I think I read this about four years ago (about 2016), although I didn't actually make my way to the end of the book. I think I acquired it as either an eBook or audio book from the library. The setting was vaguely Renaissance, I think, maybe tending toward the Victorian era. The chief protagonist was a female. Her mother was dead. Her father was a merchant who was arranging a loveless marriage for her for the purpose of alliances. She and her sister plot to escape, and somehow wind up snagging invitations (tickets?) for an event sponsored by a mysterious and somewhat malevolent (maybe only in people's minds) man. They sneak out in the middle of the night and attend the event, only to find that it's a deadly game where there can only be one winner who gains a significant boon. The sisters are soon separated. I think the protagonist happens upon an urchin in the streets who offers them more advice than they've earned, with that leading to them being in a hotel where the protagonist encounters her sister (or at that point, she thinks she's encountered her sister) who has somehow been consumed by darkness, which takes the form of a black stain on her.
There was a bit of a motif going on of locations hidden in plain sight, with the hotel in question being accessed by such a portal, with it all being part of the game.