I'm a Behemoth, an S-Ranked Monster, but Mistaken for a Cat, I Live as an Elf Girl's Pet, Vol. 13 (manga)

In their search for the newly empowered Belphegor of the Seven Great Demon Lords, Tama and Aria's path leads to the other world, Ark, and the unknown realms beyond it. But they've had no sign of their enemy's whereabouts... The one who holds the key is the thunder spirit Volt, one of the six spirits passed down in the land of Ark who was once defeated by Belphegor. To defeat the demon lord, Tama and Aria will have to undergo many more new trials...