City of Sorcery — Marion Zimmer Bradley

Haunted by mysterious images of dark, hooded figures and the cawing of strange birds, and drawn by the memory of two lost comrades, Magdalen Lorne, chief Terran operative on Darkover must pursue her quest not only to the frozen ends of the physical world, but the perilous limits of the spiritual overworld as well, for to find this city that no man had seen, she would be tested as she had never been before--by the evil sorcery of the Dark Sisterhood!