
Some monsters don't make music, instead add melodies and harmonies to other monster's music. Grillby, NiceCream Guy, and Monster Kid are good examples of this, as they wear whatever clothes they are comfortable in. The choice not to make their own music is different from the inability to make music, though in rare cases there is a monster born unable to make music. WD Gaster is the only monster known of that had the inability to make music, but he has disappeared years ago.
Fighting mechanics:
Instead of collecting better weapons or armour during the adventure, Frisk would collect different kinds of sound equipment or instruments. The better equipment they have, the better their music would sound and therefore the stronger it would be. 
When using the FIGHT option in a battle, Frisk would have to play different notes of their music to cause 'damage' to the monster's music. The lower the monster's 'music meter' is, the worse quality the music playing will be and therefore their ability to fight back would lessen. A fight will only end when either side's music has been silenced, and then the option to spare or kill the monster would be up to Frisk.
ACTing in battle would work the same as it usually does, as in trying to talk to the monster or doing specific actions that may cause them to want to spare the opponent. Two options always present in any battle would be volume + and volume -. Turning the volume up of their music would increase the 'damage' they could do to the opponent's 'music meter', but also leave them more vulnerable to having their own 'music meter' decreased. Turning volume down would decrease the 'damage' done, but also make the opponent feel more at ease with them and may increase the chance that they will want to spare them. 
ITEMS would be used the same, but they would be much different from the originals. Usable items would consist of speakers, earbuds, megaphones, etc. Items would either greatly increase or decrease the volume of their music, refill the 'music meter', or make their soul move faster to avoid the magical attacks of the opponent's music. 
MERCY works exactly the same in BeatTale, and using mercy on a monster would end the fight if they were willing to spare them as well. 
On the monster's turn of the battle, they will send musical themed attacks towards their opponent, the magic being formed out of the strength of their music. Their opponent would have to avoid these attacks to avoid having their music meter lowered. If a human's music meter is completely lowered and a monster isn't careful enough to notice, any further sound from the monster will shatter the human's soul.  
Characters and their types of music: 
Ruins enemies: Slow and simple rhythms, with not much of a beat or style to them. 
Snowdin enemies: Upbeat and light, a very happy sound. 
Waterfall enemies: Generally calm and flowing, much like the area itself. 
Hotland enemies: Slow and swinging, like the heat of the area has affected the tempo of their music. 
Frisk: Piano music. Depending on how a monster's music battles, they can either have their music be more chaotic or more lighthearted. Their main 'weapon' is a keyboard, and they often wear loose, flowing clothing. Their hair is often up in a messy bun, with two small strands handing down beside their face. 
Chara: Metal or hard rock. Their music is always violent, and their volume is almost always on maximum. Their main 'weapon' is an electric guitar, and they wear dark, punk-like clothes. Streaks in their hair are dyed hot pink. 
Toriel, Asgore, and Asriel: Classical music. The music usually slow, but depending on their emotions it can be much more violent and have a faster tempo. The whole family wears traditional royal clothing, but with a certain calm and flowing element to it. 
Sans: EDM music. His music usually has a quick and upbeat tempo, though it has an underlying slowness to it. The bass is heavy, but sometimes, depending on his emotions, it's so heavy it makes the music feel ominous or intimidating. Only when making music with his friends or his brother does his music truly shine the way it's supposed to. He wears electric blue shorts with white lighting streaks on them, neon high tops, a simple white shirt, and a brightly coloured sweater over it. The patterns and colours of the sweater are complex, though still somehow fit together nicely with his style. His hood if often up as a sort of emotional shield. He only takes it off when he's around people's he's comfortable with, like his brother Papyrus, or Frisk later on in a pacifist route. In this AU, Frisk does 'fight' Sans, but only because they are trying to become closer friends through their music. 
