Kuro No Maou Manga

A brief description of the manga Kuro No Maou:

Kurono Maou never knew how to enjoy life. Attending high school without much enthusiasm, the boy did not understand how to plan his future. He doesn’t have a girlfriend, but the hero is quite calm about this nasty little thing.
One day, sitting in the school library, the hero felt a very strong pain in the temples. From the tension he lost consciousness, and woke up in a parallel universe. Here the monsters will not let him relax, and make his life if not interesting, then dangerous exactly …