Turning into a mindless puppet of a parasite is one of the most terrifying fates that a human can come to suffer. The Aswang is a symbiotic organism made up of a Winger and a creature captured by it. To take control over the host, the Winger uses its natural magical powers. The human consciousness is then reduced to the role of a passive observer which is unable to even lift a finger. The Winger injects a mutagen into the victim’s blood which progressively changes its organism. That is why the Aswangs created from ordinary Aelion residents are incredibly strong.
Within the first several hours of injection the human can still be saved, as the mutation has not progressed too far, and the brain has not been influenced for too long. Simply killing the parasite is enough. But every passing hour diminishes the chances of survival. Once mutation is complete, it is practically impossible to save the human - if you kill the parasite, the host will die as well.