The Dungeon's Demon Lord is the Weakest!?

God left a dungeon in the hands of a boy known as the Demon Lord. After receiving multiple blessings from God, let's start my life as a Demon Lord! ...Or that's how it was suppose to be, but God messed up and created the worlds' weakest Demon Lord ever. To continue living, I have to continue making dungeons. This story is about an idiotic protagonist, a lonely demons' child, an Undine who lives in a bathtub, and a princess who fell in love with the Demon Lord, even though she was human.