Complex sentence: Wol-ryung decides to become a human in order to be with Seo-hwa, and must live 100 days without showing his true form to human, taking their lives and helping anyone that needs aid.
Split into simpler sentences: Wol-ryung decides to become human in order to be with Seo-hwa. To do so, he must live 100 days without showing his true form to a human, taking a human life, or using his supernatural powers to help anyone that needs aid.