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The story pretty much follows the canon in terms of how Monsters got stuck in the Underground; they battled against the Humans, lost, and got sealed under the Mountain as a result. Asgore and Toriel lived happily with their son for a time until Chara falls down and the chaos starts.
It was during this time that Alphys, assistant to Head Scientist W.D. Gaster, was working on her latest project. It involved creating a robotic body for Monster to inhabit as an attempt to impress both Gaster and King Asgore in hopes of getting a promotion. The body was finished when disaster struck the Dreemur family, with both their children dying and fading into dust in front of their eyes. But, instead of being overcome with rage, Asgore was broken down with grief, losing hope and putting down his mantle as King of the Monsters.
Alphys was distraught, but was determined to help the poor Ex-King. She redesigned the robot, giving it all the necessary knowledge it needs stored in its hard drives to properly rule and lead a Kingdom. When the Ghost Monster began to inhabit its body, Mettaton was thus born. Originally designed to be an entertainment robot, Mettaton was naturally used to having the center of attention. The thought of ruling the Underground appealed to him even more, and they offered just that to the King.
King Asgore agreed, desperate to leave the Throne and see any further reminders of his failure as both King and Father (even though he’s been nothing but). So he quit, just like that, and Mettaton took his place as King of the Monsters. He issued an order, that all Humans that fall through the Underground are killed, their Souls gathered in order to make him more powerful and break the barrier and take revenge on the Humans for all of Monsterkind. Toriel was outraged by this sudden declaration, and refused to help him in any way. So, before her husband could stop her, she stormed off the Castle.
When Asgore tried to follow her, Mettaton convinced him otherwise. His power could be of great use in the declaration he just issued, and appointed him the new Knight-General. With his grief for his children and contempt from the Humans still fresh on his mind, he accepted the new duty, only slightly glad that the burden is not as much now that he wasn’t King.
With the new King came new things. A Noble Family Order was created, consisting of the Dreemurs, Gasters, Mettaton’s, and Alphys’ own family lines. The members of these families managed all the laws, new or otherwise, within the Kingdom of New Home, and each family was given a specific part of the Underground to govern. Laws were made, the War had been officially passed, and the Royal Guard was established under Knight-General Asgore’s command. With this began the many centuries of slaughter against the Humans that fell through the Underground…
Until one such soul with Red as deep as the first Fallen Child came tumbling down, wearing nothing but rags as they found themselves at the mercy of an unknown world. Will Determination be enough to keep them alive?
- Rosy the Rose - Rosy the Rose is Underkeep’s version of Flowey. His color is red when friendly and has no thorns, but turns pale black and prickly when he shows his true nature. Roses are also Chara’s favorite flowers here, as is Frisk’s. it was assume that Rosy would act bashfully or coyly at first, emphasizing his attractive exterior to make sure no one is suspicious until it’s too late.
- Toriel – Toriel became a hermit in the Ruins of their old Capital, Home. She acts the way the she does in the canon, and, depending on the route they’re taking, will make Frisk either a tunic or a dress to replace the dirty rags for clothing that they wear. the creators would like to think that Toriel won’t change much in this AU, as she’s already so regal and proper that it will stick no matter what.
- Asgore – Asgore, however, is quite different. Since his initial grief at the loss of his children, he became closed off, leading the Royal Guard with a cold, iron fist. He doesn’t tolerate the failure of capturing any humans and will punish anyone who does so. Still, he has a soft spot for Undyne, Head of the Royal Guard. He still misses Toriel, but reminds himself of the burden he bears and focuses on taking more Souls.
- Jester – Jester is Underkeep's version of Sans. He, Along with Papyrus, were still children when the war ended. They grew up Underground and reached the age they’re in when Mettaton’s body was finished. As part of the Gasters, he is part of the Noble Family Order, though his personality just doesn’t click with that. He was once called a Jester by Mettaton as a joke, but it stuck to him so much that he decided to make it official. The King didn’t mind, as a good King needed a good fool, too. But, under the lazy and pun-loving exterior, something dark, mysterious and extremely powerful resides within him. His choice weapon is a cane.
- Papyrus – Papyrus, as Sans’ younger brother, is part of the Gasters, and was appointed as Second in Command of the Royal Guard by Undyne, with Asgore’s blessing. He is a master swordsman, but very soft-hearted. As such, he was never sent to any human-killing missions, and the title rings itself a bit hollow as he does nothing but set up traps and puzzles in different areas in Snowfell’s forest. Still, people admire him for either his status or energy, or sometimes both, and he’s happy with puzzles.
- Undyne – Undyne became the new Baroness of Waterfall(fell?) around the time she became the Head of the Royal Guard. She replaced Mettaton’s responsibility so the King would get more time in properly ruling the Kingdom. She trained under Knight-General Asgore as a child, and they share his resentment against Humans. Though rough and spontaneous, Undyne can carry a regal aura around her when the need arises.
- Napstablook – The Royal Maestro and Composer, Napstablook the Ghost was once seated beside Mettaton, but the pressure of attention led him back to his home at Waterfall. He continues to make music for the King while remaining as a shut-in who rarely socializes with others, much to Mettaton’s dismay.
