The Sun Warriors are the creators of Vampires.
During the Yomizuki Empire's conquest of the Nojoan lands, they were looking for ways of creating super human soldiers.
Nicknamed Oni, these soldiers were created using special magic-infused oni masks, which they couldn't remove after they were converted, as it would become part of their body.
These Oni would consume the blood and souls of their enemies in order to grow stronger during a battle.
Consuming the souls of their enemies would have adverse effects on them such as split personality disorder and conflicting memories. Due to this, many Oni would go insane during and after battles, prompting the Sun Warriors to find ways of defeating them.
After the Yomizuki empire's conquests were finished, the Oni would roam the lands, destroying villages and growing more powerful by the day, thus prompting a secret operation to be launched in order to quell this new threat.