Responses
In this case it's simply an effect of Sooin's blood. Like an animal that develops a poison to combat its predators. This poison doesn't kill, as many poisons don't, but actually cures the vampire's of being vampires. But if you look at it from the point of view in that it weakens something that is immortal to the point of being mortal, that's where you get the term of calling it a poison. Imagine it like a toxic that doesn't kill you, but weakens you so much that a simple cold could kill you. That's essentially what Sooin's human blood did.
Girl, this should be named "Everyone needs therapy" because bruhhhhh, so many of the character were lowkey crazy no offense (not everyone but a lot of them). There were too many plot holes like how did Chi-Hwan even become a vampire??? I also think Chi-Hwan and Sooin were very dependent on each other which honestly isn't healthy in my opinion.
And can someone explain to me how Chi-Hwan became a human?? It was because his powers weakened so badly to the point where he became a human? Because that doesn't even make sense honestly.
PLEASE DON'T COME FOR ME, just my thoughts hehe Σ( ￣□￣||)