- Muffet – Muffet, the Royal Baker, leads the most successful bakeries in the Underground. She raises funds to help the Spiders in the ruins, just like in the canon but doesn’t actively sell the goods herself. Aside from that, I don’t think she’d change much. She’s one of the most proper characters in the game, to be perfectly honest.
- Alphys – Alphys became the new Head of the Royal Scientist Circle after W.D. Gaster’s mysterious disappearance, and Marquess of Hotland after the Noble Family Order was created. She is one of Mettaton’s closest friends and was responsible for Rosy’s creation. She likes to stay inside her lab at times, but would come out to walk around Hotland or Waterfall with Undyne from time to time.
- Mettaton – Mettaton changed, for the better in the opinion of the story creator. His initial method of ruling the Kingdom was like that of the Neutral Ending in canon. If you don’t know what that is, the short of it is that Mettaton doesn’t like it when people complain about his dystopian rule, and would disappear mysteriously the day later. But with Asgore’s wisdom and Alphys’s conscience, he came to understand that ruling that way would not only be inefficient but would also hinder Monsterkind’s progress. His goal to break the barrier is genuine, as is his cry for revenge in the fallen Children’s names, but he wants to make it as flashy as possible, so even if he thought that taking Souls above would be easier, it didn’t matter.
- Frisk – Frisk, would be a runaway orphan from a poorly-managed orphanage above. They had enough of living in such poor conditions, and just wished that somebody would come and adopt them. They ran to Mt. Ebott to discourage people from finding them when they fell down a hole that led to the Underground.
- Gaster – Gaster is a mysterious man who was once Head of the Royal Scientist Circle but disappeared one day without a trace. No one actually knows much about him, not even his children. In this AU, the creators consider him as their father.
Roles Of The Characters In This AU
- Frisk - The Hero (shown in both their prince and princess outfit)
- Papyrus - Second in command of the Royal Guard
- Sans - The Royal Jester/Overseer of Snowdin/The Judge/The Hatter
- Muffet - Duchess of the Spiders/Royal Baking Specialist
- Mettaton - High King
- Alphys - Marquess of Hotland/Royal Scientist
- Undyne - Baroness of Waterfall/Head of the Royal Guard
Extra Details Of The AU
- Members of the Royal Circle (including the Knights) are required to learn spoken language of the Humans from the 17th Century. “Aesthetic”, declared King Mettaton.
- Papyrus has a most trusted, super secret adviser.
- Sans prefers to be called Jester, as he’s called this more often than his real name.
- Gerson is a former Knight-General once revered by the people for his participation in the war. When he retired, he became a hermit who sold scrap goods for a living in Waterfall. Only few know of his current existence and livelihood.
- Asgore is also appointed as the new Knight-General after Gerson’s retirement.
- W.D. Gaster, the previous Head of the Royal Scientist Circle and part of the Noble Order, had mysteriously disappeared one day.
- Sans and Papyrus’s mother was presumed fallen after the war. Papyrus never knew their mother, but Sans grew to resent their father for keeping this a secret.
- Sans and Papyrus’s siblinghood is as tight as it can get back in canon, though they rarely interact in public as to keep a regal face.
- Papyrus did not show any interest in Science, but established great skill in swordsmanship. Undyne appointed him as Second in Command of the Royal Knights after he challenged her to a duel and drew to a standstill, with Asgore’s permission.
- Papyrus’ enthusiastic energy, kindness, and loyalty towards Mettaton netted him a tight-knit group of friends.
- Jester made no effort (literally) to hide his laziness, more showing his comical side and his quick wit at puns than any real interest in other things, but is extremely powerful in battle.
- Papyrus is happy that his brother could make a living with his puns, but requested a better position for his brother.
- Jester, being the older brother, was appointed the position of ruling over Snowfell in his father’s place.
- Sans still kept his title, and name, as Jester. He also hid his interest in his father’s work, despite his further resentment at his disappearance.
- Alphys is the only monster who knows of Jester’s affinity to science, and appreciated his help around the lab by keeping it a secret.
- Jester has Gaster Blasters, and everyone knows this. No one knows of their true capabilities, however, as he mostly uses them for show. He even sits on one to float around when he’s too lazy to walk. He gets too lazy to walk often.
- Papyrus and Undyne are proclaimed “best buddies”, and he would sleep over at Undyne’s from time to time and take cooking lessons from her.
- Papyrus is a master at making spaghetti, but only spaghetti. He can’t bake or cook another dish to save his life.
- Alphys knew Mettaton before he was the King, and was already friends with him when she decided to make his body.
- Even being in the lab with him, Alphys never heard Gaster mention Jester or Papyrus.
- Jester likes to stay cooped up at home, and out of his uniform. He usually wears just the inner polo and pants of his outfit, foregoing the frilly decorations and belts. He only wears the outfit when it’s absolutely necessary.
- Jester is also a lot cleaner than Classic Sans, a habit he picked up from both his father and Papyrus.
- Jester is mostly never shipped with Toriel. He is mostly shipped with Frisk or The Reader