Himbaran Castle is the name of this dungeon. The dungeon's background story was about a crazy king that liked to torture his people.
Once a month, the king would select one person from his people and bring them to his castle. Inside the castle, the person would be brought to the torture room that the king had prepared for his desire.
At first, the people didn't know the king's intention for selecting the person. However, as time passed by, people started to wonder about the person he picked. It was because after getting selected, those people never be seen or back to their home.
Of course, the king gave various reasons to hide the fact. For a few months, the fact could be hidden. However, the king seemed not satisfied with the number of people he brought. The quantity began to change. From once a month to twice a month and finally to once a week.