Just read

it is the story of my AU called Familytale. in this AU, Asgore gives up fighting Frisk, keeping everyone underground. Frisk then goes to live with Toriel in the ruins. four years pass, and Sans and Toriel are a couple and Frisk has started calling him dad. everyone becomes one big happy family after that. if you want to know more about this AU and the story behind it, read my fanfic. :)