Not sure if it's a thing but I looked and couldn't find anything like it, it's not like most AU's and has a different setting.
Gaster in this AU is the Father of Papyrus and Sans, they do have a mother but when sans was 10 she was said to have died in the core, Sans Is now 16 and Papyrus is 14 years old, Im still thinking about the setting weather it should be before the war or the war never happened, Or wear there are two undergrounds being a few miles apart, do to there being too many monsters so after the war the humans like split them up, and Like Asgore can rule one and Toriel and rule the other, but the main theme will be something like sans who has a best friend, still deciding who, might be an oc or just frisk, maybe Chara and Asreil, but do to forced separation they can't see each other and as weeks go by sans learns about the violence and cruel world he lives in, he fears that he's loved ones can get hurt or even killed, so being so weak he trains in secret, putting himself through painful training in order to become strong enough to help the people he loves even if it hurts him, even if he has to change who he is, the only person who knows about he's training is Grillby, Im also thinking about Chara and Asriel being in it and also being like the main parts as mentioned before, sense i don't want sans being the only main part, but Chara and Asriel would also be Best friends or maybe they can be one person type fusion thing, but Monsters such as the amalgamate seemed to have a taken a different path and go around killing and devouring souls, some being smarter than others and some not so much, I have added or thought about the fighting and magic being different in combat and added a few more mechanics to their magic