The actual truth about Alchemy of souls
Many people are still confused about it so let me clear it.
Naksu/Cho yeong are the same person and is one soul. Cho yeong after her parents got killed became an assassin named Naksu.
Mu deok/Jin bu yeon (Jin seol ran's reincarnation) are the same person and is one soul. Jin bu yeon heir of Jinyowon got seperated from her mother and got adopted by an old lady in a village and got the name mu deok.
Naksu performed Alchemy of soul spell to shift souls with Mu deok unaware of her identity. She was able to shift her soul in my deok's body but couldn't make mu deok's soul leave the body.
Then Mu deok continued to use Naksu's energy to her advantage result of which was Naksu was able to see. The reason why Naksu wasn't able to use her powers was explained by Jin cho Yeon in an episode briefly. I think it was a conversation between Jin cho Yeon and mu deok when Jin cho Yeon was staying at Jang UK's house after she and park dang gu got caught dating. She told my deok that strong powerful priestess are unbeatable as they can lock away someone's energy or use it for them or completely destroy it.
The reason why mu deok started killing people was not because she was a soul shifter but because she was fed bell potion which helped Jin mu control her.
After naksu jumped in the lake with mu deok's body and Jinyowon retrieved it Master Lee cured it and in doing so Jin bu yeon aka mu deok used Naksu's energy which in turn changed her face to original naksu face as a result of using too much of her energy.
Why Master Lee never turned as other soul shifter is also a popular question? That was because there were two ways of changing souls. Alchemy of souls which was black magic and thus banned. The second was by Hwansu the highest level a mage can reach where the soul can be shifted with only someone who has already died(probably as it wasn't considered black magic) so people who did Alchemy of souls went berserk but people performing Hwansu weren't affected