The Beast Within. Star Sanses x reader x Bad Sanses

Long ago, two races ruled over the earth: Humans and Monsters. Over the years a great incident happened. And unknown power spread across the land. Only this effected the humans. After a few days later some of humans had been turning into humongous creatures full with huge amounts of power that were different from monsters. They were called "The Beasts". Sadly after their transformation they were unable to transform back and their inner beasts took over them causing them to act wild and vicious.

Over the centuries these beasts killed and devoured monster and human souls for their own desires. During those times the humans and monsters fought the best they could against the giants but sadly most of them fell and most of their territory were lost to the Beasts. For the safety of humans and monsters, three walls were made to keep the beasts out so they could live in peace.

Over the years the peace became undone when war broke out between two brothers: Dream and Nightmare, guardians of positivity and negativity. Boundaries were made between the walls: Sun territory and Moon territory.

During this war, the a huge beast called an "Colossal Beast" broke one of the walls causing beasts to come through. Many lives were lost during that time.
During that event a young child named (Y/N) witnessed the death of their mother thanks to a beast. After evacuated. (Y/N) swore to destroy every beast that roamed the earth. Read more to find out.