Before I realized, I had become the guardian of a dungeon, a Demon Lord. I was suddenly in a dangerous world with giant monsters loitering around, fighting for their existence, so I thought to go about developing the dungeon in order to not die but —- for some reason I was having quite the good time…. How did it turn out like this?

The protagonist, Yuki, having to live in that world now, uses his bold and sharp senses and spends time with a supreme dragon girl and a vampire girl; Sometimes laughing, sometimes being angry, passing days doing what he likes.

(Source: novelupdates)
https://ncode.syosetu.com/n2671do/