
Spoiler
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She will feel less pain and become very strong but she won't lose the black lines.
This story is more about acceptance and tolerance. The black elves, necromancers vampires, Cale wants a world where all of them can walk around freely without having to hide and be afraid.
Hannah will proudly show off her black lines and refuse to hide them because it is proof she fought for her life, endured and survived. She is proud to have those ugly scars.
That will make Mary more confident and later she will reveal her face.
I think it's better than erasing the scars.

Spoiler
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To survive, Mary had to accept the darkness attribute and had to choose to become one of the two: either a black mage or a necromancer. Black mages don't feel pain while necromancers feel constant pain.
Mary chose the latter because she didn't want to run away from the pain. She will explain more about her motivations later.
Until the end of the current novel, did anything ever happen to Mary? Did she ever get healed, or at least lose the black lines on her skin? Seriously Mary deserves to be free of pain, the poor child.