In this world there are demons and humans, but there are no peace between them demons consume blood of humans to get stronger. There are some special demons that can even make human to demon by there toxic venom, to protect themselves human have magical powers to fight against demons but they don't had it from the beginning.
Hundreds of years ago humans are helpless, desperate for someone to save them because they don't have any magical powers like today. Then someone showed up to be hope of light in that dark world for humans. He fight against the king of the demons, he's will was so strong even though demons are less with magical powers but he still managed to gain victory against the demons for the first time humanity get a hope. They say if you somehow defeat the king of demons you'll get a wish and he said
"I wish to grant powers for humans to protect themselves and fight against the demons."
.....
The people that fight against demons are called the heroes, they protect civilians.