The Underground, 21xx: a sans X reader

You, Y/n, are visiting Frisk, your somehow still alive ancestor, over the summer again for the eighteenth year in a row when they say somethings out of the norm.

"Y/n, my love."
"Yeah, GG?"
"I believe, it is time I told you of the Underground."
"GG~ you've told me that bedtime story for as long as I can remember! 'Long ago, two races ruled the earth. Humans and Monsters. Till one day, war broke out.' And long story short, we, the humans, won and trapped the monsters underground. GG-Frisk, I don'-"
"I'm not talking about the beginning! *sigh* I'm not talking about the beginning, my child, I'm talking about the middle. About my timelines and resets in the Underground, and how I shattered the already broken Monsters who lived there..."
"GG, I- I don't understand. W- what are you talking about? A- are you trying to that- that you've been there, and what is all this about 'timelines', and 'resets' an- and 'shattering monsters'? And if what you're saying is real, why are you calling it the 'middle'. I mean, what else could- no, what else WOULD happen next, if anything at all?"
"Yes, that is what I'm saying, and yes it is real. All of it is real, and as for what is going to happen next, you."
"Me?"
"You, Y/n, are what is going to happen to the Monsters and the Underground next! Whether you like it, or not!"

Please allow me to act as Narrator and Biographer of your tale, and guide you through 'The Underground, 21xx

{a/n, it's my first actual attempt at a actual story, so plz bare with me!( >.<)}