@maruuu your theory is good. But did y'all forgot about what rose said to geurim back then? He can't go back once he drink human blood and geurim understood the meaning of it after he drank chae ah bloods. The nanny turned into human again because he didn't drink human blood yet. So everyone that turned into vampire can go back to being a human again if they don't drink human blood (maybe no matter how long it is?). And only geurim can do that because that's his ability (i mean his blood). And one more thing, I think geurim's blood can hurt all the vampire not only their father (just my theory). Hope he/they realize it soon.