Now I'm a Demon Lord! Happily Ever After with Monster Girls in My Dungeon: Volume 4 by Ryuyu

Reborn as a demon lord in another world, Yuki’s living his best dungeon life! Things are more laid back than ever now that he’s returned from the royal capital. Or not—because the Dragon King, Gyuohga, shows up out of nowhere with his sights set on Lefi. “Become my mate!” he demands. This sends Yuki into a frenzy, challenging the Dragon King to a one-on-one fight! But Yuki’s got more than just that to contend with. A demon appears before him with a request: “I’d like you to meet the king of the demon world.” Intrigued by the idea of being a tourist in the demon world, Yuki accepts the invitation and heads off. What awaits him there, however, is a man with a pretty face and an incredibly shady smile...