The King? The Copy? The Soul?

A world heavily inspired by jujutsu kaisen with a power system that's pretty confusing This book is about our main character named Tori a human that's half monster after he merged with the strongest monster of all named Yoru a monster stronger than everything his sensei Jin has gotten him and let's Tori decide to Die or Live until he eats the 8 souls Tori has to eat the 8 souls of the level 8 monsters the strongest level and true monsters that will do anything to achieve there goal they want to kill Tori to gain his power and be the king of everything He has watched his sister Sanya die from a heart problem and cancer and he knows he has to live on for his sister even if he knows he'll die at the end will he carry it out or will he break the deal? Tropes/genre: fiction, dark, adventure, action, fantasy, sad, inspired by jujutsu kaisen, diseases, good characters, multiple arcs.