The Weakest Demon Lord of the Dungeon!?

A young boy appointed as the Demon Lord, entrusted with a dungeon by God. Receiving various blessings from God, he embarks on his Demon Lord life! ...or so it should be, but due to God's careless mistake, he becomes the weakest Demon Lord in the world. To survive, the Demon Lord builds his dungeon every day.

This is a story about a slightly foolish protagonist, a lonely oni child, an ondine who lives in a bath instead of a lake, and princesses who fall in love with the Demon Lord, unfolding as they bathe together.