Undertale: THE FORGOTTEN

3 YEARS AFTER THE PACIFIST RUN OF UNDERTALE:
The monsters, along with their Ambassador, Frisk Dreemurr, are living in peace on the surface.
Toriel runs a school made for both monsters and humans.
Asgore works at a botanical garden in Ebott City along with his now non plant son, Asriel.
Mettaton is a successful TV-Star and show host.
Papyrus is a well renown chef and five-star food critic.
Alphys is the head of the science faction of the city, and is working on how to cure determination poisoning to help the amalgamates.
Undyne is a highly known police chief and gym instructor.
Frisk is living with Toriel, and has a peaceful life, give or take a few governmental and presidential meetings, plus having Chara to deal with.
And Sans...well, he's Sans. What did you expect?
Everything seems happy.
But.....
Frisk has been getting the nagging feeling something is a little off. Sans is less happy than usual, and he's being secretive. Every time they asked, Sans just replied with a grin and a few puns. What is he hiding? And what is this nagging feeling that they forgot one more person to save?

(Hey guys! This is the first UT book I'll do without using my OC's! But, the thing is, this shall be your book as well! It's a choose your own adventure kind of thing! Tell me how you like it, and when something happens, vote on one of each options!)