Have you ever wondered why so many coincidences happen in a novel? Why the protagonist has to go on countless adventures or the question as to why the perfect solution to a crisis only appears whenever the protagonist arrives on the scene?
Simple—because everything is planned!
The City of Sin, the organization responsible for these tasks and is also the transit hub between parallel worlds.
The reason that it is named as such, is because the people who live here are NPCs who play the role of the villains!