Ankoku Kishi Monogatari: Yuusha wo Taosu Tameni Maou ni Shoukansaremashita

This is the tale of a protagonist who was summoned to protect the demon king from the hero.

Kuroki was suddenly summoned to another world. The one who summoned him was the demon king, Modes. And, the territory currently ruled by the demon king was on the verge of destruction because of the hero.

The demon king Modes, possessing an atrocious appearance, bows toward Kuroki.

「PLEASE SAVE US, SAVIOR-DONO！！」

Kuroki then decides to save the demon king due to some turn of events.