Don't know if anyone else has said this before, but I think the hyeokryeong were all one person before and that through an application of the demonic arts, instead of extract the soul or fragments of the soul from someone, the main hyeokryeong imparted soul fragments into others kind of like how the demon emperor fused with baek taesan or just somehow split himself into multiple person likely trying to achieve perfect form by expelling imperfections from his true self