Once upon a time, the kingdoms of Wales were rife with magic and conflict - and 18-year-old Mererid 'Mer' is well-acquainted with both. As the last living water diviner, she can manipulate water with magic - a unique elemental power many would kill to possess. For years, Mer has been running from the prince who bound her into his service - and forced her to kill thousands with her magic. Now, all Mer truly wants is a safe, quiet life, far from power and politics. But then Mer's old handler - the king's spymaster - returns with a proposition: use her powers to bring down the very prince that abused them both.