Maou ni Natta node, Dungeon Tsukutte Jingai Musume to Honobono suru

Synopsis:

Before I understood, I had turned into the watchman of a cell, a Demon Lord. I was all of a sudden in a perilous world with goliath beasts standing around, battling for their reality, so I considered building up the cell so as to not pass on but rather — - for reasons unknown I was having an incredible decent time… . How could it turn out this way? The hero, Yuki, living in that world currently, utilizes his strong and sharp faculties and invests energy with a preeminent winged serpent young lady and a vampire young lady; Sometimes giggling, now and again being irate, passing days doing what he loves.