The Tombs of Atuan — Ursula K. Le Guin

WHEN YOUNG TENAR is chosen as high priestess to the ancient and nameless Powers of the Earth, everything is taken away -- home, family, possessions, even her name. For she is now Arha, the Eaten One, guardian of the ominous Tombs of Atuan. While she is learning her way through the dark labyrinth, a young wizard, Ged, comes to steal the Tombs' greatest hidden treasure, the Ring of Erreth-Akbe. But Ged also brings with him the light of magic, and together, he and Tenar escape from the darkness that has become her domain.