According to the classification over at the Immortal Gate, a Mysterious Yang Spirit Body is equivalent to a Pure Yang Daoist Body cultivated to the Four Yang Realm.
Moreover, it innately carries within the body a strand of Xuan Yang Qi that possesses Innate Dao Rhythm, making it excellent seed material for the cultivation of the Pure Yang Scroll.
In the Dance Tool Dao Academy of the Immortal Gate, there is a Mysterious Yang Spirit Body named Pang Yingru who is the same cohort as Chen Xiaohei. She has already successfully formed her core, and under Hua Zijing's arrangements has taken over the position Ming Yuhua previously held in the Hall of Orthodox Law, and can be considered the Primordial Infant Dao Seed that their lineage will重点培养 in the future.
