Fuminori Teshima 
An Archdemon’s Dilemma: How to Love Your Elf Bride (Manga) Volume 7 [EPUB ebook] 

Having finally revealed themself before Zagan at the evening ball, Archdemon Bifrons unleashes a dreadful sludge monster upon everyone present.

Bifrons’s own servant Nephteros quickly falls prey to the beast and is absorbed as part of the monster. Despite her past antagonism, Nephy pleads with Zagan to save the poor girl from her miserable fate.

Not one to turn down a request from his bride, Zagan sets forth to do the impossible as sorcerers and Angelic Knights alike must now band together in a desperate fight for survival!