Little Star by John Ajvide Lindqvist
One autumn day in 1992, former pop singer Lennart Cederström finds something unexpected in the forest: a baby girl in a plastic bag, partially buried. He gives her the kiss of life, and her first cry astounds him; it is a clear, pure musical note. He takes her to his wife and persuades her that they should keep this remarkable child. But the baby becomes a strange girl, made more unusual by their decision to hide her in their basement to keep her from the prying eyes of government departments.
When she reaches puberty, a terrifying scene sees her kill both her parents. When her scheming adopted brother returns to find her over their bodies, he seizes the opportunity and enters her into an X Factor-style talent competition. She quickly becomes famous. In spite of this, she remains very lonely, until she befriends another damaged girl on the internet. They form a powerful bond and soon create a growing gang of other disgruntled girls and, calling themselves the Wolves, they set out to take revenge for all they’ve ever suffered.