How NOT to Summon a Demon Lord manga volume 21 features story by Yukiya Murasaki with art by Naoto Fukuda.

The war of the Fallen against the mortal Races begins. Eulerex, the Supreme Commander of the Demon Lord Forces heads one side, while the other is led by Governor Galford of Faltra. The Races put up a gallant opposition, but a Demon Overlord Cannon greatly reduces their numbers and the walls of the city begin to crumble. Klem and Edelgard come running to the rescue, but with the appearance of Demon Overlord Modinaram and the Fallen girl Ryoka, will they be able to turn the tide of battle?