I'm a Behemoth, an S-Ranked Monster, but Mistaken for a Cat, I Live as an Elf Girl's Pet, Vol. 3 (manga)

CAT-ASTROPHE STRIKES! After hearing that a demon has been preying on the town of Renald, Aria, Tama, and Vulcan have set out to save these villagers from their plight. Together with a squadron of powerful knights, they make an assault on the demon's lair-but soon find that they've walked straight into a deadly trap. To protect his master, Tama may have to steel himself and do something that would mean saying good-bye to these happy, peaceful days with Aria once and for all!