Complex sentence: The Kuchisake-onna is an enduring legend which began in the Edo period, with the woman initially covering her face with her robes, but this has evolved into a surgical mask (now that robes are less common) to stay in the popular imagination.
Split into simpler sentences: The Kuchisake-onna is an enduring legend which began in the Edo period, with the woman initially covering her face with her robes. This evolved into a surgical mask when kimonos became less common, allowing the legend to stay in the popular imagination.