The dungeon is filled with miasma. Without the Goddess's Blessing you cannot return alive.
Set a goal floor, receive the Blessing you need, and then set out on your adventure.
Clear the floor you set and you earn rewards. If you fail, you receive no rewards other than EXP, so set your goal floor carefully.
Required Blessing increases by 1 every 5 floors. Blessing recovers 1 every 2 minutes.