So, I've been slowly chipping away at like four or five different AUs and this one just kinda distracted me for a while. I had the general idea ages ago but only recently fleshed it out enough to feel comfortable showing it off.
Focusing first on the main 6 characters, there are three pairs that are inextricably linked: Sans and Papyrus, Alphys and Undyne, and Toriel and Asgore. Most swap AUs switch their roles within these pairs, like Alphys being the Captain and Undyne being the Royal Scientist, but I decided to tie these pairs together and swap them with another pair. The obvious starting point was swapping Sans and Pap with Alphys and Undyne, making Pap the Captain and Sans the Royal Scientist, and Undyne the rookie trying to get into the Guard while Alphys is the mysterious Judge. I found some concept art (so, so long ago...) that sparked this initial concept but it felt weird leaving Asgore and Toriel alone, so I had to find a pair to swap them with as well, and bellow the cut is the result.
I've got a few ideas for other pairs to swap but I haven't fully solidified any of them, especially since there aren't a lot of pairs that are quite as solid as our core characters. Let me know if y'all have any ideas for pairs and who they might be swapped with.
Angela had been married to an abusive husband and ran away when she realized she was pregnant, which made her realize she didn’t want her child to suffer what she’d been through. She fell into the Underground and Toriel instantly connected with her over motherhood, having just become a mother herself for the first time a few years ago. Over the next 7 or 8 months of pregnancy (Angela wasn’t very far along before she ran away), Toriel and Angela grew even closer, being the mother/daughter the other never had, until Angela gave birth and there are complications that monster healing magic couldn’t fix. As she’s dying, Angela begged Toriel to take her soul and free the monsters, as they had been far more kind to her than any human had been. They deserved to be free. Wracked with grief, Toriel hesitantly granted Angela’s dying wish, but couldn’t bring herself to kill the mother and child she’d found. She was then attacked by the husband/father of the mother/child and promptly died in Asgore’s arms while Asriel watched. Both are severely traumatized by this Asgore was then forced to raise the human child, Chara, on his own as best he could, while Asriel eventually ran away (I haven’t decided how long it took for him to decide that running was the best option) and holed himself up in the Ruins. Since Asriel is still alive, Asgore continued to age, twice as fast since Toriel was no longer alive to share the burden. When Asgore was about to die of old age, he told Chara to absorb his soul and lead the monsters in his stead, which effectively made Chara immortal. Toriel eventually woke up as [female Flowey] and Asriel is (the equivalent of) a young adult when Frisk eventually fell, being the self-proclaimed Caretaker of the Ruins.
Since Chara was being raised by Asgore and eventually took the throne, war was not declared and humans were not hunted for their souls if they were discovered in the Underground. Instead, they either lived with Chara in the castle or were adopted by a monster family if they were too young, and their souls were collected peacefully when they died. Chara is able to leave the Underground with Asgore’s soul but can’t take anyone else and never told anyone, though it is useful for keeping tabs on what the surface humans are doing, or getting certain resources otherwise unavailable underground. Chara also gained some non-human traits upon absorbing Asgore’s soul, including by not limited to horns and “special eyes”, as well as general access to magic.
Sans and Papyrus are estranged brothers when Frisk falls; they each blamed the other for the accident that killed their father, former Royal Scientist Dr. W. D. Gaster. Sans was appointed in his father’s stead and Papyrus became Captain of the Royal Guard. They’ve had these positions for a while and are tersely civil if ever they need to interact for their jobs.
Undyne and Alphys are childhood friends and now roommates that have caught feelings for each other but are too afraid of ruining their current Best Friends relationship to actually confess their feelings. Undyne is young and ambitious but idealistic about what she can achieve, basically throwing herself at Captain Papyrus to join the Royal Guard. He sees a lot of his younger self in Undyne and wants to protect her innocence and idealism, since he didn’t have anyone to do that for him while growing up. He knows how dark the world can be and doesn’t want to break her by exposing her to it. Alphys, meanwhile, had recently been assigned the role of Judge and isn’t handling the stress well. Having to see and Judge the worst monster kind has to offer, and not allowed to really talk to anyone about it (no one is supposed to know the true identity of the Judge), she has become more withdrawn and relies heavily of Undyne’s optimism to get through each day.
