The Erl King’s Children
From the publisher:
Imprisoned in an forlorn tower on a deserted headland, Lyllith, the last of the royal line of Érainn, prepares to use her only remaining weapon against the man who murdered her father and ravaged her land—her own death. Riv Orrsa, her kingdom’s conqueror, cannot hold the throne legitimately without her as his wife, so she waits to die as if waiting for an old friend.
But then a ghostly young boy appears in her barren cell to offer the one thing that changes it all: revenge. Accepting that chance plunges Lyllith in the middle of a contest thousands of years in the making, for the young boy is not what he seems, her new freedom is illusory, and she is the unwitting heir to an ancient bloody legacy with the power to unmake both their worlds.