Ankoku Kishi Monogatari (WN)

This legend tells the tale ofthe protagonist, Kuroki summoned toprotect the demon king from the hero. Kuroki, unbeknownst tohim, suddenly came toanother world. Hewas summoned bythe demon king, Modes. The territory which the demon king overrules was inthe midst ofbeing destroyed because ofthe hero. Hehad nochoice but tobow before Kuroki. «Please save us, Savior Dono!» Kuroki nodded, deciding tosave the demon king due toanunexpected turn ofevent. — Description from novelcodex.org