In another timeline, before there was the walls, there was a brutal and power-hungry king named Fritz who used the power of his slave Ymir to vanquish his enemies and expand his rule in an attempt to rule over the whole world. Amidst this turmoil, a hidden kingdom recognizes the danger that King Fritz is to them and sends one of their best soldiers, a young man named Flukey, to sneak onto King Fritz territory and kill him. And at his arsenal is his will and the hand of fate to help him see his mission through for the greater good his kingdom, Fritz's, and the world.
On the outside, you are pitied for being cursed with the inability to speak. Little did they know, you're carrying heavier aspects of yourself from the inside.
Disclaimer: Though the references are there, you are not completely an AOT version of Nezuko!!! Also I do not own the Attack on Titan franchise and they belong to their rightful owners.
Also, this book contains major spoilers to the AOT Franchise; DON'T read if you don't want spoilers.