How NOT to Summon a Demon Lord manga volume 18 features story by Yukiya Murasaki with art by Naoto Fukuda.

Diablo and his companions have saved the Kingdom of Greenwood from the the Demon Lord Kardia, long sealed beneath the elven wood. But at the very moment they claim victory, two more overwhelmingly powerful demons appear! To prepare for war with humanity, these demons plan to sacrifice Kardia to their master, the Overlord Modanaram, so as to bring him to a truly terrifying height of power. Diablo is the only thing standing against them…and is it possible he might wind up being appointed king of the elves, despite being a demon himself?