Just so you guys all know, I am well aware that Frisk is supposed to be a character of ambiguous gender, aka Frisk can be either gender. Since I'm a male, I will be referring to Frisk as a "he" in this story. If you want to read it as a she, by all means go ahead.
THIS STORY CONTAINS SPOILERS FOR THE ENTIRE GAME! IF YOU DO NOT WANT ME TO SPOIL THE GAME, DO NOT READ THIS STORY
Prologue: Long ago, two races ruled over Earth: Humans and monsters. One day, war broke out between the two races. After a long battle, the humans were victorious. They sealed the monsters underground with a magic spell.
Many years later…
Frisk trudged up the slopes of Mount Ebott. He soon approached what he was looking for. A cavern at the summit of the mountain. He entered inside to where the roof opened up, revealing a deep dark pit. Frisk had been dared by some of his friends to climb the mountain and peer into what was seemingly an endless abyss. He leaned over and looked in.
"There. Now that that's done with…" Frisk spoke to himself. He was about to turn around when he heard something. A stone clattering at the bottom of the pit. He remembered the old legends that inside the pit lay a whole kingdom full of monsters that had been sealed long, long ago. With the unprovoked noise, he peered in again.
"It's just a myth…right?" He thought to himself as he leaned over further. Suddenly, his foot caught a root, and just like that he was falling. Screaming, down, down, down, into the darkness.
I'm aware that the Prologue is very short. That's just because the game's intro didn't give me much to work with. Don't worry, chapters are of a reasonable length.