when the company started injuring Kuroha for "slacking" at work, the lady condensed all of the dark aura accumulated throughout the years working in the black company, resulting in a transformation into a high-ranking demon.
she then bathe in the blood of her superiors that exploited her, alerting the heaven world of her actions. she was sent to the demon realm, leaving Kuroha all alone in the Human world.
outraged by the banishment, Kuroha spent a good amount of time (decades) gathering dark energy by committing misdeeds in the Human world, allowing him to freely travel between worlds.
once he returned to the Demon realm, he found out that you got jebaited.