This is a tale back from the time when humans and demons fought over the world.

The demon king commanded not only common demons, but also, the so called “mid-bosses” with considerable power, which he deployed in strategic positions.

However, these mid-bosses kept getting defeated by the heroes, who continued to grow stronger with time. Which led to the demon king’s army needing to produce even stronger mid-bosses.
It was at that time that a certain demon stepped up.
Demonstrating great wit, she became the demon world’s founder of the official mid-boss training academy!