A peaceful world was suddenly invaded by monsters, changing the whole civilization as a result of it.

Monsters such as Dragons, goblins, and other species riddled the whole planet and humanity as a result was pushed far back, to the brink of destruction.
In these times of crisis, however, something unexpected happened. A strange energy was discovered which we called mana; it enabled our body to become far stronger and defeat those monsters; that event marked the beginning of the Evolutionary Era.
We stood our ground and fought back.
Now? If you don't have any money, you could just hunt some monsters to trade for their cores. 
Some were happy because their fantasies came true. But most were not. A boy named Ned lived with his family in a city surrounded by war, called Grendan. More than six hundred people died there every day, and Ned's father, mother and sister were included in that number. Ned cried and bled. He swore and promised to the monsters who created this mess, "You Fuckers! Even if it takes my life, I will extirminate you!"
This, was the beginning of the new era.