The one named Mona
Mona has run off somewhere after you obtained the Hougeton, go locate her.
Complete the prologue
Gain entry to the Boulevard sewers and fight the boss at the end of the dungeon.
Go to the trash heap in the alleyway next to Orcmin soft, observe it. You can now descent into the Boulevard sewers.
Explore the sewers
Fight Rustsock Warmtail to gain entry to the rest of the sewers. You will have to fight through Corpi and Mell the Soothsayer as well. Explore the dungeon until you arrive in the deepest room with the Mysterious treasure chest. Interact to start the bossfight.
Can be completed since v0.2.2.0