A Promise of Peridot by Kate Golden

The next seductive, sweeping, action-packed installment in the addictive Sacred Stones trilogy.
A prophecy of death. A weapon of hope. A sacrifice of love.

Arwen Valondale is sailing for the mysterious Kingdom of Citrine after the battle of Siren’s Bay. Still reeling from the loss of her mother and the shock of her newfound powers, Arwen directs all of her pain and rage toward the man who betrayed her: King Kane Ravenwood.
But Kane’s presence is unavoidable as he leads Arwen and her friends on an expedition to recover the Blade of the Sun—a legendary weapon with the power to kill tyrannical Fae King Lazarus. Their uneasy truce proves difficult to maintain, as Arwen battles her unresolved feelings, and Kane is willing to become darkness itself to protect her.
Arwen knows failure to fulfill the prophecy will seal both her own fate as well as the fates of those she loves most. But as Arwen faces creatures, foes, and magic beyond her wildest imaginings, her resolve is tested in ways she never could have foreseen.