Snowlily in an Urban Fantasy AU perhaps?
When Caitlin was born, a fairy told her mother that a power resided in her daughter that would be unleashed in a terrible act. The prophecy had Carla teaching Caitlin that magic was terrible and she should avoid it at all costs.
Lily used to live next door to Caitlin. At a young age, Lily learned she had a talent for magic, specifically fire magic. Caitlin loved watching her, but always kept it a secret from Carla. Alas, her mother still found out and forbid the two from playing together again. She even went so far as to move to a part of town where magic was looked down upon.
Years pass before the two women meet again in the city. Caitlin has moved back to take a job in the urban area, exposing her to magic all over again. Lily takes it upon herself to show her the sights and experiences of the world Caitlin got to miss out on.
One night, when they're heading home, a couple of lowlifes try to mug them. Panic sets in and Caitlin ends up unleashing an icy blast at the robbers. It comes as a shock to both women, since Caitlin believed she was powerless.
Lily lets Caitlin sleep over that night. As they're getting ready to sleep, Caitlin tells Lily all the things her mother told her about magic. Lily explains the lessons that she grew up with- that magic can be good and terrible. It depends on the person who uses it, and Lily believes she will use it for good.
+1 As Lily drifts off to sleep that night, Caitlin is awake enough to hear someone knocking on the window. A mage is sitting on the sill who says she knows Caitlin's father. Her name is Amunet Black, and she can teach Caitlin about her newfound powers.