Striga

"Where does it come from? Spells, magic?"
"I have no idea, sire. The Sages research these phenomena. For us witchers, it is enough to know that strong will may create them. We also like to know how to fight them."
"And kill them?"
"Most frequently, yes. That is what we are usually paid for. Few want the spell lifted. People usually just want to be protected. If the monster has killed people, revenge may be another motive."

Curse and transformation. There is not much known about the curse that turns females into striga. Such curse born from the sheer hatred of someone, those same people, in an act of vengeance often ask advice to hags, which will tell them how to prepare the ingredient for the curse. Said ingredients are a secret except to the malicious hag.

Lifting the Curse. In order to lift the curse on a striga, "someone must prevent the striga from returning to her coffin by the third crowing of the rooster. Then she would be cured, turning into an ordinary little girl".

Life After the Curse. After a person has been cured from the striga-curse, there is the chance that the person will not fully recover mentally, keeping part of the vicious and somewhat dull-witted nature of her previous striga-form. There is also the chance of a relapse, turning back into a striga. To prevent this, "cured" strigas wear sapphire amulets and participate in rituals designed to ward off the curse.