Complex sentence: Skrzetuski passes through a devastated landscape to Czyhryn and then to Rozlogi but discovers, mistakenly, that Helena has been spirited away by Bohun.
Split into simpler sentences: Skrzetuski passes through a devastated landscape to Czyhryn and then to Rozlogi, where he discovers that Helena has disappeared. He mistakenly believes that Helena has been spirited away by Bohun.