Maybe adult or could be teen story of girl being groomed for possible induction into witches’ circle that uses magical stones for healing, flying, etc. Slaves are made of abducted villagers around the lake area. The lake has monsters that attack and eat people.
Child of a Mad God is the title. It's part of the 'Coven' series by R.A. Salvatore
When Aoleyn loses her parents, she is left to fend for herself among a tribe of vicious barbarians. Bound by rigid traditions, she dreams of escaping to the world beyond her mountain home.
The only hope for achieving the kind of freedom she searches for is to learn how to wield the mysterious power used by the tribe’s coven known as the Song of Usgar. Thankfully, Aoleyn may be the strongest witch to have ever lived, but magic comes at price. Not only has her abilities caught the eye of the brutish warlord that leads the tribe, but the demon of the mountain hunts all who wield the Coven’s power, and Aoleyn’s talent has made her a beacon in the night.