The Dungeon’s Demon Lord is the Weakest

The Dungeon’s Demon Lord is the Weakest summary:

G.o.d left a dungeon in the hands of a boy known as the Demon Lord.
After receiving multiple blessings from G.o.d, let’s start my life as a Demon Lord! …Or that’s how it was suppose to be, but G.o.d 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. A story that all unfolded during a bath.