To Love a Dark Lord by Kathryn Ann Kingsley

She pulled him close, in a desperate, perfect embrace. She loved him. And she would burn this world for him…
Gwen must free the Prince in Iron, before his mind is torn apart.
For centuries, Mordred ruled over Avalon and caged its magic. Now the elementals have seized power, sentencing him to an eternity of torment in his own magical prison.
Gwen accepted Mordred’s punishment to save his life. But the love that binds them cannot be denied. She vows to find him and release him, whatever the cost.
Mordred’s time is quickly running out. Trapped inside the Iron Crystal, he burns with rage and desire, haunted by shadows of the past. His sanity hangs by a thread.
Gwen will destroy anyone who stands between her and the crystal. But is she freeing the man she loves? Or unleashing a power more vicious and terrifying than Avalon has ever known?