(I am not that knowledgeable about creepypasta, so I'm going by the official wiki and Bubblyship's stories. I apologize if I get any wrong).
This is a fanfic of the fanfics [fanfic-ception] Sans's House Guests, Shadow of Remorse, and Soul of Fire. It is based of the original idea of Soul of Fire that was scrapped. I have full permission from BubblyShip to write this story, and make sure to check the original story's out!
Also this takes place after the 2nd book.
When Sans and the creepypasta moved into underground, a barrier was brought up letting them live in peace. More creepypasta came into the underground safe haven and everyone has mostly lived in peace, humans coming in stupidly satisfying the meat eaters for the most part.
But when Frisk's kid goes into the underground to get justice for the people creepypasta have killed, they find creepypasta aren't what they thought. Going through the underground like their mother, a journey awaits them.