Nyx was born to a family of talented mages in Nohr. She was a prodigy in dark magic arts, as was the rest of her family, but quickly became too powerful. As a result, she started testing her abilities on people for fun, killing many innocent civilians. She wove a spell so dark and powerful, its magic inflicted an unbreakable curse, causing her to be unable to age. In her support conversations with Xander, she reveals that she quickly became suicidal and depressed after she was cursed. She hid herself away from the world, seeing no possible future for herself. However, Nyx decided that she wanted to live in order to atone for her crimes, and has since strived to overcome her past.

Voice Ref:  https://www.youtube.com/watch?v=fK4Do3u04u0