The Fathomless Caves (The Witches of Eileanan, #6)

In Eileanan, victory has finally been won, and those of faery kind--from the mighty dragons to the mysterious nyx--have sworn their friendship and aid. Only the sea-dwelling Fairgean have refused to sign the Pact of Peace. Driven by ancient hatreds, they have devoted themselves to destroying all who dwell upon the land.
Troubled by the darkness that may lie ahead, Lachlan agrees to release his beloved Iseult from the sacred oath that binds them together. It will then be for Iseult to decide whether she will remain in the mountains to resume life with her own kin, or return to the man who let her go.
To help bring peace to Eileanan, Iseult's flame-haired twin, Isabeau, must also face her most difficult challenge yet. But before she can prove herself worthy of both the scar of the Soul-Sage and the ring of the sorceress, she must first find a way to heal her own wounded spirit.