The Sundown Path is a relatively peaceful level, with fewer enemies and more emphasis on atmosphere and Rucks' narration. You'll encounter some familiar enemies, but they are generally less aggressive. Take your time to absorb the environment and the story being told.
The path will lead you to a large monument. Interact with it to trigger a significant cutscene and dialogue. This is where the core conflict of the game, and the nature of the Calamity, becomes clearer. Pay close attention to Rucks' words and the information presented.