Children of the Dryads - Legend of the Singer, #1

When her human father, Eldor, a member of an organization of elite warriors dedicated to protecting the Valor Alliance, is called on a mission she does not think he can survive, Tara-lin violates his orders and follows him. She is a half-elf, born with the long-lost ancestral magic of the elves. With her is Alis, a human girl desperate to avoid a marriage forced on her by her father, a colleague of Eldor, but terrified that gods Tara-lin does not believe exist will cast her into the netherhells for her disobedience.
Ahead of them is Eldor's terrifying destination - Nightshade Castle, haunt of madness and black magic.