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"—demons with considerable power, whom 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!

(Source: WhimSubs)