This is a new Undertale AU called Foiltale, and will follow Chara and Asriel in their journey through the underground. Later chapters will be longer than this, I just wanted to set the scene a little with this intro/prologue. I haven't written fanfiction in a long time, so I'm kind of rusty, but please tell me what you think! And thank you for reading!
- Fox

Foiltale runs concurrently with Frisk's True Pacifist Run, but in a mirror-like realm to the original game.

Chara and Frisk's Pronouns: They/Them

Prologue

"Can a soul so full of hatred even be redeemed?"

Chara awoke to find themself in the Ruins once more. Confused, they were alone and afraid as they try to remember what had happened. Suddenly, the memories all came flooding back: the "butterscotch" pie they consumed, the hellish fever that followed, a familiar voice piercing through their death throes and pleading them to "Stay determined!", a swift passing before being yanked back into life and traversing through the barrier.

Then, the human village… Their eyes darkened as they recalled the horror and anger on the villagers' faces. The humans had attacked them- attacked Chara and attacked Asriel! A humorless smile started to form between rosy cheeks, their fist crushing a damnable buttercup flower beneath. "I knew it… Humans and Monsters can't get along after all."

Before we could say anything, they assaulted us. Their clenched hand trembled as they remembered Asriel's consciousness, scared but unyielding, fighting against Chara's want to swing at their aggressors in retaliation. Echoes of the pain caused them to flinch, recalling the spears and fire as they penetrated and scarred their combined body. The humans had killed their brother, the kindest, most gentle creature this world had ever known.

…

… No, it was Chara that had killed their brother. In going along with their plan, Asriel met the same fate as his sibling. Chara hadn't wished for this; the human just wanted to find a way to break the barrier and set the peaceful Monsters free.

"But it seems humanity just can't help but kill everything it touches." Chara slowly got up. "I guess there's only one thing left for me to do now: kill the humans." Including me.

"Humans and Monsters can get along!"

That voice…! I-it couldn't be! Asriel?!