Complex sentence: The book takes place on the planet of Worlorn, which is itself dying; it is a rogue planet whose erratic course is taking it irreversibly far from its neighboring stars into a region of cold and dark where no life will survive.
Split into simpler sentences: The book takes place on the planet of Worlorn, a world which is dying. It is a rogue planet whose erratic course is taking it irreversibly far from its neighboring stars into a region of cold and dark where no life will survive.