A world is a sequence of zones. Worlds contain series of stages of enemies, a Miniboss and a boss. 8 worlds are played through in a run. Worlds with the player character as bosses are omitted from being in a run. Shopkeeper is an exception to this, as she gets an additional world in her route.
When the player ends the previous world with full health, for the first branching path of a world, the path furthest down will be denoted with a red line and have a red zone icon along the bottom path, denoting that said path will change the world to its dark variant. When entering a dark path, all zones for the rest of that path become dark. Dark zones have the following properties:
"Dark" is appended to the end of the world name
The background changes to a more ominous background
All enemies become raised by one tier, up to a maximum of 4
If the zone is a Shop, the Shopkeeper will sell items for HP instead of Money and the price is doubled.
If the zone is a Treasure zone, each chest will become replaced with a Cursed Chest.