In Leigh Bardugo’s fantasy novel Shadow and Bone, the dark barrier dividing Ravka is the Shadow Fold.
The Shadow Fold is a vast region of unnatural darkness that cuts across Ravka and separates its western territories from the capital and eastern lands. It is filled with volcra, winged creatures that attack travelers, making crossing the Fold extremely dangerous.
The barrier was created centuries before the novel’s main events by the Darkling, also known as General Kirigan, when his magic went out of control. Its existence weakens Ravka politically and militarily because supplies and people cannot move safely between the divided regions.
The Fold is also called the Unsea, but “the Shadow Fold” is the novel’s standard name and the most precise answer. It should not be confused with the Little Palace, where Grisha train, or with the True Sea, the normal body of water bordering Ravka.