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He was weakened by drinking Sooin's blood for a long time but it didn't kill him. I think his vampirism (idk what to call it hehe) became fragile because of it especially when he was on the brink of death (if you'll think abt it vamps cannot die so maybe it's canon that when vampires are about to die they somehow 'regain' their humanity. idk if thay makes sense but yeah) and that's when vampire Sooin did the conversion thingy to bring Hwan (who is now human) back to life and made him his link.
Sorry for the lengthy explanation. I hope u understand it in some way. (((:
How did Chihwan become human again?