Now I'm a Demon Lord! Happily Ever After with Monster Girls in My Dungeon

Yuki finds himself reincarnated as a demon lord in another world, and he’s been put in charge of a dungeon to boot! Once Yuki accepts that his fate hinges on the dungeon’s survival in a world where powerful monsters roam free and danger lurks at every corner, he begins gathering allies and bolstering his defenses. Within a few days, he takes on a legendary dragon, an adorable pet slime, and a vampire girl, but despite all his precautions, life as a demon lord in a dungeon constantly under threat...is surprisingly mellow? Of course, with such a colorful cast of characters (and possibly more on their way), Yuki’s bound to wind up in some trouble. Will he be able to pull off a reckless rescue mission? And how will he and his monster girls survive an invasion by a hostile human kingdom?! Join him as he juggles daily life, responsibilities as a freshly minted demon lord, and his growing family of monster girls and pets!

(Source: J-Novel Club)