Konosuba: God's Blessing on This Wonderful World!, Vol. 9 (light novel) Crimson Fate

A fateful encounter! In its endless war against the Demon King's army, the capital faces a new crisis in the form of the enemy's latest general-the dark god Wolbach. When Magumin learns of Wolbach's approach, she drags Kazuma (against his will) to a fortress just on the edge of the front line so they can mount a defense. But to their horror, they discover that Wolbach is also a user of Explosion magic! Just what is this Explosion master's relationship to Megumin..? And what could she possibly want with Chomusuke?!