The Serpent's Tooth — Diana L. Paxson

She is called Cridilla--the youngest daughter of Leir Blatonikos, the mighty Celtic warlord who conquered three kingdoms. Beautiful, loyal and warrior-trained, she grows to womanhood in a magnificent, barbaric land--and learns lost secrets of the Goddess from Crow, enigmatic mystic and King's fool. But poisonous treachery festers within the bosom of her own family. And in the end it is Cridilla who must be strong in the face of her sisters' dread perfidy--to defend a noble liege blinded by a father's love from the inspiring storm that threatens to tear his house and his realm asunder.