Papyrus: Surprisingly, brass band music. His music is very brash and bold, but always upbeat and happy. If any monster is killed, his music will sound ominous and scary, with a certain element of timidness to it. His favourite people to make music with are Sans and Undyne, as he is closer with these two monsters than any others. He wears clothes that are themed after a band master's suit, but instead the pants are cut short into loose caprees. The vest and undershirt are instead replaced by a loose white tank top, with a tie printed at the collar of the shirt. He wears boots similar to his original ones, instead in black and white. His 'fight' with Frisk is when he's trying to bring them to Undyne, not to capture them but to inquire about how they could fit into the society of the Underground. 
Undyne: Rock music, though not nearly as violent as Chara's. In this AU, Undyne is still the head of the royal guard but she's less bent on capturing humans and more inclined to want to protect her friends. Her music is generally loud, but in a good way. If any monster is killed, her music will become much more chaotic, and she will fight much harder. She loves making music with Papyrus and Alphys, but especially Alphys. She wears black leggings and light grey slip on sneakers. Her shirt is a black tank top with a comic book explosion effect that says 'BOOM' in the middle in big white letters. Her 'fight' with Frisk is when she first meets them, when Papyrus brings them to her in an attempt to try to fit them into society. Without listening to his explanation first, she engages Frisk in a fight to test them if their music is strong enough. 
Alphys: Pretty much any music that sounds like it could be in an anime. Her music is often lighthearted, but changes to being very slow and terrified if any monster is killed. Her favourite person to make music with is Undyne. She used to make music with Mettaton, until he became the star of the Underground and he was swamped with work. She still wears her traditional labcoat and glasses, but her glasses have a small pink star charm hanging off the end. Her labcoat is unbuttoned, revealing her pink 'Mew Mew Kissy Cutie' shirt that she wears underneath. She never 'fights' with Frisk, but during their hangout time she plays her music for them and they watch anime together. 
Mettaton: Hip hop / pop music. His music is always, ALWAYS high energy, with lots of posing while he plays it. If any monsters are killed, his music becomes dark and foreboding, with an energy that would make almost anyone burst into tears. He makes his music on his own, and performs it in front of the many monsters of the Underground. He practically only plays his music for performances until Frisk comes to the Underground. He wears black short shorts, his original knee length pink boots, and a pink cropped top with white hearts on it. The words 'Oh yes!' are printed in hot pink on the back. A pair of MTT brand headphones hand from his neck. His hair is styled to be much more wavy, but still covers his right eye. His 'fight' with Frisk was when they accidentally stumbled onto stage during one of his performances in his EX form. When he finally collapsed from low battery st the end of the battle, they reminded him of how he needed to take enough time to rest and relax himself. After their first 'fight', Frisk and Mettaton began making music together often, just for fun and to help him unwind from work. 
Napstablook: Jazz music. His music is smooth and slow, giving a calm and somewhat sad feel to it. If any monster is killed, he stops making music completely. He used to make music with Mettaton when he was still a ghost like him, but now only makes music with the Waterfall enemies and Frisk when they arrive. He wears his 'Dapper Blook' hat at all times because it makes him feel a little bit happy. He never fought with Frisk, but if they enter his home they can catch him making his music with Shyren. He'll seem slightly embarrassed, but them opens up to Frisk and the three make music together.  
Shyren: Sad and haunting chords. The only time her music is heard when she makes music with Napstablook and Frisk. 
Muffet: Electro swing. Her music is swing with the elements of electric synthesizers. If monsters are killed, she refuses to play music for Frisk, instead commenting on how her music was to calm her spiders in the trying times they were in. She plays music with her spiders and Toriel, seeing as in this AU the queen hasn't locked herself in the ruins. She wears a top hat and a tight black dress. She wears white gloves on all hands, and a black bowtie. She 'fights' with frisk because they tripped and spilled the baking her spiders had made, so she makes them play music for them as an apology. 
Temmies: Their music is mostly just whatever bouncy, happy tune they feel like playing. They mostly make music with each other. 
Burgerpants: Doesn't really make music, or even play along with others. He works tirelessly at the MTT resort, giving him no time to make music.  
(If there's anything you think I missed, comment it below! I know other people have thought of this concept, but I wasn't able to really find any detailed information about this AU so I'm claiming it as my own! I'm currently working on drawing the character designs for this AU, So bear with me!)